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Analysis Volume I

Editor

Zeev B. Alfassi, Ph.D. Professor

Department of Nuclear Engineering Ben Gurion University of the Negev Beer Sheva

Israel

NIC

LIBRARY

CRC Press, Inc. Boca Raton, Florida

Library of Congress Cataloging-in-PublicationData Activation analyis / editor Zeev B. Alfassi. p. cm. Includes bibliographical references. ISBN 0-8493-4583-9 (v. 1). -- ISBN 0-8493-4584-7 (v. 2) 1. Nuclear activation analysis. I. Alfassi, Zeev B. QD606.A252 1990 543' ,0882--dc20

89-24021

CIP This book represents information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Every reasonable effort has been made to give reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. All rights reserved. This book, or any parts thereof, may not be reproduced in any form without written consent from the publisher. Direct all inquiries to CRC Press, Inc., 2000 Corporate Blvd., N.W., Boca Raton, Florida 33431.

" 1990 by CRC Press, Inc. International Standard Book Number 0-8493-4583-9 (Volume I) International Standard Book Number 0-8493-4584-7 (Volume 11) Library of Congress Card Number 89-24021 Printed in the United States

PREFACE Elemental analysis is done best by nuclear methods since these are determined only by the nuclei and are not affected (in most cases) by the surrounding electrons, i.e., the chemical environment. Activation analysis is a method of quantitative chemical analysis of the elemental composition of the samples based on the nuclear activation of the atoms of the chemical elements present in the analyzed sample. Activation analysis usually has the following advantages: (1) simultaneous multielement analysis, (2) very high sensitivities (detection of limit in the range of ppm and ppb or less), (3) nondestructive analysis, and (4) easy and fast analysis which in many cases can be automated. The book describes both prompt measurements (both y and particles) and delayed activities (mainly y-ray spectrum). The book treats the various methods of activation, i.e., activation by neutrons, accelerated charged particles, and high-energy photons. Special chapters are devoted to the application of these methods in the fields of life sciences, biological materials, coal and its effluents, environmental samples, archeology, material science, and forensic studies.

THE EDITOR Z. B. Alfassi, Ph.D., is a professor and the chairman of the Nuclear Engineering Department in the Ben Gurion University, Beer Sheva, Israel. Professor Zeev B. Alfassi received his B .Sc. and M.Sc. degrees from the Hebrew University in Jerusalem in 1964 and 1965, respectively, in the fields of chemistry and biochemistry. He received his Ph.D. from the Weizmann Institute of Science and the Soreq Nuclear Research Center in 1970. Professor Alfassi is a member of the council of the Israel Nuclear Society. He has published more than 100 scientific papers and edited the CRC book Chemical Kinetics of Small Organic Radicals. His current research interests include chemical analysis by nuclear methods, radioisotope production and uses, radiation chemistry and chemical kinetics of radicals in solution, and solubility of electrolytes in water-miscible organic-solvents mixture.

CONTRIBUTORS Volume I D. D. Burgess, Ph.D. Manager Chemex Labs Ltd. Mississauga, Ontario, Canada

Mariana Mantel, Ph.D. Senior Radiochemist Soreq Nuclear Research Center Yavne. Israel

William D. Ehmann, Ph.D. Professor Department of Chemistry University of Kentucky Lexington, Kentucky

S. Iraj Najafi, Dr. Ing. Senior Research Scientist Department of Engineering Physics Ecole Polytechnique Montreal, Quebec, Canada

John J. Fardy, M.S. Leader Radiochemistry Group Centre for Advanced Analytical Chemistry CSIRO Division of Fuel Technology Meani, New South Wales, Australia

Nobuo Suzuki, Dr. Sci. Professor Department of Chemistry Tohoku University Sendai, Japan

Vincent P. Guinn, Ph.D. Professor Department of Chemistry University of California Irvine, California

Masuo Yagi, Dr. Professor Institute for Materials Research Tohoku University Sendai, Japan

To my parents Arieh and Lea the lion and the lioness

TABLE OF CONTENTS Volume I I. GENERAL Chapter 1 Introduction - Principles of Activation Analysis ........................................3 Z. B. Alfassi Chapter 2 Computerized Analysis of y-Ray Spectra.. ...............................................9 , S. I. Najafi Chapter 3 Optimization of Instrumental Activation Analysis .......................................39. D. D. Burgess Chapter 4 Limits of Detection in Instrumental Neutron Activation Analysis .......................55 V. P. Guinn Chapter 5 Radiochemical Separations in Activation Analysis J. J. Fardy

......................................61

Chapter 6 Use of Delayed Neutrons in Activation Analysis.. ...................................... 9 7 Z. B. Alfassi Chapter 7 Use of X-Ray Emitters in Activation Analysis .........................................1 1 1 M. Mantel Chapter 8 .I31 Stable Isotope Dilution Activation Analysis ........................................... M. Yagi Chapter 9 Substoichiometric Radioactivation Analysis N. Suzuki

............................................145

Chapter 10 Utilization of Chemical Derivatives in Activation Analysis.. ...........................165 W. D. Ehmann Index ................................................................................... I 7 3

VOLUME OUTLINE Volume I1 ACTIVATION METHODS Activation with Nuclear Reactors 14 MeV Neutrons Activation Analysis Prompt Activation Analysis with Charged Particles Photon Activation Analysis Activation Analysis with Isotopic Sources Activation Analysis with Small Mobile Reactors APPLICATION OF ACTIVATION ANALYSIS Activation Analysis of Biological Materials Activation Analysis of Coal and Coal Effluents Activation Analysis of Water Samples In Vivo Activation Analysis Activation Analysis in Archaeology Activation Analysis in Forensic Studies Activation Analysis in Atmospheric Environmental Samples Activation Analysis in Agriculture and Botany Activation Analysis of Semiconductor Materials Depth Profiling of Silicon by Nuclear Activation Methods INDEX

General

Volume I

3

Chapter 1

PRINCIPLES OF ACTIVATION ANALYSIS Zeev B. Alfassi

TABLE OF CONTENTS I.

Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

11.

Basic Nuclear Chemistry.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4

Activation Analysis

I. INTRODUCTION Activation analysis is a method of quantitative chemical analysis based on the nuclear activation of the chemical elements present in the analyzed samples. The method is one of the most sensitive methods of chemical analysis. The discovery of nuclear activation, i.e., the reaction of elements with other nuclei or subnuclei particles to give radioactive substances, was made in 1934 by Irene Joliot Curie and Frederic Joliot who bombarded aluminum, boron, and magnesium with naturally occurring alpha particles. The suggestion to use the activation method for elemental analysis was done in 1936 by Hevesy and Levi who used neutrons as the bombarding projectiles to activate dysprosium and europium. The method could be used either if newly produced nuclei were radioactive with appropriate half-lives and emitted radiation (by measurement of this delayed radiation) or even if the newly produced nuclide was stable or had undesired radiation or half-life by the measurement of the radiation (photons or small particles) emitted in the time of activation - a prompt radiation measurement. The use of delayed measurement is the more common one. The use of prompt method is not limited to cases where the formed nuclide has undesired nuclear properties but also in some other cases where its advantages (e.g., less activation or smaller sources) overcome the disadvantages, (e.g., less sensitivity or selectivity), e.g., on line measurement of bulk elements in coal by isotopic source neutron irradiation or in vivo measurements. Activation analysis relies on the nuclear reaction between projectiles and target nuclei. In delayed measurement, the nuclear reaction leads to production of radioactive nuclides in the samples and the amount of the radioactive atoms of each element are measured subsequently by nuclear radiation detectors. In prompt measurements, nuclear radiation (photons or particles) emitted simultaneously with the nuclear reaction are measured, independent if the product of the nuclear reaction is radioactive or not. Activation analysis can be used to determine either trace elements or bulk elements. The trace elements are determined usually by delayed radiation since the prompt measurements are less sensitive. The analyzed samples can be activated by bombardment with thermal neutrons, fast neutrons, energetic charged particles, or high energy photons. The activation with thermal neutrons is the most widely used form of activation analysis due to larger flux and large cross-sections. The amount of emitted radiation, either prompt or delayed, depends on the number of atoms which have been activated. The number of activated atoms is proportional to the number of atoms in the target sample, and consequently, the activation method can be used to analyze quantitatively the elemental contents of the sample. The rate of nuclear reaction in a given irradiation system is proportional to the flux of the incident radiation and the number of the target nuclei. The proportionality constant depends on the probability that an incident particle will react with a target nucleus. This probability expressed in terms of area is called the cross-section s of a particular reaction. cmZ.Besides these factors (flux of projectiles, The unit used for s is the barn 1 b = number of target nuclei, and activation cross-section), the induced activity depends on the duration of the irradiation and the half-life of the formed radionuclide. The nuclear activation of the analyzed samples is followed by the quantitative detection (counting of radioactive emission) and identification of the induced activity as to the type of radiation emitted, its energy, and its half-life. The most common and sensitive method of activation analysis is by activation with thermal neutrons in the high flux of a research reactor and measuring the radioactivity induced in the newly formed radionuclides from the stable isotopes present in the sample. For example, the amount of bromine in a sample is measured by the 616 keV gamma emission of 'OBr formed from 79Br(n,y)80Br(abundance of 79Br = 50.7%, cross-section = 8.5 b, half-life of 80Br(t,,2) = 17.6 min, intensity of y

Volume I

5

rays = 6.7%) or by 82Br777 keV gamma line from 81Br(n,y)82Brand 81Br(n,y)82mBr+82Br 0.26)b, half-life of 82Br = 35.3 (abundance of 81Br = 49.3%, cross-section = (2.43 h, intensity of y rays = 83%). When all the parameters determining the induced activity (the flux of the projectile and its energy spectrum, the cross-sections and their energy dependence, the time of irradiation, half-lives, and detector efficiencies) are known, the quantity of the elements can be calculated from the measured radioactivity. However, in most analytical cases, a use of a comparator, which is irradiated under the same conditions as the sample, is preferred. The comparator consists usually of a mixture of elements in which the concentration of each element is known quite accurately. The quantity of an element X to be determined is given by the equation

+

quantity of X in sample

=

quantity of X in comparator .

radiation of X in a sample radiation of X in comparator

(1)

The use of multielement standards cancels the errors due to unknown energy spectrum and excitation functions (energy dependence of cross-sections), although other errors, e.g., self-shielding and interfering peaks are not removed. These errors depend not only on the contents of the determined elements, but also on the concentration of the main elements in the matrix, and unless the two matrices are quite similar, they are not removed. A great deal of effort is done to increase the accuracy of the measurements using a monostandard and to prepare reliable multielement standards. The irradiated samples usually contain more than one element which becomes radioactive and hence the accuracy and the sensitivity of the analysis are often dependent on the ability to distinguish between the radiation of the different radioisotopes or to separate the different elements in the irradiated mixture. If a complete separation of the elements is done, their radioactive emission can be measured very accurately with simple instrumentation. However, this method requires a lot of chemical work and this method is almost neglected in most cases and a purely instrumental activation analysis (IAA) is used or at least the separation of the mixtures to small number of fractions (see Chapter 5) in which several elements are measured simultaneously. The purely IAA has the advantages of (1) multielement determination in one measurement, (2) lower costs of analysis although it requires more expensive equipment for measurement, (3) ease in automation, (4) ability to use short-lived radioisotopes which for some elements are the only ones available, (5) being nondestructive and being able to be used for precious samples or those which do not dissolve easily, (6) no question of efficiency of the separation process and the errors due to this process are involved. Activation analyses are done mainly with the measurement of gamma rays due to their high penetration (in contrast to charged particles and X-ray photons), the large number of elements which produce gamma-emitting nuclides, and the existence of high resolution gamma-ray spectrometers which allow the determination of the energy of the gamma rays. The different radionuclides produced in activation analysis are distinguished by their different gamma-ray energies (see Chapter 2 for computerized analysis of gamma-ray spectra) and sometimes also by their half-lives. There are two kinds of interferences in IAA by gammaray analysis besides the interference from highly radioactive products due to major constituents which can mask the gamma-ray spectra. The first kind of interference is when the same radionuclide is produced from two different elements, e.g., 28A1which is produced from 27Al(n,y)28A1, 28Si(n,p)28A1,or 31P(n,a)28A1.This interference can be solved if one of the elements also produce another radionuclide, e.g., natural silicon also produces 29Alvia constant, ~ S i the amount of the contribution the reaction 29Si(n,p)29A1.Since the r a t i ~ ~ ~ S iisl ~ of silicon to '*A1 can be calculated from the activity of 29Al which is formed only from

6

Activation Analysis

silicon. Another way is in the case where the cross-sections of the interfering reactions depend differently on the energy of the activating particles. In that case, the irradiation with projectiles at different energies allows the measurement of both elements. For example, in the case of 28A1,the (n,y) reaction is done mainly by the thermal neutrons, while for the (n,p) and (n,a) reaction, fast neutrons are required. Thus the use of thermal neutron absorbers (cadmium or boron) enable the determination of both elements (see Volume 11, Chapter 1). Another example is the determination of lithium and boron by proton activation analysis via 7Be formed by the reaction 7Li(p,n)7Beand l'B(~,a)~Be. It was found that, with 1.%-MeV protons, only the second reaction occurs, while at 7.4-MeV protons, the first reaction has higher cross-section than the second one. Thus by the use of proton irradiation at both 1.55 MeV and 7.4 MeV, both Li and B can be determined. In most cases, the radiation measured is gamma radiation. However, in several cases, use of other radiation, such as beta particles, X-rays, and neutrons, are used. If it is impossible to separate the activities due to the undesired elements from the overall spectra; radiochemical separation of the irradiated sample is required in order to get rid of the interferences. The separation of the desired elements from the interfering ones is preferably performed after the irradiation and not before irradiation to eliminate the possible contamination of the sample by the chemicals used (see Chapter 5). Some techniques were devised to facilitate activation analysis where it is not possible or not sensitive enough, e.g., the use of derivative techniques or substoichiometric activation.

11. BASIC NUCLEAR CHEMISTRY In this chapter, only the basic ideas of nuclear reactions are dealt with. The notation for nuclear reaction is similar to chemical reaction notation, however, in a more concise form. For example, the first nuclear reaction which was studied was the reaction of radioactive nuclides of alpha particles with aluminum to give 30Pand neutron which the chemists write as 4He "Ab3OP In, while the nuclear chemist writes it in the concise form 27Al(4He,1n)30P or 27Al(a,n)30P.In this notation, the bombarding particle and the light product are written in parentheses between the initial and final nuclei. The atomic numbers are usually omitted and neutron, proton, deutron, triton, and alpha particles are represented by the symbols n, p, d, t, and a , respectively. In nuclear reactions, there is conservation of the number of protons and the number of neutrons (i.e., both the sum of atomic numbers and mass numbers are conserved). energy are conserved in nuclear reaction and the energies of the reaction The mass can be calculated from the differences between the masses of the products and the masses of the reactants. When a positively charged particle reacts with a target nuclei, the bombarding particle must have a sufficient energy to overcome the Coulombic repulsion energy - the Coulombic barrier:

+

+

+

E,,

= 1.109

A, + A2 Z,Z, (MeV) AF3 Al Akt3

+

where A is the mass, Z the charge, and 1 and 2 are the particles and target, respectively. The probability for a nuclear reaction is expressed in terms of the reaction cross-section -a. This term is derived from the elastic scattering geometric cross-section mz,which a target nucleus of radius r presents to an incident particle. Consider the irradiation of a target of area A, thickness X, and which contains n atoms per cm3 by a homogenous perpendicular beam of particle. The sum of cross-sections of all particles in the target is A X n a where a is the geometric cross-section. Assuming that the probability of an incident particle to react within the target is the ratio of the geometric cross-reactions of all target nuclei to

. .

Volume I

7

- -

the area of the target, the probability equals X n a. This use of u in the term of the probability is used for all reactions even if u is not always given by m 2 . The rate of a specified nuclear process, R, (events per unit time), induced by a beam of particle impinging on a thin target in which the beam is very little attenuated, is given by the equation

where ui is the cross-section for the specified process in units of cm2. The usual unit for cross-section is the barn where 1 b = lOPz4cm2.I, the fluence, is the number of bombarding particles per unit time. When the sample is situated in a uniform flux of particles bombarding it from all directions, as for example in a nuclear reactor, I is replaced by the flux 4, being the number of bombarding particles per unit time per unit area, (+ = VA). nXA gives the total number of nuclei in target, N,, and Equation 2 is transformed into the equation

+

+

the flux is given in units of number of impinging particles per unit area per unit time. When several reactions can occur between the incident particle and the target nuclei, each process has its own partial cross-section which gives the rate of formation of the specified product. The cross-section for disappearance of the target nuclei is the sum of the partial cross-sections. -

Volume I

9

Chapter 2

COMPUTERIZED ANALYSIS OF GAMMA-RAY SPECTRA

.

S Iraj Najafi

TABLE OF CONTENTS I.

Introduction ...................................................................... 10

I1.

Analytic Approximation of Gamma-Ray Spectra .................................10 A. Physical Aspect .......................................................... 11 B. Analytical Representation ................................................ 11 C. Fitting Method ...........................................................14 D. Calculation and Comparison ..............................................16 1. Distortion Is a Symmetrical Gaussian .............................17 Exponential Distortion ............................................17 2. 3. Asymmetrical Distortion Function ................................17 4. Deformed Gaussian ...............................................17

111.

Analysis of Gamma-Ray Spectra by Nonlinear Least-Squares Fit ................ 18 A. Analysis by the Program Developed by Najafi ...........................20 1. Smoothing of the Experimental Data ..............................20 Peak Searching ...................................................21 2. Selection of Fitting Intervals ......................................24 3. 4. Peak Energy Calculation ..........................................24 B. Analysis by SAMPO Program ............................................24 Analysis by SPAN .......................................................25 C. Calculation and Comparison .............................................. 25 D.

IV .

Analysis of the Spectra Using a Summation Method .............................27 A. Peak Search .............................................................. 27 B. Peak Integration .......................................................... 28 Application ............................................................... 28 C. 1. Single-Element Analysis ..........................................28 2. Multielement Analysis ............................................28

V.

Gamma-Spectra Analysis by an Empirical Method ...............................31 Parameters of a Photopeak ...............................................31 A. B. Experimental Study of Variation of Parameters ..........................31 C. Translation of Standard Peak .............................................33 D. Fitting Method ........................................................... 34 Application of the Method ...............................................34 E.

References ...............................................................................38

10

Activation Analysis

I. INTRODUCTION With the use of high-resolution semiconductor detectors and multichannel analyzers, gamma spectrometry has become an important tool for numerous applications: detection of natural and artificial radioactivity and activation analysis. The energy resolution of these systems allows the selective detection of radiosotopes and the determination of their activity. To benefit from the high resolution of these detectors, the gamma spectra should be digitized using a large number of channels. These spectra, especially those of fission products and those from samples activated by irradiation, can be very complex. It is not practical to analyze these spectra by manual numerical techniques. Use of computers is essential. Automatic analysis of the gamma-ray spectra has been studied by several investigators. The reported analysis approaches are based on different principles depending on the type of available computer and the complexity of the gamma spectrum. Many algorithms and computer programs will be mentioned in this chapter and quantitative comparison of several methods will be performed when different phases of numerical analysis are considered. A gamma spectrum consists of a series of rays which are characteristic of the radioisotopes present in the sample, and a continuous background. The two principal roles of the numerical analysis are the automatic peak search and the measurement of gamma-ray energies and intensities. In the literature, a number of methods for gamma-ray spectra processing have been reported. The methods of peak search are based on second derivative operation,' resolution enhancement using Fourier Tran~formation,'.~resolution enhancement transformation by convolution operati~n,~.' statistical comparison of channel inten~ities,~.~ and the first derivative method.8-" A few of these techniques will be examined and compared in this chapter. For energy and intensity calculation, many codes are based on the principle of leastsquare fitting of a portion of the spectrum using an analytical f ~ n c t i o n . ~ , ~ -The " ~ 'form ~-~~ of the peak depends on the physical process taking place in the detection of the gamma ray and varies from one detector to the other. One has to select an analytical function and then determine its parameters. This is generally performed by nonlinear least-square fitting. These methods involve the use of large and fast computers. In this chapter, we will discuss these techniques. Another class of codes uses more simple in which the peak area is simply calculated by a sum of channel contents with subtraction of an estimated background. These codes give the possibility of using small size and low cost computers. However, they cannot analyze the overlapped peaks. The performance of this kind of technique will also be studied and will be compared to the methods which use nonlinear least-square fit. Finally, we will discuss a technique36 which conciliates the advantages of the abovementioned methods: resolution of overlapped peaks, but using small computers. In this approach, several parameters defining the gamma ray can be fixed experimentally. The remaining parameters can be determined by linear least-square fit which can be done by a small computer. The performance of this technique will be examined and compared to the nonlinear least-square method using examples of activation analysis.

11. ANALYTIC APPROXIMATION OF GAMMA-RAY SPECTRA In analysis of the gamma-ray spectra by a germanium detector, the precise determination of the energy and the intensity is based on fitting of the photopeak by an analytical function. This fitting is performed by nonlinear least-square method. In order to achieve a fast and high-precision analysis, a minimum number of parameters should be utilized to define the peak. In this section, several analytical representations for the line shape of photopeak will will be used. be studied. In the calculations, a test spectrum, that of 177mL~,

Volume I

11

A. PHYSICAL ASPECT The exact calculation of the response of the detector to the photons is very difficult because of the complex physical and statistical phenomena involved in the process. This calculation requires a precise knowledge of the property and the structure of the detector which are not well understood in most cases. For this reason and because the shape of the peaks can be sensitive to small variations in the experimental parameters, such as counting rate and degree of collimation, one has to determine directly an analytical representation for the photopeak. However. to define an analytical representation, one has to take into account the physical mechanism involved in the detection of gamma rays. The first contribution to the photopeak in a semiconductor detector is due to the statistical fluctuations in sharing the absorbed energy between ionization and heating the crystal network. This gives rise to a Gaussian distribution with a small width. The average energy to create a pair of electron-hole in cooled germanium is E = 2.98 eV. The variance, crZ,of the produced electron-holes is given by

where E is the incident energy and F is Fano factor." F is about 0.13 for germanium. The contribution from statistics to the full width at half maximum is

The difference between the calculated value given by Equation 2 and the measurement depends on factors such as the quality of the detector and the associated electronics. The physical properties of the material and the impurities in the detector affect the charge collection and the electronic background. Incomplete compensation of the impurities and partial collection of the charges deteriorate the energy resolution and cause a distortion in low energy side of the peak. In low energies, the contribution of the preamplifier to the background is high. At higher energies, the instability of the amplificator and the analyzer affects the width of the photopeak. The continuum under the peak is due to Compton effect from the gamma rays of higher energies and the background.

B. ANALYTICAL REPRESENTATION Following the considerations mentioned in the preceding section, the analytical representation of the photopeak should have a principal Gaussian form. However, a deviation from this Gaussian form should be included in the analytical description. Moreover, the approximately linear region in two sides of the peak have different heights. The contribution to the low energy side of the peak can be taken into account using a step function. The second part of the distortion is usually eliminated; if included, it has to be described by a function which joins smoothly the background situated in each side of the peak. The functions proposed by different authors fall into two categories: (1) supplementary terms are simply added to a dominant Gaussian form, and (2) the principal Gaussian function is deformed by convolution, by multiplication by another function, or by joining it to another function. In general, the photopeak can be represented by

where n = channel number, D = tail distortion, G = Gaussian function, B = linear or polynomial background, and S = step. The symbol * indicates that the distortion should be added, multiplied by, or convoluted

12

Activation Analysis

FIGURE 1 . Analytical components of an isolated photopeak (see Equation 3). In (a) the tail D is incorporated in the main Gaussian G by multiplication or convolution; in (b) D is simply added.

with the Gaussian. These four components are shown in Figure 1. In most cases, the tail component is represented by an exponential or a second Gaussian in the low energy side. Routti and Prussinl have used a Gaussian form for the central part of the photopeak. Both left and right tails are approximated by exponential functions. These exponentials are joined to the central Gaussian so that the line shape and its first derivative are continuous. The function has the following form,

where n = channel number, PI = constant parameter in continuous background approximation, P, = slope of background approximation, P, = height of Gaussian, P4 = abscissa of Gaussian, P, = width of Gaussian (full width at half maximum = 2.354 P,), P,2 = distance to the lower junction point, and P,Z = distance to the higher junction point. Robinson15 has studied the gamma-ray peaks using an analytical form which consists of two Gaussians of the same width and an arctangent function. The adapted expression is fn

=

Aexp [-(n2i21b)2]

+ ~ [ +f

Volume 1

+

ex,[

- (n - no

2u2

+ Pu)

13

I

P a is the distance from the abscissa, n, of the main Gaussian to the abscissa of the secondary Gaussian of lower energy. A and u are the height and standard deviation of the main Gaussian. The second term in the equation is an arctangent function with the center at channel n, bu and with height B. The last term is the second Gaussian having the same width as the first one but with a height D and the maximum at n, - P a . An error function has been used to generate the continuum in the work of D o j ~The . ~ ~ photopeak is represented by a Gaussian with an exponential for low energy tail joined such that the function is continuous. The line shape is approximated by,

A,%, and u are, respectively, the height, the peak position, and the standard deviation of the Gaussian; the distance from the center of Gaussian to the junction point is t. The error function is defined by its height, h, and its width a,,

Sasarnoto et aL8 have utilized a similar functional form replacing the error function by another function. The analytical form is given by 2

f, = Aexp

, n,anan, -t

f,, = Aexp

fn = Aexp

t(2n - 2%

+ t) ] + : { 2 - e x P [ $ ( n - n o ) ] ) ,

n
where h is the height of the used function. More complicated functions have also been used to approximate the photopeaks. KemI3 has suggested the peaks are approximately oblique Gaussians with higher tails at the low energy sides than at the high energy sides. These characteristics are approximated by fn = Aexp [-(n2i2 %)2]

[I

+ P,(?)~

+p2(7)12]

14

Activation Analysis

The first term is a deformed Gaussian: the Gaussian is multiplied by a polynomial with coefficient P, and P,. A tail of constant amplitude P, and a tail of exponential amplitude P, and slope P, are added to each peak. These two tails are multiplied by -\/2;; A which is proportional to the height of the peak and by an expression which vanishes at n,. Ciftcioglul1 has used an asymmetrical function for the tail which is obtained by introducing an exponential function in the Gaussian. The function is given by f,,

=

[

Aexp - (n - no).] 2u2

+

e,t[(n

"o)]

with 6 = 1 for n 5 Cn, and 6 = 0 for n > Cn,, where h is the height of step, D the coefficient for the tail function, and a, the width of this function. B determines the exponentially decreasing part in the tail function. This function vanishes beyond Crb. Campbell and Jorch2, have proposed a function with the following form to approximate the photopeak f,, = Aexp

This function has a long exponential tail on the left and a Gaussian type behavior on the right with the junction point at n, = C - 2 a;/@ The first derivative of this function is not continuous at the junction point and, therefore, cannot be justified physically. However, it has the advantage that its four parameters, D (maximum height), C (Channel where height is maximum), a, (width of Gaussian), and P (exponential slope), are not intercorrelated. Najafi36has utilized the following mathematical form in order to approximate the line shape of a photopeak,

The tail function has a behavior similar to that of Campbell and Jorch but with a continuous first derivative. Its junction point is at n, = c - u & / P The step function used in this equation has been suggested by Baba et al." The advantage of this expression is in its flexibility and simplicity. These two characteristics allow one to use this analytical form for a variety of semiconductor detectors.

C. FITTING METHOD Least-square technique can be used to minimize

Volume 1

X2

=

'

15

[Data, - Fiti(Xi)12weight degree of freedom

Suppose that Yi(X,) is a dependent variable of an ensemble of m independent variables, X,

with f(X,a), a function fitted to Y with an ensemble of n adjustable parameters, a,

One obtains the equation

=

f(ri, aO) +

f:af(x''a q

(a, - a:)

j=l

+ R(Aa)'

where a" is an ensemble of selected values for a. The last term in Equation 14 represents the residue. The minimization condition for the sum of the square of the differences is then given by the simultaneous equations

where Pi is the weight for the ith measured value. The value of the parameters to be employed after the first iteration is consequently given by aj = a;

+ Aaj

where Aa, is the solution of the equation A.Aa= B with

a,. =

2 Pi af(x,,aajao)

i=l

af(xi, a")

a%

The fitting process is repeated until the convergence is obtained.

(16)

16

Activation Analysis

1 06,

1

CIIANNEL NUNHLK

FIGURE 2. Gamma-ray spectrum of "'"Lu from a 32.2-cm3Ge-Li coaxial detector.*' (From Najafi, S . I . , J . Radioanalyt Chem., 78, 391, 1983. With permission.)

D. CALCULATION AND COMPARISON To evaluate the performance of different methods, seven well-isolated peaks of 1 7 7 m L ~ spectrum have been analyzed. This spectrum has been obtainedz7using a Ge(Li) detector of 1 cm2 (surface) x 5 mm (depth). Figure 2 depicts the spectrum of '""Lu. The analyzed peaks are numbered from 1 to 7. The performance of a particular analysis is determined by the agreement between the experimental data and the calculated values. This is performed by calculating the value of

xZ

where v and u are the channel numbers defining the fitting internal, i is the channel number, fi is the analytical approximation at channel i, ni is the count number at channel i, N is the degree of freedom, and NPAR is the number of variable parameters used in the analytical form. As was earlier mentioned, the totally absorbed component of an isolated photopeak is given by Gaussian function and the deviation with respect to this form is considered by using tail and step functions. A number of functions have been suggested for these components. The step function is always added to the main function, but the tail function is employed in different ways. Depending on the form of tail function, the analytical forms studied in this section can be divided in three groups.

Volume 1

17

TABLE 1

xZ Values Using Different Threshold Functions Step function

Approximate

maximum

Robinson

erf

Baba(K)

Note: The distortion is a symmetrical Gaussian hav-

ing the same width as the main Gaussian. K is the parameter in the threshold function of Baba. From Najafi, S. I. and Kikindai. T., J. Radioanalyt Chem., 68(1--2), 127, 1982. With permission.

1. Distortion Is a Symmetrical Gaussian

The calculation is performed using three different peak functions (see Equation 3). In all of these cases, G and D are Gaussians with the same width parameters. They are simply added to each other. B is assumed to be linear. Three different forms are considered for S and the calculation is repeated for each one. The results are summarized in Table 1. It can be seen that the step function proposed by Baba results in a better fit to the experimental values due to the good flexibility of this function.

2. Exponential Distortion This distortion is realized by adding an exponential function to one (or two) extremes of the main Gaussian. The calculations are performed using two different step functions: those of Dojo and Sasamoto. The results are compared with that of Routti' which is obtained without assuming a step function. K* values are summarized in Table 2. It can be seen that fitting improves if one uses a step function.

3. Asymmetrical Distortion Function In this group, we have compared the analytical forms proposed by Campbell et al., Ciftcioglu" and Najafi et al. In each case, the particular step function suggested by the authors is utilized. The calculations are repeated for each case and the results are reported in Table 3. The comparison of K' values indicates that the function proposed by Najafi et al. better fits the experimental data.

4. Deformed Gaussian A good example of this kind of function is the analytical form used by Kern, Equation 9. Although this form gives relatively good results, it is difficult to estimate initial values for the parameters and very often the least-square fitting routine does not converge. The results of calculation for this functional form are reported in Table 4. Figures 3, 4, and 5 show a few examples of the step and distortion functions discussed for photopeak Number 1. The asymmetry of the tail function of Campbell et al. and the symmetry of the Gaussian form and that of Najafi et al. can be seen in Figure 3. The functions of Kern and Robinson are expected to underestimate the experimental data in the high energy

18

Activation Analysis

TABLE 2

x2 Values for Gaussian with an Exponential Tail Step function Approximate maximum

Dojo

Sasamoto

Routti (without step)

241.8 328.1 420.5 701.7 781.1 852.9 990.9

6.32 4.24 5.89 6.68 1.61 1.65 0.72

6.89 5.05 5.97 7.21 1.73 1.70 0.75

7.19 5.84 6.60 9.79 1.95 2.11 0.86

From Najafi, S. I. and Kikindai, T., J . Radioanalyt. Chem., 68(1-2), 127, 1982. With permission.

TABLE 3

x2 Values Obtained Using the Functions with Asymmetrical Distortion Function Approximate maximum

"

Campbella

Najafi8 (K)

Ciftcioglu

crD=p=a.

A polynomial of degree m is used for background. From Najati, S. I. and Kikindai, T . , J . Radioanalyt. Chem., 68(1-2), 127, 1982. With permission.

side of the peak because of its asymmetry (see Figure 4); we have observed this effect in practice. The study of the values indicate that the choice of the threshold and distortion functions has a significant effect on the quality of the fitting. Table 5 summarizes the results of calculation using only one Gaussian plus a linear background. The comparison of these results with those obtained using other forms (Tables 1 to 4) shows that the line shape function should include one Gaussian, one step function, one symmetrical or asymmetrical tail function, and a linear or polynomial background. A simple function of additive form gives a good fit with a fast convergence.

111. ANALYSIS OF GAMMA-RAY SPECTRA BY NONLINEAR LEAST-SQUARES FIT In the spectra obtained by gamma-ray spectrometry, the total absorption photopeaks contain enough information in order to identify the nuclei present in the sample and to

Volume 1

19

TABLE 4 Comparison of the Results of Fit to the Three Peaks of 1 7 7 m L Spectrum ~

Function

Accurate peak position

u

2.055 x lo6 2.068 X 1@ 2.068 X lo6

Robinsona Dojo Sasamoto Routti Kern Ciftcioglu Campbell Najafi Robinsona Dojo Sasamoto Routti Kern Ciftcioglu Campbell Najafi Robinsona Dojo Sasamoto Routti Kern Ciftcioglu Campbell

-

2.096

X

lo6

2.062 2.056 1.036 1.053 1.053 0.928 1.047 0.996 1.039 1.038 2.399 2.522 2.520

x x x x X X X

lo6 lo6 lo6 10' lo6 lo6 lo6

X

x lo6 lo6 lo4 x 104 X 104

X X

-

Najafi a

Gaussian area

2.401 2.305

X

2.443

X

2.390

X

X

lo4 lo4 lo4 lo4

Modification: threshold = h(n - 4)1/(2K - 1). A polynomial of degree n is used for background.

From Najafi, S. I. and Kikindai, T., J . Radioanalyt. Chem., 68(12). 127, 1982. With permission.

measure their radioactivity. However, the multiplicity of the peaks requires automatization of this process and use of computers for numerical analysis. A computer code should localize the peaks and calculate their principal characteristics (area, resolution, and energy) without making these parameters sensitive to the statistics or to the complexity of the spectra. This is especially essential for the laboratories which analyze a large number of different types of samples. The radioactivity of the radioisotopes can be determined from the area of the peaks and the efficiency curve of the detector. In this section, we will study different codes which use nonlinear least-square fit technique. ~ be used to compare the performance of three computer codes: The spectrum of ' 7 7 m Lwill SAMP0,27SPAN,9 and that developed by Najafi.37 The main parts of a numerical method for analysis of gamma-ray spectra are 1. 2.

3. 4. 5.

Smoothing of experimental data in order to damp the inherent local fluctuations Automatic peak search and the selection of limits for each one Choosing the fitting internals Fit of each internal by a shape functianCalculation of the peak surface

-

The performances obtained in different steps of analysis depend strongly on the accuracy

20

Activation Analysis

CHANNEL NUMBER

FIGURE 3. Comparison of the three tail functions used in calculation of peak number one.

of the peak search technique. The most important criteria in a peak search program is to detect as few as possible false peaks, but do not neglect the peaks with low intensity. Many methods have been utilized. The principal ones are summarized herein (see Table 6). There are also different methods to calculate the area of the peaks. A comparison of different techniques has been done in Section I1 and it was concluded that an analytical form consisting of a main Gaussian, a tail function riding on a linear background, and a step function are needed to define the photopeak.

A. ANALYSIS BY THE PROGRAM DEVELOPED BY NAJAFI 1. Smoothing of the Experimental Data To obtain the smoothed point at each channel, a zone of the gamma-ray spectrum is fitted to a polynomial function of degree m,

FIGURE 4. Comparison of the three step functions used to fit peak number one.

where N, is the degree of the polynomial. Yulez8 has proposed that the number of points in each smoothing, No., should be the largest odd number which is smaller than the full width at half maximum of the photopeak. N, is determined by N, = 2

for

No < 9

N, = 4

for

No 3 9

(20)

The smoothed value is then calculated for the central channel of the zone. This procedure is repeated for all of the points of the spectrum. 2. Peak Searching The method suggested by Sasamoto et a1.* is adapted for peak search routine because it appears to be simpler than the other methods. There are three conditions to localize a real peak.

1.

The first derivative of the spectrum passes from a positive value to a negative value. n, is the channel number where the first derivative is zero. The first derivative at each channel can be obtained from the smoothed data points using Equation 19.

where N is the distance, in channels, between the local minimum and the local maximum of the first derivative. This condition is used to eliminate the maximums due to the Compton effect because they are larger than the total absorption peaks.

22

Activation Analysis

FIGURE 5. Comparison of the step function of Baba with erf function in the case of peak number one. The influence of parameter K on the results of calculations is shown below:

K = l K =2 K =3 erf

Surface

u

Channel

x2

2056080.68 2056449.24 2056420.80 2058381.78

2.026 2.028 2.028 2.037

240.85 240.86 240.87 240.86

3.66 5.07 6.05 5.36

TABLE 5 Results Obtained for the Seven Peaks of the Spectrum Using a Gaussian Plus a Linear Background Peak

u

Surface of Gaussian

X'

Volume 1

23

TABLE 6 Principal Peak Search Methods

Statistical methods Does the channel content of a given window follow a statistical low compared to the channel content of the window? Example: section 4.4

First derivative methods Y' = constant no peak Y' passes through a maximum at left If there is a peak passes through zero passes through a minimum at right Y' is calculated by fitting a function to m channels centered on the considered channel Example: section 4.3

Second derivative methods Y" f 0 If there is a peak Y" = 0 There is no peak Example: Routti et al.'

Resolution enhancement methods To determine the components of a multiplet the resolution of each one is enhanced by a numerical technique: Sterlinski5 and Shiokawa et al.4

Y ( n )

3.

3

a

)

,

R(n,,)

=

e-0.5'u(~)

(22)

where Y(n) and Yf(n)are, respectively, the smoothed data points and their derivatives in channel n. a , an integer number, is the peak detection sensitivity parameters. a(%) is the resolution of the peak in position h. The spectrum in the neighborhood of a photopeak can be approximated by

where G(n) = Gaussian function, A = constant, and B(n) = background. Since dB(n)/dn G dG(n)/dn for all n values, the following formula can be obtained:

therefore,

where n, is the position of the peak determined by condition 1. The statistical value of Y(n), being the measured value, is because Y(n) follows a Poisson distribution. In this case, the condition to detect a photopeak is

Condition 3 can be obtained combining Equations 25 and 26.

24

Activation Analysis

3. Selection of Fitting Intervals The selection of portions of the spectrum for fitting by the analytical function is done after peak detection. A peak sufficiently away from the other peaks is considered isolated. Overlapped peaks have to be analyzed simultaneously. The fitting intervals are selected such that their left limit is higher than their right limit. The left limit of a peak is obtained by comparing the first derivative and a imerical parameter proposed by Baba.16 This parameter, FACT, has different values depending on the width of the peak and the number of counts. For the number of counts N, the parameter FACT is defined by FACT

=

A. F

(27)

with

and F depends on the number of points utilized for the fitting of the data, No. A parameter, IN0,is introduced to control No 2-INo+5

for for for

OSINoSIO INo= 11 IN, = 12

The values of F are given in the following table

4. Peak Energy Calculation In order todetermine the radioactivity of the nuclei present in the sample, it is necessary to know the detection efficiency of the detector for the energies of different peaks identified in the spectrum. The efficiency of a detector is defined as the number of pulses in a total absorption peak per the total number of photons emitted by the sample. It can, therefore, be determined by measuring the radioactivity of the samples with known radioactivity. The shape and measurement conditions of the known samples should be identical to those with unknown activity. The efficiency curve can then be obtained. For energy calibration, one considers linear interpolation in linear scaleyJ1 and for efficiency calibration, linear interpolation in log-log scale is an acceptable approximation for the energies higher than 100 keV.9.11,2y

B. ANALYSIS BY SAMPO PROGRAM The peaks are approximated with a main Gaussian and two exponentials representing the high and low energy zones (Equation 4). The program performs an automatic peak search by second derivative method. Then, it determines the limits by searching the minimums at right and at left sides of the peak.

Volume 1

25

TABLE 7

Comparison of Peak Search Results by Three Programs Najafi

SAMPO

SPAN

The program has then two options for background evaluation: linear and parabolic. The importance of the choice of the limits is only to have a zone large enough to define this line or parabole. The area of the peak can, therefore, be calculated from one limit to the other.

C. ANALYSIS BY SPAN The photopeaks are approximated by one or two Gaussians superposed to a polynomial background. The parameters of these functions are determined by fitting to the experimental data by nonlinear square method. In the first step, the experimental data are smoothed by a quadratic polynomial. The peak search is then performed by first derivative method. The analysis of the peaks are finished by calculation of area and linear background. D. CALCULATION AND COMPARISON '""Lu spectrum is utilized to perform a comparative study of the above-mentioned three programs. Table 7 summarizes the peak search results obtained by these programs. One can see that the first derivative method performs better than the techniques developed in the other two programs. The second derivative method used in SAMPO does not detect the peak at Channel 996 and detects a false peak at Channel 816. Table 8 presents energy and relative intensities calculated by the three programs. The

26

Activation Analysis

TABLE 8 Calculated Energies and Intensities by the Three Programs

105.308 112.922 115.876 117.064 121.610 128.503 136.726 145.765 147.167 153.292 159.812 168.702 171.887 174.386 176.787 181.934 195.524 204.096 208.364 214.421 218.097 228.468 233.835 249.655 262.349 264.184 268.756 28 1.765 283.798 291.585 292.574 296.442 299.066 305.513

Intensity 100 185.94 4.44 1.50 52.98 136.60 12.60 30.76 148.05 3.20 1.06 43.54 108.77 26.08 6.14 8.98

SPAN

SAMPO

Naiafi

Energy (keV)

1 1

508.58 177.77

1

27.30 ) 54.55 301.31 44.41 49.19 1.35 27.62 14.22 3.55 9.35 6.23 39.58 11.43 13.36

1

\

Energy (keV) 105.317 112.920 116.001 117.376 121.607 128.492 136.722 145.748 147.153 153.273 159.759 168.598 171.844 174.388 176.981 182.029 195.545 204.102 208.359 214.426 218.092 228.452 233.831 249.640 262.856 264.079 268.758 281.758 283.674 291 S46 292.582 296.431 299.029 305.490

Intensity

Energy (kev)

Intensity

98 175.8 4.53 1.27 48.84 125.4 11.49 7.15 27.69 134.4 4.09 1.28 37.89 98.5 26.4 0.58 6.5 106.0 462.4 49.71 25.97 281.5 42.62 46.53 0.77 1.45 25.69 106.1 3.22 8.22 6.55 37.46 12.25 13.63

105.36 112.94 115.93 117.25 121.59 128.48 136.67 145.69 147.10 153.24 159.68 168.29 171.83 174.34 176.97 181.85 195.52 204.12 208.34 214.42 218.11 228.49 233.86 249.69 263.80 266.97 268.80 281.80 283.77 291.57 292.43 296.44 299.10 305.49

100 182.9 7.0 3.5 51.8 129.9 11.9 8.3 29.2 139.2 4.0 2.2 40.4 101.0 26.9 1.8 6.2 116.4 505.1 50.3 26.4 298.5 45.6 45.0 1.69 0.71 26.5 110.5 2.1 7.2 5.7 36.9 11.2 12.4

Note: The intensity of each peak is normalized to that of 105 keV calculated by the

first method.

three programs determined accurately the position of the peaks (with one exception where SPAN overestimates the position of peaks of the doublet 262-264 by one channel). The intensity calculations by our program are generally higher than those by SPAN and SAMPO. This may be due to the fact that no step function is considered in these two programs. The intensities attributed to some peaks by the three programs disagree considerably in the case of low intensity peaks and when the peaks are overlapped. This comparative study reveals a few characteristicsof the nonlinear least-square method: The position of the peak is determined with good precision regardless of the algorithm used. The intensity calculation is very sensitive to the type of analytical function especially in the case of low intensity and overlapped peaks. At least seven parameters (three for Gaussian, two for tail function, one for step function, and one for background) are needed to define a good analytical form.

Volume I

27

It is more difficult to get divergence when a large number of parameters is used. The choice of the initial values for the parameters play a very important role when several peaks are fitted simultaneously.

IV. ANALYSIS OF THE SPECTRA USING A SUMMATION METHOD Another class of programs utilize simpler algorithms6 in which the area of the peak is simply calculated by adding the channel contents and subtracting an estimated background. These programs can be run using small and low cost calculators. The problem of a gamma-spectrometry user is to choose among these numerous codes. Is it necessary to use an elaborate method of analytical fitting? Is such a method more accurate for peak position and area calculation than a simple summation method? In order to answer these questions, in this section, two methods of gamma-spectra evaluation for neutron activation analysis are compared. We have selected two methods completely different in their principle and in the calculator size. This comparison is performed on concrete examples of material analysis.

A. PEAK SEARCH The peak detection method is based on a statistical comparison of the intensity in the channel close to the center of the peak. A group of five channels is considered in this operation. This is repeated along the spectrum. If n is the central channel number in the group with f(n) as the channel content, n is considered to be the maximum of the peak by the following conditions

where K is an integer number generally equal to 4. This condition prevents false peak detection in the zones with very low count rates

where a is generally equal to 3. If the condition in Equation 29 is not satisfied, the following two conditions are applied

If the conditions in Equations 29, 30, and 31 are satisfied, similar conditions are tested with channel n 1 and n 2:

+

+

or, if the condition in Equation 32 is not satisfied, the conditions

are tested. If the conditions in Equations 33 and 34 are not satisfied, the group of five

28

Activation Analysis

channels are shifted one channel and all of the conditions are tested again. If the conditions in Equations 28, 29, or 30 and 3 1, 32 or 33 and 34 are satisfied, the channel n is considered as a maximum of peak and the following quantities are calculated

where c = position of peak, h = peak intensity estimation, b = background estimation, and r = value related to peak width.

B. PEAK INTEGRATION The total number of counts in a peak is calculated by adding the channel content of a fixed number of channel

where n,, is the channel number of the maximum.

C. APPLICATION Single- and multielement samples and standards are irradiated in a neutron accelerator. The gamma-ray spectra of the samples and standards are measured by a 100 cm3 Ge(Li) detector with a resolution of 2 keV at 1332 keV. A 2000 channel analyzer is used. The measured spectra are analyzed by two methods: the method explained above and the nonlinear least-square technique. Main features of the two programs are summarized in Table 9. 1. Single-Element Analysis The results of the analysis of single-element samples are reported in Table 10. There is excellent agreement between the calculated energies by the two methods. The simple method based on barycenter calculation from five points (Equation 35) is very precise in the case of the isolated peaks. The calculated intensities by nonlinear least-squares method are about 5% higher than those calculated by simple summation technique. This is expected since the latter does not calculate the entire area of the peak.

2. Multielement Analysis A total of 20 elements are detected in the multielement samples. The samples are analyzed

29

Volume I

TABLE 9 Main Features of the Two Methods Used for the Analyses Peak Detection Method of section 4.4

Method of section 4.3.1 Yes First derivative

No Statistical comparison of channel intensities near peak center Barycenter of five channels Approximate height and form factor

Smoothing Detection method Centroid calculation Other parameters

Change of first derivative sign Approximate parameters for fitting program

Peak Integration Smoothing Background determination Integration method Calculation of standard deviation Calculation of detection limit Correction for dead time Resolution of peak overlapping Radioactivity at the end of irradiation Other calculations or controls

No Linear function

No

Extrapolation from a four channel zone after peak 1.5 Sum of channel intensities in peak width at half-height From statistics of peak and background

-

+ step function

Area of main Gaussian from fitting of line to 1 or 2 Gaussian background Not considered

+

From statistics of background in peak zone Through pulse generator peak intensity No

Not considered

Automatically calculated using half-life

Performed in another step of calculation

Control of background variations and peaks vicinity

1.Peak centroids and FWHM 2. Plot of calculated and experimental values for each peak

Through pulse generator peak intensity Yes

From Najafi, S. I. and Federoff, M., Radiochem. Radioanalyt. Lett., 56, 305, 1983. With permission.

TABLE 10 Analysis of Single-Element Samples, A, and A, Energy (keV) Sample A, A2 Standard

Method I

Method I1

41 1.93 41 1.91 41 1.92

411.93 411.89 41 1.92

Peak intensity (count per minute) Method I 650612.4 300440.5 6495.2

+ 6840.5 5

-C

3044.3 61.6

Concentration (P&)

Method I1

Method I

Method I1

685715.9 316908.9 6826.9

23.04 5 0.46 10.64 2 0.21

23.10 10.68

Note: Method I, summation method explained in section 4.4; Method 11, nonlinear least-square method of section 4.3.1.

by both methods and the comparison is performed in two steps: (1) peak position and intensity and (2) concentration of the elements. Several results of the first step are shown in Table 11. As in the case of single-element analysis, we observe a very good agreement in peak positions. The difference between the calculated intensities by the two methods is a function of energy. The effect can be explained by the fixed number of channels used in the summation method which does not take into account the variation of peak width with energy. There is a very good agreement between the quantity of elements determined by the two methods (Table 12). This means that in the two methods, the intensities of the peaks, although different, are proportional to the quantity of the elements.

30

Activation Analysis

TABLE 11 Examples of Calculation of Position and Intensity for the Multielement Sample

Element

Gamma energy (keV)

Peak position (channel) Method I

Method I1

Peak intensity Method I

Method I1

Difference (I1 - I)%

Note: Method I , method of section 4.4; Method 11, method of section 4.3.1

From Najafi, S. I. and Federoff, M., Radiochem. Radioanalyt. Lett.. 56,305, 1983. With permission.

TABLE 12 Comparison of the Concentrations (pg . g-') Calculated by the Two Methods in Multielement Analysis of Samples B, and B, B2

B1

Element

Method I

Method I1

Method I

Method I1

Note: Method I , method of section 4.4; Method 11, method of section 4.3.1.

From Najafi, S. I. and Federoff, M., Radiochem. Radioanalyt. Lett., 56, 305, 1983. With permission.

Volume I

31

The main advantage of the nonlinear least-square method over summation method is its capability to solve the multiplets. The main advantage of the summation method is its simplicity which allows use of small calculators.

V. GAMMA-SPECTRA ANALYSIS BY AN EMPIRICAL METHOD In this section, we will develop a numerical method for gamma-ray spectrum analysis capable of solving the problem of multiplets, but in a simpler way than nonlinear squares fit. Furthermore, this method does not depend on the choice of analytical function. In this approach, the analytical function is replaced by an empirical definition of the gamma ray. A standard peak is defined numerically by measuring the gamma spectrum of a radioisotope having a well-isolated gamma ray. This peak can be later translated to the position of the sample peak. The translated standard peak can then be used for fitting of the sample peak. The method of fitting is very simple because there is only one parameter, intensity of peak, to determine. The width of a photopeak, however, varies with energy. Therefore, it is necessary to be able to define the width of the translated standard peak at any energy. In order to perform this transformation, one has to define one or several mathematical functions and calculate the channel contents of the translated standard peak using those of the original standard peak. Following these considerations, the principal steps of this method are choice of the function in order to define the parameters of a gamma ray, experimental study of the variation of these parameters with energy, translation of a peak, and fitting method of a sample peak with the standard peak.

A. PARAMETERS OF A PHOTOPEAK In the previous sections we have seen that a photopeak can be defined by a Gaussian modified on the lower side

where A,gij(ni) = value of Gaussian in Channel ni, hiSij(ni) = value of step in Channel ni, and Bj = background. The Gaussian

has three parameters: rb, = peak centroid, ui = standard deviation (related to the width of photopeak), and A, = peak intensity. An arctangent function is selected to define the step function,14

We have observed that this form gives a more precise value for the height, h, of the step. A constant contribution is considered from the background.

B. EXPERIMENTAL STUDY OF VARIATION OF PARAMETERS We have seen in the previous sections that the centroid of a peak can be calculated with

32

Activation Analysis

1.4

1

FIGURE 6. Variation of standard deviation with energy. (From Najafi, S. I. and Federoff, M . , J . Radioanalyt. Nucl. Chem. Art., 89(1), 143, 1985. With permission.)

FIGURE 7. Variation of ratio of height of step function to intensity of Gaussian with energy. (From Najafi, S. I. and Federoff, M., J. Radioanalyt. N w l . Chem. Art., 89(1), 143, 1985. With permission.)

the barycenter method. We have used this method because of its simplicity for calculation of centroids at different energies. The variation of the standard deviation with energy is determined experimentally. Wellisolated photopeaks emitted by different radioisotopes are measured under identical conditions. Each peak is fitted by Equation 40. For each peak, we have determined the standard deviation of the Gaussian, a,and the ratio of the parameter h of the arctangent to the intensity A of the Gaussian function. These results are summarized in Figures 6 and 7. The variation of the square of standard deviation with energy is linear and can be expressed by

Volume I

where E is the gamma-ray energy. The value of polation of the closest points.

h/,

33

at any energy is determined by inter-

C. TRANSLATION OF STANDARD PEAK The original standard peak with centroid n, is translated to the position n', by conserving the area of peak. The first operation is to subtract the step part of the background. Then, the channel content of the net standard peak can be approximated by a Gaussian

The channel content of the translated peak is

Using the approximation

and imposing the condition A

=

A', we can arrive at the following formulas for translation

In general, this translation does not end with the integer numbers for the channels. Since the form is approximately Gaussian, the variation of the logarithm of the channel content with energy is parabolic. Therefore, we apply a three-point Lagrange i n t e r p ~ l a t i o nto~ ~ the logarithm of the transformed net standard peak to calculate the form in integer channels

In this equation y(n,), y(n2), and y(n3) are the logarithms of the channel contents in positions n,, n,, and n,. n in the nearest integer number to n, and y(n) is the interpolated value for the position n. We calculate y(n) replacing n by n = n, - s and taking the approximation n2 - n, = n, - n, =. 1. This leads to

34

Activation Analysis Standard peak measurement

Substraction of step and background

measurement

FIGURE 8. Flow chart of the analysis of a spectrum by the empirical photopeak. (From Najafi, S. I. and Federoff, M., J. Radioanalyt. Nucl. Chem. Art., 89(1), 143, 1985. With permission.)

D. FITTING METHOD The background should be defined in order to complete the model. Here we approximate it by a locally constant.value. Therefore, the descriptive model for Channel i is given by m

+ step,] + B

Fi = j=, [AjGji(ni)

(50)

where j defines the number of the peak in an interval containing several peaks. A, is the intensity of the peak, Gji is the antilogarithm of the value calculated by Equation 48, and B is the background. Stepj is the value of step for the peak j obtained using the curve of Figure 7. We can easily see that the only parameters to determine are B and Aj. This can be done using a linear least-square method by minimizing the value of chi-square.

where Ei is the content of Channel i of the spectrum and Fi is the value obtained by empirical Equation 50. This method is always convergent. Figure 8 shows the flow chart of the method.

E. APPLICATION OF THE METHOD As an example, we have taken the doublet formed by the photopeaks of 559 keV of 76Asand that of 564 keV of lZ2Sb.We have compared the results obtained for each of these peaks by the method developed in this section, the nonlinear least-square method, and the summation method.

Position

Intensity

Standard peak Au n, = 410.88 keV; a = 0.748 Position Intensity

Position

Net standard peak Intensity

Standard peak transformed to the position of the sample n', = 558.93 keV; a = 0.807 Position

Intensity

Standard transformed peak (integer channel numbers)

TABLE 13 Example of Transformation of an Empirical Standard Peak

Position

+

Intensity

Standard transformed step peak

As Sb As Sb As Sb

Element 558.93 563.92 558.96 563.90 559.09 563.94

0.807 0.809 0.807 0.809 0.807 0.809

u (channel) 29122.77 31845.11 28582.38 31946.78 2902.95 31985.05

Intensity

Method developed in this section Centroid 558.93 563.94 558.97 563.96 558.97 563.99

0.784 0.787 0.813 0.766 0.859 0.770

o (channel) 27876.96 30730.17 28222.57 29201.24 3469.76 3058 1.40

Intensity

Nonlinear least-square method Centroid

From Najafi, S. I. and Federoff, M., J. Radioanalyt. Nucl. Chem. Art., 89(1), 143, 1985. With permission.

IV

I 11 111

Sample Centroid

o (channel)

Summation method

TABLE 14 Energy, a, and Intensity Calculated for the Four Samples by the Three Methods

Intensity

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TABLE 15 Ratio of the Quantity of As and Sb Determined by the Three Methods in the Samples Empirical method

Nonlinear least-square

Summation

Theoretical value

From Najafi, S. I. and Federoff, M., J . Radioanalyt. Nucl. Chem. Art.. 89(1), 143, 1985. With permission.

We have compared the results obtained in the case of the doublet As-Sb: Sample Sample Sample Sample

1: 11: 111: IV:

10 cm3 of 10 cm3 of 10 cm3 of 1 cm3 of

As Sb As As

+ +

10 cm3 of Sb 10 cm3 of Sb

The spectra of these samples were measured under the same experimental conditions of the standard peak of 41 1 keV of ' 9 8 A ~and the 16 peaks used for the determination of the variation of u2 and h/A with energy. Table 13 summarizes different steps of calculation of empirical peak obtained by transformation of the standard peak. The photopeak of Au (41 1 keV) is utilized as the standard peak and the net standard peak is obtained by subtracting background and step from the experimental values (Column 2). The net standard peak is transformed using Equations 46, 47, and 49 (Column 3). The value of step is added to the transformed standard peak in order to obtain the channel contents of the empirical peak, Gji(ni), in Equation 50 (Column 4). The background and the height of the peak can then be determined by fitting this peak to the sample peak. The surface (intensity) of the photopeak can, therefore, be calculated. The spectra of the four samples are analyzed by the method developed in this section, the method of nonlinear least square, and the summation method. Table 14 summarizes the results of the calculations. The values obtained for energy are in very good agreement except for As in Sample IV where there is a small difference between the calculated value by the barycenter method and the nonlinear least-square method. When the peak of arsenic is small compared to that of antimony, the resultant deformation reduces the precision of the barycenter method. However, this impression has no effect on the calculation of peak intensity. The method explained in this section analyzes perfectly the doublets: the intensities obtained for the peaks coincide with the intensity of the individual peaks (Table 15). The nonlinear least-square method gives good results for Sample 111, but it overestimates the value of arsenic when its peak is ten times less intense than that of antimony. It seems that this method is less accurate than the empirical method since the components of a multiplet are very inequal. It also gives a less exact value for width of the photopeak. The summation method gives relatively good results for the peak of antimony but cannot evaluate the peak of arsenic.

38

Activation Analysis

REFERENCES 1. Routti, J. T. and Prussin, S. G., Nucl. Instrum. Methods, 72, 125, 1969. 2. Horlick, G., Anal. Chem., 61A, 43, 1971. 3. Horlick, G., Anal. Chem., 44, 943, 1972. 4. Shiokawa, Y., Misugashira, T., and Suzuki, S., J . Radioanal. Chem., 54, 267, 1979. 5. Sterlinski, S., J . Radioanal. Chem., 31, 195, 1976. 6. FMoroff, M., Blouri, J., and Revel, G., Nucl. Instrum. Methods, 113, 589, 1973. 7. Philippot, J. C., IEEE Trans. Nucl. Sci., NS-17, No. 3. June 1970. 8. Sasamoto, N., Koyama, K., and Tanaka, S. I., Nucl. Instrum. Methods, 125, 507, 1975. 9. Basu, S. K. and Patro, A. P., Nucl. Instrum. Methods, 125, 115, 1975. 10. Baba, H., Baba, S., and Suzuki, T., Nucl. Instrum. Methods, 145, 517, 1977. 11. Ciftcioglu, O., Nucl. Insrrum. Methods, 174, 209, 1980. 12. Kern, J., A Fortran nonlinear least square fit code for nuclear spectra, Physics Department, University, Fribourg, Switzerland, 1970. 13. Kern, J., Nucl. Instrum. Methods, 79, 232, 1970. 14. Robinson, D. C., Report Atomic Energy Research Establishment, AERE-R6144 Harwell, 1969. 15. Robinson, D. C., Nucl. Instrum. Methods, 78, 1970. 16. Baba, H., Okashita, H., Baba, S., Suzuki, T., Umezawa, H., and Natsums, H., J . Nucl. Sci. Technol., 8, 703, 1971. 17. Baba, H., Sekine, T., Baba, S., and Okashita, H., JAERI 1227, 1973. 18. Steyn, J. J. and Beier, M., J . Radioanal. Chem., 15, 699, 1973. 19. Weigel, H. and Dank, J., J . Radioanal. Chem., 23, 171, 1974. 20. Dojo, M., Nucl. Instrum. Methods, 115, 425, 1974. 21. Baedecker, P. 4., Anal. Chem., 43, 405, 1971. 22. Najafi, S. I. and Kikindai, T., J. Radioanal. Chem., 68, 127, 1982. 23. McNelles, L. A. and Campbell, J. L., Nucl. Instrum. Methods, 127, 73, 1975. 24. Jorch, H. H. and Campbell, J. L., Nucl. Instrum. Methods, 143, 551, 1977. 25. Campbell, J. L. and Jorch, H. H., Nucl. Instrum. Methods, 159, 163, 1979. 26. Nielsen, P., Nucl. Instrum. Methods, 192, 433, 1982. 27. Routti, J. T., UCRL-19452, Lawrence Radiation Laboratory 1969. 28. Yule, H. P., Nucl. Instrum. Methods, 54, 61, 1967. 29. Hollander, J. M., Nucl. Instrum. Methods, 43, 65, 1966. 30. Currie, L. A., Anal. Chem., 40, 586, 1968. 31. Mosulishvili, L. M., Kolomi'tsev, M. A., Dundua, V. Yu, Shonia, N. I., and Danisaval, 0. A., J. Radioanal. Chem., 26, 175, 1975. 32. Ritter, G. L. and Currie, L. A., Conf. Computers in Activation Analysis and Gamma-Ray Spectrometry (Mayaguez, Puerto Rico, 1978), Publ. 780421 U.S.Department of Energy, 1979, 39. 33. Rektorys, K., Survey of Applicable Mathematics, MIT Press, Cambridge, 1969. 34. Nelder, J. and Mead, R., Comput. J., 7, 308, 1965. 35. Deming, S. N. and Morgan, S. L., Anal. Chem., 45, 278, 1973. 36. Najafi, S. I., Dr. Ing. thesis, Ecole Centrale de Paris, March 1983. 37. Najafi, S. I., J . Radioanal. Chem., 78, 391, 1983. 38. Najafi, S. I. and Federoff, M., Radiochem. Radioanalyt. Lett., 56, 305, 1983. 39. Najafi, S. I. and Federoff, M., J. Radioanalyt. Nucl. Chem. Art., 89(1), 143, 1985.

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Chapter 3

OPTIMIZATION OF INSTRUMENTAL ACTIVATION ANALYSIS

.

Donald D Burgess

TABLE OF CONTENTS Introduction ...................................................................... 40 Experimental Parameters .........................................................40 A. Irradiation Time ..........................................................41 B. Decay Time ..............................................................41 C. Radiation Measurement ...................................................42 1. Counting Time ....................................................42 2. Activation Product ................................................42 3. Gamma-Ray Energy ..............................................42 4. Geometry .........................................................42 5. Special Methods ..................................................43 D. Irradiation Conditions ....................................................43 1. Flux ...............................................................43 2. Energy ............................................................43 3. Radiation Damage ................................................44 E. Sample Size .............................................................. 44 Response Functions .............................................................. 44 A. Single-Element Determination ............................................44 B. Multielement Determination .............................................. 45 Performance Prediction ..........................................................48 A. Direct Computation .......................................................48 B. Simulation ................................................................ 48 C. Experimentation .......................................................... 49 Optimization .....................................................................49 A. Analytic Solution ......................................................... 49 B. Search Methods ..........................................................49 1. Manual Inspection ................................................50 2. Automatic Search .................................................50 C. Expert Systems ........................................................... 51 D. Constraints ............................................................... 51 Future Directions ................................................................ 51 References ............................................................................... 52

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Activation Analysis

I. INTRODUCTION When setting out to solve an analytical problem by means of activation analysis, or any other method, the analyst must select or devise an appropriate procedure. An important property of instrumental activation analysis is the strong dependence of the quality of the results on the matrix activities present at the time of radiation measurement. The procedure to be followed must, therefore, be adjusted to suit the type of sample analyzed. It is rarely the case that nothing is known about a sample of interest. More commonly, considerable information about the general composition of a sample is available. For instance, knowing that a sample consists of vegetation, one can obtain a typical composition by consulting the scientific literature. Activation analysis is one of the most well-understood methods available to the analyst. It should, therefore, be possible to infer, from prior information about the sample, what procedure should be followed in its analysis. The accuracy of this process is naturally limited by the extent and accuracy of the prior information available. Better results should be obtained in this way, however, than by ignoring prior information. It is the task of optimization to discover the analytical procedure that best suits the sample being analyzed. In any method of chemical analysis, there are experimental conditions that can be controlled by the analyst and that affect the performance of the method. In chromatography, an example would be column temperature. Here, these conditions are referred to as experimental parameters or simply as parameters. The goal of optimization is to discover the set of parameter values that yields the desired performance for the analytical problem at hand. Optimization can be conveniently conceptualized if each experimental parameter is considered as a dimension of a geometric space. In activation analysis, if only irradiation and decay times are to be adjusted, the parameter space will be two dimensional. Each point in the parameter space corresponds to a possible procedure for carrying out a determination and each such procedure will perform more or less satisfactorily than others. Optimization, then, consists of a search for a point or a region in parameter space where performance meets the analyst's requirements. Practicality is an important consideration in designing a procedure for activation analysis. There are limits to the amount of radioactive material that can be handled safely and to the count rate that be accurately measured. Circumstances often impose further limits. It is, therefore, necessary to constrain the search of parameter space to those regions that correspond to practical procedures. In attempting an optimization, one must consider a number of aspects. A set of experimental parameters must be chosen for adjustment and others set at fixed values, often due to practical constraints. The way in which quality of analytical performance (the response function) is to be evaluated must be decided. A means of locating the optimum must be chosen and, finally, this optimization scheme must be implemented in a practical, convenient manner. These aspects are discussed in the following sections.

11. EXPERIMENTAL PARAMETERS It is useful to examine the well-known' equation for activation analysis. This equation models the activation of a single element and radiation measurement for a single activation product.

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TABLE 1 Adjustable Parameters in Activation Analysis Method

Parameter Irradiation time

All

Decay time

All

Counting time Number of cycles Irradiation flux Irradiation energy Sample size

All Cyclic AA All All, especially charged particle methods All

Counting geometry

All

Activation product Gamma-ray energy

All All

Constraints Mechanical, sensitivity, radiation damage, radiation safety, longlived interferences, cost Mechanical, sensitivity, short-lived interferences Sensitivity, isotope half-lives, cost Experiment time Adjustability Adjustability Packaging, availability of sample, heterogeneity, radiation safety, sensitivity Mechanical, count rate limits, sensitivity Half-life, sensitivity, interferences Sensitivity, interferences

C, counts registered; W, weight of target element; t,, irradiation time; t,, decay time; t,, counting time; E,, energy of activating particles; A, Avogadro's number; M, atomic weight; 8, target isotope abundance; X, decay constant; y, gamma-ray branching ratio; E,, garnrnaray energy; +(E,), flux of activating particles; u(E,), reaction cross-section; g, counting geometry factor; e(E,,g), detector efficiency. A, M, A, u(E,), 0, and y are physical constants and cannot be changed. The available adjustable parameters are, therefore, t,, t,, t,, W, +(E,), and e(E,,g). y can be regarded as adjustable if choices among the gamma-ray energies emitted by the activation product are considered. The chief adjustable parameters of activation analysis are summarized in Table 1.

A. IRRADIATION TIME During the irradiation time, samples are exposed to activating particles, such as neutrons. The induced activity of each activation product asymptotically approaches a limit. Shortlived activities approach this limit more quickly than long-lived ones. The irradiation time should be adjusted to provide the required sensitivity for a given analyte and to minimize matrix activities that are longer lived than the analyte activity. Radiation damage to the samples and their containers and limits to the amount of radioactivity that can be handled set the upper bound of the allowable range of irradiation times. The procedures used to insert, rotate, and remove samples for irradiation set the lower bound. For single-element determinations, the minimum irradiation time consistent with sufFor multielement determinations, however, more ficient sensitivity has been rec~mmended.~ complex calculations are required. B. DECAY TIME During the decay time, all induced activities decay, each with its characteristic halflife. The decay time should be adjusted to minimize the matrix activity at the time of counting while retaining sufficient analyte activity for the required sensitivity. In principle, there is no upper bound to the decay time. Practical requirements may, however, impose an upper bound. The speed with which samples can be transferred from the irradiation site to the radiation measurement apparatus sets a lower bound to the decay time.

42

Activation Analysis

For single-element determinations, a decay time of one half-life of the analyte activity has been rec~mmended.~ It has been shown that, unless the half-life of the analyte activity is more than twice that of the shortest-lived matrix activity, the optimum decay time is zero.3 For multielement determinations, the computation of the decay time is again more complex.

C. RADIATION MEASUREMENT 1. Counting Time Several requirements must be met in fixing the duration of radiation measurements. The counting time must be sufficient for the registration of the number of counts per unit analyte mass defined by the required sensitivity. Unless special methods are e m p l ~ y e dthe , ~ counting time should be small compared to the rate at which the radioactivity of the sample is changing in order to preserve the accuracy of dead time corrections. Finally, the counting time must be sufficiently short to permit the efficient processing of sets of samples. Where the contribution of detector background activities is important, the proper relationship between the times used in counting background and samples can be c ~ m p u t e d . ~ Under most circumstances, the counting time is not an important parameter in achieving discrimination against the effects of matrix a~tivities.~ It has been shown that, if the background activity is greater than the analyte activity, a measuring time of twice the half-life is desirable and that otherwise, the optimum lies at longer times.' These times, however, conflict with the limitation mentioned above. 2. Activation Product For many elements, irradiation produces more than one active isotope. Each such activation product is produced with a particular combination of cross-section, isotopic abundance, and so forth, and decays with a characteristic half-life. Consequently, it may be possible to determine a given element by means of more than one indicator isotope. This presents an opportunity for optimization. In most cases, it is sufficient to select the analyte activation product that yields the highest sensitivity and that decays with a convenient half-life. Matrix activities, however, may require that a different activation product be used. In multielement determinations, proper choice of activation products can be used to minimize the number of irradiations and radiation measurements that must be carried out. 3. Gamma-Ray Energy For each analyte, each activation product emits gamma radiation at one or more energies, producing peaks in the observed spectrum. The intensities of these peaks vary from one to another. It is desirable to select peaks of high intensity in order to obtain high sensitivity. The corresponding detection limit, however, may be degraded by a high underlying Compton andlor Bremsstrahlung continuum and by other, overlapping peaks. The optimum selection of peaks, therefore, depends on the expected Compton continuum distribution and on the presence or absence of interfering peaks. These in turn depend on other parameters of the procedure.

4. Geometry The effective measurement efficiency during counting depends on the solid angle that the detector presents to the sample. The shape of the sample and its distance from the detector, therefore, affect the detection efficiency. This can be used to control the count rate presented to the spectrometer. In optimization, counting geometry can be used to compensate for a lower limit on sample size. If, at the lowest allowed sample size, the conditions that yield the best discrimination against interferences also yield an unacceptably high count rate, increasing the

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sample to detector distance will reduce the count rate without changes in other parameters except counting time.3

5. Special Methods Procedures in activation analysis are not limited to a single irradiation followed by a single radiation measurement. Multiple irradiations and multiple countings can be employed. For example, the measurement of short-lived activities by subtracting the results of two countings has been explored and the optimum separation of the radiation measurements has been examined. A further example of such a procedure is cyclic activation analysis. Here, a sample is repeatedly irradiated and counted and the spectra are summed. The transfer, irradiation and counting times, and the number of cycles are important adjustable parameters. Al-Mugrabi and Spyrou9 used a computer model of activation and spectrometry to optimize the analyte signal-to-noise ratio for single-element cyclic neutron activation analysis. The number of cycles and the irradation and counting times were adjusted. Sample transfer time was set as short as possible and the total experiment time was fixed before optimization. The strategy used in optimization was not described. Variations of the cyclic method include pseudocyclic analysis, where samples are permitted to decay before reactivation, and replicate activation, where separate aliquots of each sample are irradiated.I0 The possible selection of these special methods should be considered during the overall optimization process.

D. IRRADIATION CONDITIONS 1. Flux The incident flux of the irradiating particles, neutrons, photons, protons etc. directly affects the level of radioactivity produced. For a particular analyte, the flux determines the irradiation time required to obtain the required sensitivity. Where long-lived interferences or matrix activities are of concern, an increase in flux can permit the use of reduced irradiation times and, therefore, reduced saturation of these undesirable activities compared to that of the analyte(s). The flux employed cannot always be changed freely. Instead, it may be necessary to choose among available irradiation positions. 2. Energy The effective cross-section of a nucleus in an activation reaction generally depends on the energy of the bombarding particle. Some reactions, "threshold reactions", do not occur below a certain energy. Consequently, the distribution of analyte and matrix element sensitivities and the possibility of certain interferences depend on the energy distribution of the particles used in irradiation. Proper selection of this energy distribution, when this is possible, can improve the performance of analyses. An important example of energy selection in irradiation is the use of a thermal neutron filter, such as cadmium or boron to perform epithermal neutron activation. This favors elements with activable isotopes that possess significant resonance peaks in their crosssection vs. energy functions over those elements that do not. The effective energy cut-off of such filters can be adjusted to some degree by changing the thickness of the energy filter. The use of the neutron filter thickness as an adjustable parameter in optimization could be desirable, especially in multielement determinations, but has not been systematically explored. In irradiations with charged particles, where the reactions are of the threshold type, energy is an important parameter. Proper irradiation energy can prevent interference through reactions that require higher energies. Similar considerations apply in photon activation.

44

Activation Analysis

The feasibility of optimizing irradiation energy, and the range of energies that can be considered, is naturally dependent on the irradiation facilities available.

3. Radiation Damage Nuclear activation is a highly energetic process. Large amounts of heat can be generated and radiolysis of samples and sample containers can occur. The permissible flux and irradiation time are often limited by the damage to samples and containers that can be safely tolerated. The type of container used and preirradiation sample preparation, such as ashing, are options that should be considered in devising an optimized activation analysis procedure. The correct choices may permit the use of advantageous irradiation regimes that otherwise would not be permissible.

E. SAMPLE SIZE The amount of sample that can be used in a single measurement is constrained by several restrictions. The capacity of the container or the irradiation facility used imposes an upper bound. Neutron self shielding may also limit the size of samples. Samples that are too small may be too difficult to handle or to provide standards for. Most sample materials cannot be sampled below a certain quantity because of inhomogeneity. Sample size must be adjusted to produce a level of radioactivity that does not overload counting equipment and that can be handled safely. These constraints, together with those mentioned in the preceding paragraph, can impose additional constraints on other parameters, such as irradiation time.

111. RESPONSE FUNCTIONS In order to search parameter space for an optimum procedure, it is necessary to be able to compare procedures with respect to the quality of their performance. That is, one must be able to compute some numerical quantity that expresses how well procedures solve the analytical problem at hand. Here, this quantity is called the response of the analytical system. The variation of the response with changes in parameter values defines the response surface of the system. Optimization then consists of a search of the response surface for the set of parameter values that yields the most favorable value of the response. It is important to realize that the function used to compute the response determines what aspect of analytical performance is optimized. Common responses are measurement precision and detection limit." A variety of response functions have been proposed for use in the optimization of activation analysis and are discussed below. An important distinction is that between the optimization of single-element determinations, for which response functions are relatively easily defined, and of multielement determinations for which this task is more difficult.

A. SINGLE-ELEMENT DETERMINATION In optimizing single-element determinations, the goal is to obtain the best measurement for the radioactivity produced from the analyte element in the presence of other activities produced from the sample matrix. Early studies of the optimization of activation analysis employed a response function based on the relation between analyte and matrix activities and incorporating a model of activation and often also radiation measurement. These methods use sets of simultaneous equations whose solution yields the desired parameter values. Where many matrix activities are present, these methods require substantial computational resources.

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Okadaj2 derived an expression for the optimum decay time in the presence of another isotope. The response used was the activity of the activation product of the analyte element. The irradiation time was presumed to have been selected beforehand and no assumptions were made concerning the identity of the matrix activity. Isenhour and M o r r i ~ o noptimized '~ the ratio of the count rate of the analyte activity to that of the matrix activities in the energy region of the analyte peak by solving a set of two simultaneous equations. The irradiation and decay times were the parameters considered. An estimate of the composition of the sample matrix was required as input to the computation and an iterative method was used to solve the set of equations. ZikovskyX studied two different counting schemes; one in which each sample is counted once and another in which it is counted twice and one spectrum is subtracted from the other. Two response functions were employed: the difference between the analyte activity and the matrix activity, and this difference multiplied by the ratio of the matrix activity to the analyte activity. An iterative solution of sets of simultaneous differential equations was used to compute optimum irradiation, decay, and counting times. The effect of the energy discrimination of the spectra was not included in the calculations. Other authors have adopted the same approach, but have optimized quantities of direct analytical interest. The optimum point in parameter space was still located by the solution of simultaneous equations. Tyurnin and Smakhtini4 optimized precision and analysis time by adjusting the irradiation, decay and counting times. The method of optimization was not stated. Watterson3 optimized the standard deviation of the measurement due to counting statistics. The parameters considered were the irradiation and decay times and the counting geometry. It was pointed out that the optimum procedure may be unusable due to practical limitations. Quittner and Montvai7 optimized the accuracy of the measurement of the analyte net peak area in the presence of a second activity and a constant background. Various equations were used to suit different relationships between the half-life of the analyte and that of the interfering activity. Irradiation and decay times were the parameters adjusted. Fedoroff optimized detection limits and precision in the presence of another activity.I5,l6 Optimization was carried out graphically. A different approach uses actual measurements rather than mathematical models of activation and radiation measurement. Davydov and Naumov17minimized the cost of analysis using a series of experiments guided by the simplex search method.'Vhis was demonstrated in photon activation with irradiation, decay, and counting times, and the energy of the Bremsstrahlung spectrum employed in irradiation as adjustable parameters. Guinn and c o - ~ o r k e r s " -employed ~~ a computer model of activation and spectrometry, instead of actual experiments, to compute precisions and detection limits for stepped sets of irradiation, decay, and counting times, and sample size. The optimum was found by inspection of printed lists. Burgess and Hayumbu6 also employed a computer model of activation and spectrometry together with the simplex method to adjust irradiation, decay, and counting times, and sample size. Two response functions were studied: the peak to baseline ratio and the detection limit. Obrusnik and EckschlageP have proposed a response based on information theory. This response is discussed in the next section.

B. MULTIELEMENT DETERMINATION In designing a procedure for multielement activation analysis, one must normally accept compromises. To avoid performing a series of single-element determinations, analytes must be determined in groups. For example, after a given irradiation, various groups of elements may be determined by counting at various decay times. Efficiency demands that the number of irradiations and countings be minimized. Consequently, it is not possible to employ the optimum procedure for each element and a compromise procedure must be used.

46

Activation Analysis

Achieving the best compromise conditions for multielement activation analysis is not a trivial task. One of the most severe difficulties lies in evaluating performance since simple measures, such as average detection limit can lead to good performance for some analytes at the expense of unacceptably poor performance for others.26 Formulations given in the literature are reproduced here for clarity, because of the complexity of the problem. One approach, devised by Davydov and N a ~ m o v , ~is' to minimize the analysis time subject to the condition that the analytes determined together provide signals that satisfy a limit of quantitative determination. The response function used was:

Where t is the analysis time, q is the total number of elements to be determined, n, is the number of elements determined simultaneously, and X,,, Xzi,X,,, and Xdiare the irradiation, decay, counting and delay (see below) times for the determination of the ith element. The adjustable parameters were the irradiation, decay, and counting times, the delay between counting and reirradiation, and the maximum Bremsstrahlung energy used for the photon activation treated in the paper. Sets of parameter values were evaluated by carrying out actual measurements on a representative sample and the simplex method was used for optimization. The strategy employed for optimization involved three steps. First the location of the optimum detection limit for each element, considered alone, was found. Next, the region of parameter space in which each element could be determined was obtained and the elements were divided into groups whose regions overlapped or were closest. Finally, the optimum (lowest) analysis time was sought for each group using the optimum parameter values of the individual elements to define the starting position of the search. In this scheme, no weighting was required. Obrazovskii et a1." minimized analysis time by computing the optimum decay time, subject to constraints on detection limit. Multielement optimization using three responses was proposed by B u r g e s ~ .These ~~,~~ were

1. 2. 3.

The average detection limit The average uncertainty in peak area The number of elements simultaneously detectable

Sample size and irradiation and decay and counting times were adjusted. Parameter sets were evaluated by the simulation of gamma-ray spectra and optimization was carried out by simplex optimizationz6and interactive computer graphics.29 Response Number 3 was used to group elements for simultaneous determination. Response functions Numbers 1 and 2 were the averages of the single-element responses. Weighting was accomplished by using the reciprocal of the optimum single-element response obtained for each element as the weighting factor for that element and combining the single-element values in a weighted sum. The weighted functions were, therefore:

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(4)

where F1 is the detection limit, F2 is the peak area standard deviation, n is the number of elements, primed quantities refer to the multielement optimization, and the superscript " refers to the single-element optimization. Response surfaces were investigated. Where the active isotope and the gamma-ray energy were selected for each analyte before optimization, these surfaces were found to be smooth and simple. Automatic selection of isotopes and gamma rays, on the other hand, produced highly complex surfaces. Attractive response functions have been proposed by Obrusnik and E c k s ~ h l a g e r In.~~ formation theory was used to derive functions based on the information content (gained by analysis) and information profitability. Simulation of spectra was used to evaluate parameter sets. Only decay time was varied in testing this concept, although this does not reflect a limitation of the approach. This general method could be used in the case of single-element determination also. The basic concept in the use of information theory is that the analyst's knowledge of sample composition is improved from the a priori state, provided by knowledge of the sample type, to that obtained by carrying out the determination. The amount of improvement depends on the quality of the analytical procedure employed. The information content can thus be used as a response function in optimization. Information profitability includes further considerations, such as cost. Information content was expressed for an individual determination as follows:

where I is the information content, X, is the detection limit, x, is the determination limit, x, and x2 are, respectively, the lowest and highest expected concentrations of the analyte in the sample, p is the true expected concentration, x is the concentration found, n is the number of replicates, and u2 is the variance of the measured values. One virtue of this response is that the detection limit, the measurement precision, and the analyst's degree of familiarity with the sample type are combined in a single function. As is required, the information content increases as the detection limit and precision are improved and as the number of replicates increases. Information profitability (IP) for multielement activation analysis was expressed as:

48

Activation Analysis

where 7 is the cost of analysis, m is the number of groups of elements (grouped according to importance), E is a selectivity parameter, k is the relevance, 1 is the number of elements in group j, e is an effectivity parameter, and I is the information content for element i. The cost of analysis, T , was divided into fixed cost and variable cost that depends on such considerations as counting time. The relevance, k, is set according to the importance attached to each group of elements. This quantity is not necessarily arbitrary. For example, relevance factors could be set on the basis of discriminating power for eventual pattern recognition operations. The selectivity parameter, E, was defined, for each group, as the ratio of the number of elements required to the number satisfactorily determined under the conditions under consideration. The effectivity parameter, e, was defined as follows: (1) if the relative standard deviation of the measured values is less than or equal to 5 % , e is set to one, otherwise, (2) e is set to the ratio of the logarithms of the required and obtained standard deviations. This function is remarkably complete. Its behavior as the adjustable parameters are varied, however, has not yet been extensively studied. It should be noted that, for multielement optimization, the separation of the elements to be determined into groups for irradiation and counting is an important step. As this grouping entails decisions concerning the active isotopes and gamma-ray energies used for determination, the complexity of the response surface searched in the subsequent optimization stage can be reduced.28

IV. PERFORMANCE PREDICTION To carry out an optimization, it is necessary to predict the results of measurements made under varying values of the adjustable experimental parameters. These data must then be evaluated for the quality of the corresponding procedure and the resulting response value must then be passed to the optimization process. Three approaches have been employed for this sequence: (1) the prediction equations are imbedded in the response function, (2) activation and spectrometry are modeled separately to yield a gamma-ray spectrum which is then evaluated, and (3) actual activation and spectrometry are carried out to yield a spectrum for evaluation. Each of these methods exacts costs in time, expense and computational complexity.

A. DIRECT COMPUTATION Early work on the optimization of activation analysis involved the formulation of equations that expressed the quality of procedures in terms of physical constants, instrumental factors such as efficiencies, and adjustable parameters. These equations were then differentiated and solved to obtain parameter values corresponding to an extremum of the response function. Examples may be found in Section 1II.A. Direct computation has not been attempted for multielement determination. As the number of adjustable parameters is increased, the complexity of the equations to be solved grows rapidly. Consequently, direct computation has required either the separate handling of special cases and the use of large computers or consideration of a limited number of parameters. Further, this method is not well suited to the inclusion of discrete parameters, such as the choice of gamma-ray energy.

B. SIMULATION Most recent studies of optimization in activation analysis have separated the process into three interacting modules: computer simulation of activation and spectrometry, computation of the response from spectra, and adjustment of parameter values. This approach

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avoids the solution of complex systems of equations and permits a more thorough treatment. While the computer programs required may be fairly large, the operations involved are relatively simple and optimization can be performed in a reasonable time on a small computer. Simulation of thermal activation and spectrometry were pioneered by Guinn and coworkers. 19-24 A simplified detector response was adopted that employed rectangular Compton distributions and a simple functional form for peaks. This model has since been extended by several researchers to include other modes of activation,' accurate dead time computation,'(' Brems~trahlung,~ and an extended form of the Compton di~tribution.~' The construction of a useful computer model for the prediction of the results of irradiation and spectrometry requires substantial effort but need be done only once unless the equipment used is changed. A library of physical constants, such as cross-sections, must be assembled, preferably in computer-readable form. If the variation of cross-section with irradiation energy is to be considered, this information must be incorporated in this library. The irradiation site must be characterized in terms of flux and preferably also energy distribution. Radiation detectors must be thoroughly calibrated with respect to efficiency (both total efficiency and peak to Compton ratio) and resolution as a function of energy. It is desirable to include calibrations for geometry, dead time, escape peak production, backscattering, and Bremsstrahlung to obtain a realistic model. Finally, a computer program is required that will compute a description of the spectrum that results from a given set of sample composition and size, irradiation conditions, and radiation measurement conditions. It should be noted that an actual spectrum need not be computed unless this is specifically required. Peak areas, baseline areas, and information regarding interferences are the data usually required as input to response functions.

C. EXPERIMENTATION The simplest was to obtain the result of' a proposed procedure is to execute it with either a portion of the sample in question or a sample of similar material. The solution of complicated equations and the construction of elaborate computer models are thereby avoided. Optimization by experimentation, however, is often impractical for several reasons: (1) sufficient sample material may not be available, (2) if long decay periods are involved, too much time is required, (3) the cost of irradiation and use of equipment may be too high, and (4) the effort involved may discourage the use of optimization. Where the experiments required can be carried out quickly, easily, and cheaply, this method should not be overlooked since it is likely to provide more accurate results.

V. OPTIMIZATION In this section, the actual location of the optimum is discussed. In all cases, this amounts to a search of parameter space. There are two main approaches to this task: solution of equations embodying the response function and performance prediction, and a search of parameter space guided by response values obtained by performance prediction.

A. ANALYTIC SOLUTION Optimization by the solution of equations has been discussed above. As iterative algorithms are required for the more complex equations, this process is then equivalent to an automatic search of parameter space.

B. SEARCH METHODS In all optimizations that do not employ analytic solution, parameter space must be searched. This search should be carefully organized to provide efficient operation.

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Activation Analysis

1. Manual Inspection The simplest approach to searching parameter space is to obtain responses from points in parameter space that form a grid and to then inspect these responses to find the best one. For efficiency, a series of grids of increasing resolution and decreasing extent can be used. The initial grid must be fine enough to permit reliable location of the region of the optimum. Guinn and c o - w ~ r k e r s ' ~used - ~ ~this approach, but did not formulate a multielement response function. Inspection was done by means of printed lists of responses. Burgessz9 employed interactive computer graphics for manual inspection. The response surfaces were displayed at a computer terminal and could be explored and refined interactively. Multielement responses were employed. Plots of the number of elements detectable at each grid point were used for element grouping and plots of response surfaces were used for location of optima. The advantage of a graphic presentation is ready comprehension of the structure of each surface. A serious difficulty, however, is the limited number of adjustable parameters that can be displayed. Remaining parameters must be optimized automatically for each point plotted. Only two parameters can be displayed in a conventional three-dimensional projection. The third dimension is required for the response. In the study cited above, a pseudo four-dimensional representation was investigated in which three parameters were shown as a three-dimensional grid and the response was encoded as a sequence of colors. In activation analysis, fortunately, the irradiation and decay times are responsible for most of the important structure of response surfaces. Parameters, such as sample size and counting time, exert more simple effects on responses, are more easily optimized automatically, and can be employed as "hidden" parameters.

2. Automatic Search It would be desirable to automate the process of locating optima. For efficiency, a minimum number of points of parameter space should be evaluated. The most popular strategy for accomplishing this is the modified simplex search algorithm devised by Nelder and Mead.18 This algorithm has been extended by a variety of aut h o r ~ .In~ this ~ - ~approach, ~ a geometrical figure (simplex) having a number of vertices one greater than the number of parameters is employed. The procedure corresponding to each vertex is evaluated and the simplex is modified so as to move it away from the worst vertex. The basic operation is reflection of the worst vertex through the opposite side of the simplex. The simplex eventually circles about the optimum point. Constraints are enforced by assigning an unfavorable response to all points that lie outside the permitted region of parameter space. The simplex method generally performs well, with two important exceptions. Constraints may impose a shape on the border of the permitted region of parameter space that can trap the simplex and prevent convergence at the optimum and, if the response surface possesses local optima, the simplex may converge to one of these, missing the global optimum. It is generally recommended that several optimizations be carried out from different starting points to verify that the global optimum has actually been found. Simplex optimization is not applicable when one or more parameters is not continuous. If considerations, such as the use of epithermal or thermal neutrons are to be included in an optimization, sufficient vertices cannot be generated and a separate search must be made in each case and the performances at the optima compared. The other search method employed for the optimization of activation analysis is the Here, the direction of the steepest local gradient method of steepest descent (or a~cent).~' of the response surface is followed in order to locate the optimum. This method, like the simplex algorithm, is subject to failure at boundaries and at local optima. The simplex method, however, samples more points on the surface at each step.

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C. EXPERT SYSTEMS An alternative to the methods already discussed is the use of an "expert system" computer program. Herc, the knowledge of an expert analyst is placed in a computer-readable knowledge base that consists of information, procedures, and rules. A computer program employs this knowledge base to solve problems presented by users. The object is to simulate the reasoning processes of an actual expert analyst. The existence of an expert system for neutron activation analysis has been reported.38 Expert systems find their most appropriate application in situations in which the system under consideration is only partly understood theoretically and, therefore, requires the use of the experience accumulated by experts for optimization. Where the system is well understood, conventional programming should be applicable and the expert system is red~ndant.~' It is the author's opinion that the latter situation applies in activation analysis.

D. CONSTRAINTS The search of parameter space must be limited to those regions that correspond to practicable procedures. Unfortunately, it is not always known beforehand where these regions are located. For example, a combination of irradiation and decay times that is compatible with the facilities for irradiation, transport, and handling may nevertheless yield too high a count rate for accurate spectrometry. Consequently, constraints fall into two categories: 1.

2.

Limits to parameter values Limits to the results of the application of procedures

Constraints of the second type may enforce further constraints on parameter values. Parameter constraints, Type 1, are generally set for practical reasons. Considering irradiation time as an example, the minimum value will depend on the facilities available for transferring samples to and from the irradiation site and the maximum value will depend on the permissible radiation damage, costs, and analysis time requirements. These constraints are easily found and pose no problems in locating the starting point of the search. Secondary constraints, Type 2, may also arise from considerations of practicability. Safety considerations, for example, may limit the amount of radioactivity that can be tolerated. They may also be derived from analytical requirements. Two possibilities are maximum detection limit and the severity of interferences. These constraints often require careful consideration of the analytical problem at hand and can lead to difficulty in locating a starting point for optimization at which they are satisfied. Davydov and Naumov2' have emphasized this operation in their method. The careful setting and enforcement of constraints is critically important to success in the optimization of activation analysis. One or more parameters, such as decay time and counting time, frequently yield impractical values if the true optimum is sought. The analyst, however, is only interested in the best procedure that can be obtained using reasonable parameter values. Consequently, some parameters can be expected to converge to values enforced by one or more constraints.

VI. FUTURE DIRECTIONS Further development of methods for the optimization of activation analysis can be expected to occur in several areas. The most important will be the study of response functions for multielement determinations, refinement of models of activation and spectrometry, and investigation of strategies for optimization. Comprehensive response functions, such as those of Obmsnik and Eck~chlager~~ should be formulated and their behavior studied. The success of each function in locating satisfactory

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Activation Analysis

procedures, especially for complex sample types, should be verified. The suitability of the response surfaces for automatic optimization, especially with regard to the occurrence of local optima, should also be investigated. Computational models for predicting gamma-ray spectra can be expected to be extended to all of the important modes of activation. Realistic, complete, and practical models of the detection process are required. These should account for the various features of gamma-ray spectra and also for the effects of sample and counting geometry. Many of these aspects have already been studied in isolation, but the formulation of a comprehensive model that is readily implemented is still needed. The strategy used in carrying out optimizations also requires study. Methods for determining the best combination of irradiations and radiation measurements, and for grouping elements for simultaneous determination need to be refined. Studies of interactions among parameters would be useful and could show which parameters can be relegated to a secondary role in optimization as mentioned above in connection with graphical optimization. An important topic which has not yet received attention is the degree to which a procedure can depart from the optimum and still produce satisfactory results. This property has been This ~ is particularly important in activation analysis where large termed " ~ g g e d n e s s " . ~ numbers of samples can be irradiated together but usually cannot all be counted at the optimum time. The optimum analysis of large sample sets has also not yet been addressed. Activation analysis is no longer used only as a research tool but is being camed out as a commercial service. Practical aspects, such as costs and speed of analysis, are more than ever important in the design of procedures. Further research in the optimization of activation analysis will yield both more powerful methods for research and more efficient and profitable procedures for commercial and service analyses.

REFERENCES 1. De Soete, D., Gijbels, R., and Hoste, J., Neutron Activation Analysis, Wiley-Interscience, London, 1972, 445. 2. Heydorn, K., Neutron Activation Analysis for Clinical Trace Element Research, Vol. I, CRC Press, Boca Raton, FL, 1984, 101. 3. Watterson, J. I. W., Optimization of irradiation and decay times in nuclear activation analysis, J. Radioanal. Chem., 26, 135, 1975. 4. De Soete, D., Gijbels, R., and Hoste, J., Neutron Activation Analysis, Wiley-Interscience, London, 1972, 492. 5. De Soete, D., Gibels, R., and Hoste, J., Neutron Activation Analysis. Wiley-Interscience, London, 1972, 527. 6. Burgess, D. D. and Hayumbu, P., Simplex optimization by advance prediction for single-element instrumental neutron activation analysis, Anal. Chem.. 56, 1440, 1985. 7. Quittner, P. and Montvai, A., Determination of optimum schedule and sensitivity for non-destructive activation analysis, Acta Chim. Acad. Sci. Hung., 51, 371, 1967. 8. Zikovsky, L., The optimum irradiation, decay and counting times in activation analysis for single and double counting and for different minimization functions, J. Radioanal. Chem., 22, 165, 1974. 9. Al-Mugrabi, M. A. and Spyrou, N. M., The use of simulation for the optimization of the signal-to-noise ratio in cyclic activation analysis, J. Radioanal. Nucl. Chem., 110, 67, 1987. 10. Egan, A., Detection limits and precisions in various irradiation and counting regimes, J. Radioanal. Nucl. Chem., 110, 47, 1987. 11. Currie, L. A., Limits for qualitative detection and quantitative determination,Anal. Chem., 40, 586, 1968. 12. Okada, M., Optimum "cooling time" to minimize interfering activity in non-destructive activation analysis, Anal. Chim. Acta, 24, 410, 1961. 13. Isenhour, T. L. and Morrison, G. H., A computer program to optimize the times of irradiation and decay in activation analysis, Anal. Chem., 36, 1089, 1964. 14. Tyurnin, G . A. and Smakhtin, L. A., Optimum time programming in instrumental activation analysis, Ind. Lab., 37, 1574, 1971.

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15. Fedoroff, M., Contribution au calcul des conditions optimals d'irradiation et de mesure de la radioactivite en analyse par activation, Nucl. Instrum. Methods, 91, 173, 1971. 16. Fedoroff, M., Calcul des conditions optimales en analyse par activation, J . Radioanal. Nucl. Chem., 15, 435, 1973. 17. Davydov, M. G. and Naumov, A. P., Optimization in quantitative activation analysis, At. Energy., 40, 417, 1976. 18. Nelder, J. A. and Mead, R., A simplex method for function minimization, Compur. J . , 7, 308, 1965. 19. Guinn, V. P., Instrumental neutron activation analysis limits of detection in the presence of interferences, J . Radioanal. Nucl. Chem., 15, 473, 1973. 20. Guinn, V. P., Garzanov, E., and Cortes, E., Further studies in the advance prediction of gamma-ray spectra and detection limits in instrumental neutron activation analysis, J . Radioanal. Nucl. Chem.. 43, 599, 1978. 21. Guinn, V. P., The INAA advance prediction computer program - its uses in environmental and energy research, in Nuclear Methods in Environmental and Energy Research, Vogt, J . R., Ed., University of Missouri, Columbia, 1980, 2. 22. Guinn, V. P., Leslie, J., and Nakazawa, L., Performance of the updated INAA advance prediction computer program, J . Radioanal. Nucl. Chem., 70, 513, 1982. 23. Guinn, V. P., Nakazawa, L., and Leslie, J. C., The effect of other activities on INAA detection limits of detection, J . Radioanal. Nucl. Chem., 84, 103, 1984. 24. Hsia, H. S. and Guinn, V. P., New forms of the INAA advance prediction computer program, J . Radioanal. Nucl. Chem., 112, 223, 1987. 25. Obrusnik, I. and Eckschlager, K., Optimization of the information properties of NAA with respect to information content and profitability of results, J. Radioanal. Nucl. Chem., 112, 233, 1987. 26. Burgess, D. D., Optimization of multielement instrumental neutron activation analysis, Anal. Chem., 57, 1433, 1985. 27. Davydov, M. G. and Naumov, A. P., Optimization of the multielement quantitative activation analysis, Radiochem. Radioanal. Lett., 35, 77, 1978. 28. Obrazovskii, E. G., Kostrovskii, V. G., and Gil'bert, E. N., Calculation of optimum decay time in multielement neutron activation analysis, Zh. Anal. Khim., 39, 617, 1984. 29. Burgess, D. D., Optimization of neutron activation analysis by interactive computer graphics, J . Radioanal. Nucl. Chem., 110, 51, 1987. 30. Burgess, D. D., Dead time correction for the prediction of gamma-ray spectra, Nucl. Instrum. Methods Phys. Res., A236, 368, 1985. 31. Zikovsky, L. and Schweikert, E. A., Use of computer simulated gamma-ray spectra in activation analysis, Nucl. Instrum. Methods, 155, 279, 1978. 32. Gustavsson, A. and Sundkvist, J.-E., Design and optimization of modified simplex methods, Anal. Chim. Acta, 167, 1, 1985. 33. Aberg, E. R. and Gustavsson, A. G. T., Design and evaluation of modified simplex methods, Anal. Chim. Acta, 144, 39, 1982. 34. van der Weil, P. F. A., Maasen, R., and Kateman, G., The symmetry-controlled simplex optimization procedure, Anal. Chim. Acta, 153, 83, 1983. 35. Ryan, P. B., Barr, R. L., and Todd, H. D., Simplex techniques for non-linear optimization, Anal. Chem., 52, 1460, 1980. 36. Burgess, D. D., Rotation in simplex optimization, Anal. Chim. Acta, 181, 97, 1986. 37. Massart, D. L., Dikstra, A., and Kaufman, L., Evaluation of Optimization of Laboratory Methods and Analytical Procedures, Elsevier, New York, 1978, 279. 38. Dessy, R. E., Expert systems, Anal. Chem., 56, 1200A. 1984. 39. Youden, W. J. and Steiner, E. H., Statistical Manual, Association of Official Analytical Chemists, Benjamin Franklin Station, Washington, D.C., 1975.

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Chapter 4

LIMITS OF DETECTION IN INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS Vincent P. Guinn

TABLE OF CONTENTS I.

Introduction. .................................................................... .56

11.

General Requirements for Tabulations of NAA LODs ...........................56

111.

The INAA Advance Prediction Computer Program (APCP) .....................57

References.. ............................................................................ .59

56

1

Activation Analysis

I. INTRODUCTION 1

Lower limits of detection (LLODs), frequently referred to simply as limits of detecticn and abbreviated as LODs, often appear in the literature of analytical chemistry - for numerous different methods of elemental and/or molecular analysis. In this chapter, one particular method of quantitative elemental analysis, that of instrumental neutron activation analysis (INAA), is the subject discussed, with reference to LODs. Particularly in the literature of neutron activation analysis (NAA), many tables of "interference-free" NAA LODs are available. Not all of these are of much use, because (1) for many the definition used for LOD is not clear, or reasonable, (2) for many, the analysis conditions used are not clearly specified, and (3) for many, the analysis conditions used are specified, but not very practicable for most laboratories. For NAA work, such tables of "interference-free'' LODs are, in any case, only applicable to samples in which, at the time of counting, only one radionuclide is present to any significant extent in the activated sample. Thus, such LODs generally only apply, for the conditions specified, to (1) radiochemical-separation NAA (RCS-NAA) of individual elements, or (2) preirradiation chemical separation of individual elements. There are, of course, some sample matrices that are activated only to a negligible extent by therrnal/epithermal neutrons (at least to form gamma-emitting products), and that contain essentially only one detectable product. Such samples, analyzed by INAA, can be considered to have LODs essentially as low as those in various published tables, but there are relatively very few such samples. Matrices for which such a favorable possibility for the use of INAA may exist include, for example, beryllium, carbon, water, hydrocarbons, many other organic compounds, lead, and bismuth. In some other instances, such tabulated LODs may be usable for INAA even for sample matrices that become highly activated by thermaUepitherrna1 neutrons, but in which the predominant induced activity in the matrix is a very short-lived activity, and the major induced activity is the one of interest and it has an appreciably longer half-life. With these exceptions, however, it is important to note that tables of INAA LODs, per se, do not exist - since the LOD for a given element, under stated analysis conditions, can vary by orders of magnitude, depending on the elemental composition of the matrix in which it is present. For any given element, its INAA LOD will always be as large as, and usually much larger than, its tabulated "interference-free'' NAA LOD -how much larger depending upon the elemental composition of the matrix in which it is present. As discussed in a later section of this chapter, however, an INAA computer program exists that can calculate realistic INAA LODs for any elements of interest, in any kind of specified sample matrix, under any given set of analysis conditions.

11. GENERAL REQUIREMENTS FOR TABULATIONS OF NAA LODs Although not always done, tabulations of LODs for sample components determined by any specified analytical method should be explicit in (1) defining the method used and the exact conditions under which it is used, (2) specifying any limitations on minimum and maximum allowable or usable sample sizes (especially if concentration, rather than absolute LODs, are to be tabulated, i.e., ppm rather than k g LODs), and (3) defining clearly the definition used for LOD. Now considering the case of "interference-free" NAA absolute LODs, usually expressed in micrograms of element, and usually limited to (n,y) products, the conditions that should ( ), and the epithermal-neutron flux (Q,,,), the be specified are the thermal-neutron flux @ irradiation time (ti), the decay time (t,,), the counting time (t,), the type and size of detector used, the counting geometry, and the type and size of detector shielding used. In addition,

1 f

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the type of counting employed should be specified, i.e., beta counting, gamma-ray counting, or gamma-ray spectrometry (measuring a product y-ray of a stated energy). For tabulations of interference-free absolute LODs, specification of sample size is not important, whereas it is if concentration LODs are to be tabulated. For example, a 1-ng absolute LOD corresponds to a 1-ppb concentration LOD for a I-g sample, but a 10-ppb value for a 100-mg sample, or a 100-ppb value for a 10-mg sample. In some tabulations, the LLOD is merely defined as that amount of the element (under the stated measurement conditions) that will produce a specified net counting rate above background, or a specified number of net counts obtained during the stated counting period. Such definitions are not statistically very meaningful, especially if the detector background counting rate is not specified. It is more meaningful to define an LOD as that amount of the element that will produce a number of net counts that is measurable to some reasonable relative standard deviation (e.g., + 30%), based upon the counting statistics. For example, one such tabulation of interference-free NAA LODs is that by Guinn and Hoste.' Since INAA involves gamma-ray spectrometry measurements (in some cases including X-rays emitted in radioactive decay) of multielement activated samples, the "background" above which a given gamma ray is measured depends on the cumulative Compton-continuum baseline under the peak that is produced by all sample gamma rays higher in energy than the particular one of interest. Typically, the number of baseline counts under a given "photopeak" varies widely from one sample matrix type to another. For this reason, no single table of INAA concentration LODs is possible.

111. THE INAA ADVANCE PREDICTION COMPUTER PROGRAM (APCP)

As mentioned earlier, it is possible to calculate INAA LODs for all elements of interest, under the conditions specified, for any matrix of exactly or approximately known elemental composition, via the INAA Advance Prediction Computer Program (APCP). This program has been developed by the author and certain of his students during the past several years. Copies of the program have been distributed to many different laboratories who have re'quested them, and many have put the program to effective use. The APCP and its various facets have been described in a number of publication^*-^ and hence is only briefly summarized here. Basically, the APCP calculates, from "best" literature values of all the relevant nuclear parameters (atomic weights, stable-isotope abundances, thermal-neutron isotopic (n,y) cross-sections (a,) and epithermal-neutron resonance integrals (I,), radionuclide half-lives, gamma-emission energies and yields -and experimental curves of the responses of any available y-ray detector (detection efficiency, photofraction, resolution, escape-peak fractions, 51 1-keV production in the shielding, counting geometry each as a function of y-ray energy), all details of the predicted pulse-height spectrum for any set of input parameters for a sample of the input elemental composition. The calculations are based upon a simplified, but quite acceptable (and experimentally verified) model of detector pulse-height spectra. The input elemental composition may be either one that is accurately known (e.g., for various reference materials) or is simply an approximate/typicaYor average composition for such a sample matrix (e.g., igneous rocks, coal, etc.). The program first calculates the maximum allowable sample weight, and then calculates and prints out, for that sample weight, all of the relevant details of the y-ray pulse-height spectrum, for the specified input ,, and @,pi). Usually, the maximum parameters (ti, t,, t,, detector, counting geometry, @ allowable sample weight is defined as the sample weight that would give a y-ray counting rate of 5000 cps at start-of-count (SOC), or 1 g, whichever is smaller. The counting-rate limitation on sample weight is very important and is necessary if one is to avoid excessive analyzer deadtime, pulse pileup, and peak broadening.

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Activation Analysis

For any one set of conditions, the APCP output lists the cumulative Compton-continuum level from one Compton edge to the next throughout the pulse-height spectrum, and each significant ( a , < 50%) photopeak. For each such photopeak, the y-ray energy, radionuclide identity, number of net photopeak counts (NPPC), and standard deviation (a) and relative standard deviation (percent a,,) are printed out. Cumulative Compton levels (in counts per keV) and photopeaks (in keV) are listed in descending order of energy, down to 50 keV. The program also includes an optional routine to calculate the LOD for any specified input or noninput elements of interest. For any given set of conditions, it calculates and prints out the ppm LOD for each specified element, for each of its various gamma rays. This calculation is easily performed, since the main APCP has already calculated the cumulative Compton-continuum level (in counts per keV) on top of which each photopeak will fall. For simplicity of calculation, each photopeak is considered to have a width at its base equal to three times the FWHM of the peak. The number of underlying baseline counts (BLC) is taken as the sum of counts on either side of the photopeak, taking a 3/2 X FWHM range on each side of the peak. For this particular choice, since a, = [a:, a;, J'I2, a,,, = [GPPC BLCI1/', and hence a, = [NPPC + 2BLC]'I2. The percent a,, of the NPPC then equals 2 100 X a,,/NPPC. For LOD calculations, the LOD (for the given set of conditions) is usually set as the ppm concentration of the element that would give a a,, of + 30%, in that sample matrix. To cite an example, one may consider a sample of the NBS SRM- 1571 Orchard Leaves standard reference material (38 input elements), activated for 3 min in a a, of 10" plus aePi of 10" n s - I , decayed for 3 min, and counted for 3 min 2 cm above a "15%" Ge(Li) detector. Under such conditions, the maximum allowable sample weight is 84.7 mg. The INAA LOD for vanadium, via the 1434 keV y-ray of its 3.76-min "V (n,y) product, is calculated from the cumulative Compton-continuum level in the vicinity of such a 1434keV peak (202 counts per keV) and the FWHM of such a peak (2.55 keV), for barely measurable detection ( ? 30% a,,), using the quadratic formula, to be that amount of vanadium that would give 191 NPPC. The program then calculates that 2.18 X lo-' pg of V would be required to give 191 NPPC under these conditions. Hence, for a 0.0847-g sample, the INAA LOD for V in this matrix is calculated to be 0.257 ppm V. This INAA LOD for vanadium is markedly higher than the interference-freeLOD for the same conditions and a 0.0847-g sample: 0.0149 ppm V (for + 30% a,,), or for a 1-g sample: 0.00126 ppm V. Thus, the INAA LOD for vanadium in this matrix is 17 times higher than its interferencefree value, for the same sample weight, or 204 times higher, when compared with the interference-free value for a 1-g sample. For (n,y) products that significantly emit gamma rays of several different energies, the LOD program output for a given set of conditions shows the LOD attainable with each. In some instances, the lowest LOD may not be very useful, if the photopeak overlaps with another sample photopeak or falls on a significant Compton edge. The APCP flags those peaks that have either or both of these complications. Where one is not merely interested in the INAA detectability of one element in a given type of sample matrix, but instead is interested in information on all detectable elements, the APCP is usually run under a series of stepped conditions that cover the typical practical ranges of ti, t,, and tc. These increase in steps of about 3 for ti (up to a practical maximum of 300 rnin, for t, (up to 30,000 min., i.e., about 3 weeks), and for tc (up to a practical maximum of 100 min). For the short- and medium-lived induced activities, the INAA optimum condition set is that one close to ti = t, = t, = T (where T equals the radionuclide half-life), for most multielement sample matrices. By optimum is meant the condition set that gives the smallest percent a,,, or the lowest LOD. Thus, the 12 condition sets usually run are the following:

+

+

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Times (in min) Set No.

tt

t,

t,

When one inspects (manually, at present) all 12 APCP outputs (for a given a,,, a,,,, detector, and counting geometry), one can readily ascertain (I) the best product radionuclide, if the element produces more than one gamma-emitting product, and (2) the best gammaray energy for its detection. An excellent and recent discussion of detection limits by various analytical methods is the book edited by L. A. C ~ m e . ~

REFERENCES 1 . Guinn, V. P. and Hoste, J., Elemental Analysis of Biological Materials, IAEA Technical Report No. 197, Pam, R. M., Ed., Vienna, 1980, International Atomic Energy Agency, chap. 7. 2. Guinn, V. P., Gsrzanov, E., and Cortes, E., J. Radioanal. Chem., 43, 599, 1978. 3. Guinn, V. P., Leslie, J., and Nakazawa, L., J . Radioanal. Chem., 70, 513, 1982. 4. Guinn, V. P., Dahlgren, L. N., and Leslie, J. C., J. Radioanal. Nucl. Chem., Articles, 84, 103, 1984. 5. Guinn, V. P., J. Radioanal. Nucl. Chem., Articles, 110, 5, 1987. 6. Currie, L. A., Ed., Detection in Analytical Chemistry - Importance, Theory, and Practice, American Chemical Society, Washington, D.C., 1988.

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Chapter 5

RADIOCHEMICAL SEPARATIONS IN ACTIVATION ANALYSIS

.

John J Fardy

TABLE OF CONTENTS Introduction ......................................................................62 General Radiochemical Considerations...........................................62 A. Hazards of Activation ....................................................62 Low Chemical Concentrations ............................................63 B. Carriers ................................................................... 63 C. Preseparation Treatment of Sample ..............................................64 A. Sample Preparation .......................................................64 B. Irradiated Sample Transfer ............................................... 64 C. Carriers Addition .........................................................65 D. Mineralization ............................................................65 Radiochemical Separation ........................................................66 Separation Characteristics ................................................66 A. Geological Samples ......................................................68 B. 1. Rare Earths ....................................................... 68 2. Noble Metals .....................................................70 3. Other Trace Elements .............................................74 C. Biological Materials ......................................................76 1. Single-Element Separation ........................................77 2. Group Separation .................................................82 Ion-Retention Media ..............................................87 3. D. Automated Radiochemical Separations ...................................87 Future Prospects ..........................................................90 E. Acknowledgments .......................................................................90 References............................................................................... 9 1

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Activation Analysis

INTRODUCTION Since the introduction of high-resolution Ge(Li) detectors, instrumental neutron activation analysis (INAA) has been the preferred version of neutron activation analysis (NAA). However, often the gamma-ray spectrum from an activated sample is dominated by only a few radioisotopes. Thus, the less abundant radioisotopes arising from activation of many elements at low concentrations are either difficult to detect or not detected at all. This often happens when INAA is used to analyze biological materials and interfering radionuclides produced by activation of the matrix (24Na, 32P, 38Cl, 42K, and 82Br) severely limit the analysis. In this case, the full sensitivity of NAA is not realized unless radiochemical separations after activation of the sample (RNAA) are used either to remove interference or to isolate the elements of interest before measurement by gamma-ray spectrometry. Preirradiation chemical separations are used in NAA and do concentrate the elements of interest to remove those matrix elements which seriously interfere with subsequent INAA detection. However, they do not eliminate the conventional problems of trace element chemistry, i.e., adsorption losses and reagent contamination. Thus, one of the main advantages of NAA is lost. This technique is not discussed any further here. Those interested in preirradiation chemical separations are referred to the book on environmental radioanalysis by Das, Faanhof, and van der Sloot. ' The main radiochemical operations involved in RNAA include the transfer of the irradiated sample, the addition of carriers, the mineralization of the sample, chemical separation of the interfering radionuclides, and/or the radionuclide of interest, and the determination of the chemical yield. These are discussed below with special reference to published literature during the 1980s and proceedings from the 1981 and 1986 Conferences on Modem Trends in Activation Analysis (MTAA) in Toronto and Copenhagen, respectively, and the 1987 Conference on Methods and Applications of Radioanalytical Chemistry (MARC) in Hawaii. Pre-1980 reference to RNAA can be found in the books of KrugerZ and De Soete et a1. , 3 in the reviews of Girardi and Pietra4on multielement and automated radiochemical separation procedures, Lau15 on applications in geological materials, and Bowen6 on its use for the analysis of biological materials.

11. GENERAL RADIOCHEMICAL CONSIDERATIONS A. HAZARDS OF ACTIVATION In NAA, the process of activation with neutrons gives rise to several hazards that dictate the course of the radiochemical procedures followed. Some of the radioisotopes formed by (n,y) reactions may be present in a chemical form that differs from the form before irradiation. This transition occurs during the activation process and is caused by the recoil energy which is imparted to the newly formed radionuclide during the emission of prompt gamma rays and which often results in chemical-bond rupture and the ejection of the nucleus. When the nucleus slows to thermal energies, it reacts in some way with the sample molecules. Care must be taken, therefore, during the addition of carriers to overcome this difference in chemical form. This is discussed more fully in Section 1I.C. Further difficulties can arise from The effect of radiolysis on organic material High temperatures leading to loss of particular trace elements from sample by diffusion and volatilization Pressure build-up due to evolution of gaseous products Radionuclide migration from container material as a consequence of recoil effects

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Ionizing nuclear radiation emitted by the radioactive products of neutron irradiation can cause rupture of chemical bonds and so destroy living tissue. Thus, there is a hazard associated with handling these radionuclides. This hazard may be of two types; a radiation hazard or a contamination hazard. Since the former is caused primarily by handling high gamma-ray activities, it will dictate the method of handling. The latter, however, is caused by quite small activities of alpha and beta emitters and will, therefore, require techniques that minimize possible ingestion of traces of active materials. In general, radiochemical procedures require methods that contain the radioactive material; for example, methods involving volatilization or which produce dust hazards are best avoided, and those involving "wet" handling are preferred. Even where solutions are concerned, methods that involve a minimum of transfers are preferred to those that require multiple transfers.

B. LOW CHEMICAL CONCENTRATIONS When elements are present in extremely small concentrations, they behave quite differently than when present in macroscale. In RNAA, where the concentrations of the elements being determined are often less than 1 kg g-', laws governing chemical equilibria and kinetic phenomena no longer apply. Moreover, in extremely dilute solutions, surface effects and particularly adsorption processes may play a key role in determining behavior. A further complication at these low concentrations can be colloid formation which often causes ions to behave in ways that are difficult to control. Such behavior includes inhomogeneous distribution in solution, irreversible adsorption on filters, particles or walls of containers, and nonspecific adsorption on ion exchangers. Most of these problems can be overcome in radiochemistry by the addition of carriers. C. CARRIERS A carrier is a substance with identical or similar chemical properties to the trace element in solution that is added in macroscopic amounts to increase its concentration and so minimize those problems and assist macrochemical separation procedures. If the carrier is isotopic with the activated element of interest, then it is classified as an isotopic carrier. However, it will fulfill its function as carrier only when both are in the same valence state and chemical form. Where the nature of these forms is uncertain, the mixture must be subjected to chemical procedures to ensure that both are converted to a single form and that complete isotopic exchange occurs. The use of isotopic carriers in radiochemical separation procedures makes it possible to determine individual chemical yields. Nonisotopic carriers are used in radiochemistry where a radioactive element must be finally recovered without its carrier. Their use in RNAA is generally limited to group separation by precipitation; for example the use of calcium as carrier for rare-earth elements by fluoride precipitation. If, however, isotopic carriers of each of the rare-earth elements were used in place of the calcium, the individual chemical yields would be obtained. Alternatively, a single rare-earth element, for example lanthanum, could be used as a carrier for all the rare earths. In this role, it is termed a group carrier; however, the chemical yield is obtained only for lanthanum. When carriers are added to prevent contamination of the desired radioactive trace element by other trace elements not of interest, they are classified as hold-back carriers. The small known amounts of carriers added usually range from 0.1 to 20 mg. Larger amounts may be used where separations are performed from large solution volumes, or if the procedure involves complicated, multistep operations.

64

Activation Analysis

111. PRESEPARATION TREATMENT OF SAMPLE A. SAMPLE PREPARATION The preparation of samples for NAA is independent of whether instrumental or radiochemical techniques will be used. Generally, procedures for geological samples are simpler than for biological materials. However, sample preparation for all types of material should be done in a "clean room" in which there are no chemicals and which is equipped with a laminar flow hood to restrict contamination from air particulates. Geological samples are either crushed into small aggregates or a powder in an agate or stainless-steel mortar to ensure homogeneity. However, the use of stainless-steel equipment would negate analyses for Cr, Mn, and Ni because of cross-contamination. Procedures for sample preparation of biological materials are more complex and vary according to the nature and physical form of the sample. Iyengar and Sansoni,' I ~ e n g a r , ~ and an IAEA Advisory Groupg recently examined the problems associated with these procedures and recommended methods to avoid losses and contamination. Often a number of steps are required to prepare solid biological materials, such as washing, drying, ashing, and homogenization. Hair, nail, and autopsy samples are often washed prior to analysis to remove external contamination. However, there is no universally accepted washing procedure and some elements may even be leached by such treatment. Drying and, to a lesser extent dry ashing, are often used before homogenization. Freeze drying is preferable provided that samples are frozen before being placed in the unit and an adequate vacuum is maintained. The alternative procedure of oven-drying can lead to loss of such metals as Sn and Hg if temperatures exceed 100°C. Dry ashing at about 500°C is convenient for handling large samples but can cause a large number of elements, such as Ag, As, Co, Cr, Hg, I, K, Na, Pb, Sb, Se, Sn, and Te, to be lost by volatilization. The merits of different types of ashing have been covered in more detail by Bock.lo Homogenization of biological materials is often necessary because of their complex and intricate structure. This can be achieved for both wet and dry specimens by the use of cryogenic homogenization techniques."-l4 The brittle fracture technique using a "microdismembrator" is useful for handling small sample sizes up to 10 g. However, the disk mill type used in the National Bureau of Standards is a vast improvement on this technique. B. IRRADIATED SAMPLE TRANSFER The transfer of irradiated sample from the irradiation container is the first step in radiochemical separation. This operation varies in complexity with the nature of the material irradiated, the type of irradiation container used, and the length of irradiation and/or thermalneutron fluence. The simplest procedure is to transfer powdered or liquid samples irradiated in polythene containers, where no changes in physical state occur and nonvolatile elements are not being determined, to a nonirradiated container and to measure the weight or volume after the transfer. Since no washing of the inner container is necessary, contamination from this source is avoided. However, if double containment is not used during irradiation, any contamination present on the outside of the irradiation container may also be transferred even in this simple operation. External cleaning of the container before opening the sample minimizes this. Transferring of samples irradiated in silica ampules can pose several problems. Pressure build-up from radiolysis of organic material which has already been mentioned (Section II.A), or volatilization of some radionuclides can cause loss of material unless care is taken during opening. These losses can be reduced by either cooling the container in liquid nitrogen before opening or carrying out the operation under water, suitable acid, or alkaline solution. Biological material may decompose under irradiation causing the material to char to a tarry substance that sticks to the container walls. Physical recovery of the material will

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65

require dissolution which is generally the first step in the mineralization of the complete sample. To do this, the outside of the irradiation container must first be cleaned with the same solution used for dissolution before proceeding with recovery of the sample. Added contamination arising from leaching of radionuclide from the inner wall of the container can be reduced by thoroughly cleaning the inner surface before irradiation. Errors may still occur when radionuclides recoil from the inside surface of the container and penetrate the samples. Where silica ampules are used, breaking open the container reveals new surfaces which can contribute to the blank. Experience shows this is only a problem for a few elements at the lowest concentration (Mn, Cr, As, and Sb). All pieces of the irradiated container should be removed quickly to avoid continued leaching and increasing in the blank value. C. CARRIERS ADDITION The use of camers to control variations in physical or chemical processes has been discussed (Section 1I.C). Therefore, carriers should be added as soon as possible in the separation process to help quantitative recovery. Because of the difficulties already discussed, addition to the irradiation vial assists quantitative transfer of the irradiated sample. However, the carrier is usually added at the start of the mineralization. Heydom15 recommends that the quantity of carrier be at least two orders of magnitude larger than the concentration of element being determined; i.e., for trace elements use 5 mg of carrier per gram of sample, and 1 kg gpl for ultratrace elements. The activated trace element and the carrier of the element must be converted to the same form and oxidation state early in the separation to ensure that they behave identically. Alternate reduction and oxidation steps are recommended. Many radiochemical procedures, especially those involving biological materials, have this as a part of their mineralization process when reductive conditions exist at the beginning and oxidation at the conclusion. Addition of a camer permits the determination of chemical yield for the separation procedure. Chemical yield determinations may use any of the conventional analytical techniques or radiochemical methods. Radiochemical methods involve either reactivation of the sample, preferably after some decay of the activity, or labeling the carrier with a known amount of radionuclide isotopic with the activated radionuclide of interest. Most modem radiochemical procedures have high and reproducible recovery of the carrier added, and de Goeij16 points out that yield determinations are often unnecessary, Here, the coefficient of variation of several percent falls within other variations in the NAA procedure. However, de Goeij stresses that, for accuracy and precision, individual chemical yields are essential.

D. MINERALIZATION Solid samples can be dissolved by several methods including addition of acid or, acid mixtures, bisulphite fusion, peroxide fusion. Fusion processes generally increase the risk of contamination, often add ions that interfere in subsequent separation, and are more likely to result in losses of volatile elements. However, this dissolution technique has been applied successfully to the solution of geological material before the rare earths are separated. Acid or acid mixtures are usually preferred for the dissolution of samples and can be used in open, semiclosed, or closed systems. The choice depends on whether the element of interest is lost by volatilization. Losses by volatilization depend markedly on the dissolution conditions, e.g., reagent, temperature, and time. Mercury and halogens are elements that are easily lost, while other element losses depend on the presence of particular reagents - Cr in hot perchloric acid or Sb, Sn, As, Hg, and Se in strong halide systems. However, this volatility has been used as the fust step in radiochemical separations during the destruction of the sample." Dry biological material can be dissolved rapidly by fust boiling in HNO, and then adding boiling HClO,. l o The danger of explosion is negligible if a few milliliters of H,SO, is added

66

Activation Analysis

and the beaker never allowed to boil dry. No loss of selenium occurs if temperatures during destruction stay below 210°C.'8.19However, losses of more volatile elements, such as Hg and Os, can be minimized by refluxing in a Bethge apparatus. Many laboratories avoid the use of HClO, because of the dangers of explosion and the lack of fume hoods suitable for this acid. Alternative acid systems used to dissolve biological materials include H3P0,1 HN03/H202,20 HN03/HC1/H202,21 HN03/H2S04,22.23 and HN0,1H202.24Again, loss of volatile elements can be reduced by using a reflux system such as a Bethge apparatus. Both concentrated Mg(NO,), solutionz5and mixturesz6of HN03/Mg(N03),have been used to wet ash biological materials in an open system. The search for a rapid dissolution technique has led several laboratories to use a microwave oven as a heat source in low-temperature digestion system^.^'-^^ If open and closed vessel microwave digestion systems with HClIHN0,IHF mixtures are used, dissolution of metals from powdered coal, fly ash, oil shales, rocks, sediments, and biological materials can be completed within 3 min. An alternative way to decompose biological material is to bum them in oxygen by the Shoniger t e ~ h n i q u e . ~ ~ , ~ ' However, the possible loss of such elements as Mn, As, Cu, Sb, and Zn on the platinum or quartz sample holder limit applications of this method. Other methods of wet ashing in a closed system involve the use of Teflonm-coveredstainless-steel pressure bombs32.33 or a Carius tube. 34 Care must be taken that mineralization is complete, since if it is not, residues and colloids that may still be present will retain a substantial part of the radionuclide. Moreover, the mineralization agent may interact with the sample constituent to cause precipitation (e.g., use of H2S04with Ca-rich samples) which in turn can cause possible co-precipitation of other elements. de Goeij16 advises that all such solids should be removed before chemical separation to avoid complications. Furthermore, they should be analyzed to ascertain whether they contain radionuclides of interest. Where necessary, the solids should be dissolved and returned to the mineralized sample.

IV. RADIOCHEMICAL SEPARATION A. SEPARATION CHARACTERISTICS The aim of a radiochemical separation is to reduce interferences and improve counting statistics. It usually also improves the precision and accuracy of the analytical determination, compared to instrumental techniques, but this depends on the detector system used to measure the separated samples. For the high-resolution Ge detectors, the purity of the radionuclide fraction can be less than that required for the more sensitive but lower resolution Nal(T1) detectors. Although gamma-ray spectrometry permits group separation, analytical results are less precise because of the poorer signal measurements. The effectiveness of the various separation techniques used is assessed by how well they isolate elements from interfering elements and how long they take. Separation schemes are designed either to separate the interference or to separate one or more of the radionuclides of interest, often into a number of groups, each containing one or more elements. The use of carriers permits nonquantitative recovery, with emphasis on speed of separation and radionuclide purity, and yield corrections are applied to ensure quantitative measurements. Many radiochemical separation procedures must meet specific conditions and criteria, e.g., specific trace elements in specific matrices, sensitivity, precision, accuracy, yield, speed, possible automation. A measure of the efficiency of a separation scheme in radiochemistry is given by the decontamination factor. This factor is merely the ratio of the activity of the contaminant to the desired radionuclide in the original sample compared to the same ratio after the separation. Thus

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67

where A: and A; are the activities of the contaminant and the desired radionuclide, respectively, in. the original sample and A, and A, are their activities in the sample after the separation. Sometimes this factor is termed the separation factor, but usually the separation factor is defined as the reciprocal of F?

Another important factor in radiochemical separations is the recovery R, or yield which is related to F by the equation.

when R, is fractional recovery or yield for the desired radionuclide. Heydorn15 defines the ratio (A:/A,) as the decontamination factor in his discussion of performance characteristics. He further introduces another term to describe the interferences in activation analysis called the effective value f. Interference is then defined by

where f is a measure of the contribution by the contaminant to the error in determining the concentration of the desired element. This is expressed as the microgram of desired element per microgram of interfering element. This measurement is important especially where counting procedures involve the use of the low resolution NaI(T1) detector. While the decontamination factor is a specific separation performance characteristic, the effective value of an interfering element depends on irradiation and decay times, selective counting equipment, and the type of data reduction techniques used. Where a number of procedures are used in the separation scheme, the overall decontamination factor is a product of the decontamination factors for each step. The majority of separations involve the distribution of an element between two phases (I and 11), and the fraction in Phase I1 at equilibrium is given by

where D is the distribution between Phase I1 and I and V, is the ratio of the volume or amounts of these phases. Equation 1 can then be written as

where D, and D, are the distribution ratios for the desired element and the contaminant respectively. Separation is best achieved when D, V, > 1 but D, V, < 1. The choice of a radiochemical separation technique depends largely on a number of factors including sample type, the nature and number of elements to be separated, the type and extent of interferences, and the time available for separation. Often, the final procedure involves a combination of a number of procedures. The development of multielement radiochemical separations for NAA peaked in the mid-1970s with Girardi and Pietra4 listing more than 100 methods. However, since then there has been a reduction in published methods. Ion-exchange chromatography methods dominated this early period, closely followed by solvent extraction. Precipitation and distillation procedures accounted for the bulk of the remaining procedures.

68

Activation Analysis

TABLE 1 Distribution of Radiochemical Methods between Geological, Biological, and Other Samples Source

Geological

Biological

Metdothers

1981 MTAA 1986 MTAA 1987 MARC 1981-1987 Journals

5 7 2 9

13 14 11 45

2 1 2 1

Note: MTAA, Modem Trends in Activation Analysis Conference; MARC, Methods and Applications of Radioanalytical Chemistry Conference.

TABLE 2 Distribution of Radiochemical Methods between Group and General Elements in Geological Samples Source

Rare earths

Noble metals

General

1981 MTAA 1986 MTAA 1987 MARC 1981-1987 Journals

3 2 4

3 2

2

2

2 8

Note: MTAA, Modern Trends in Activation Analysis Conference; MARC, Methods and Applications of RadioanalyticalChemistry Conference.

This contrasts with the present review of the literature since 1980 which showed an even distribution for solvent extraction, precipitation, and adsorption techniques. The slower ionexchange technique was reserved for the more difficult separations or when high precision and accuracy were required. Not surprisingly, radiochemicalprocedures were applied more often to biological samples as seen in a summary of conference and journal papers listed in Table 1. Three times as many papers described development of biological systems (74%) than those that referred to geological samples (20.5%), with analysis in other systems being only 5.5% of the total. This differs markedly from a review of earlier published methods4 where approximately 40% of separation schemes were developed for biological samples, 25% for geological materials, and 30% for high-purity metals and materials. 34,

B. GEOLOGICAL SAMPLES Radiochemical procedures that have been developed for geological samples are evenly divided between those separating specific groups of elements and those used for single or general, multielement separations (Table 2). The two main groups analyzed by these procedures are rare earths and noble metals. The following subsections survey procedures used for these groups of elements. 1. Rare Earths Although.general use of organic ion exchangers in radiochemical separation procedures had decreased, most of the procedures for rare-earth isolation (see Table 3) use these in combination with precipitation techniques. Laul et al.36reported a group separation method for the rare-earth elements (REE). Their separation scheme is outlined in Figure 1. Irradiated samples are fused in nickel crucibles

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TABLE 3 Radiochemical Separation Schemes for Rare Earths in Geological Materials Matrix Rocks, soils

Mineralization

Separation technique

36

Na,O,/NaOH fusion

Rocks

Na,O, fusion

Rocks Rocks Rocks

(1) HClIHF (2) Na,O,lNaOH fusion Na,O,/NaOH fusion Na202fusion

Rocks

Na,O,/NaOH fusion

Monazites

Predissolution-H,SO,

Rocks Rocks

Na,O,lNaOH fusion HCl/H,SO,/HF

Ref.

F-pptn (1) OH -pptn, Dowex 2, S C N extraction, OH-IF-pptn (2) OH -IF- pptn, TBP extraction, F-pptn (1) Dowex 50-X8 (2) F - P P ~ ~ OH- pptn, AG 50W-X8 Dowex 1-X8, HDEHP extraction, F- pptn O H pptn, Dowex 1-X8IHC1, Dowex I-XIISCN, O H pptn HREE-MnO, pptn, Dowex l/HNO,/ CH,OH. LREE-MnO, pptn, F-/OH pptn, Dowex I/HCl. Sc pptn, F-pptn Dowex 1-X8IHC1, TBP extraction Ox- pptn

37

38 39 40 41 42

43 44

Note: pptn, precipitation; TBP, tributyl phosphate; HDEHP, di(2-ethylhexyl) phosphoric acid; HREE, heavy rare earth elements; LREE, light rare earth elements.

with an Na202-NaOH mixture and in the presence of REE carriers. The fused cake is decomposed with water, neutralized with HC1, and the REE precipitated as a group hydroxide with excess NH,OH. After dissolution of the precipitate in HCl, the hydroxide precipitation is repeated with NaOH to remove any insoluble silica. The precipitation is repeated a third time with NH40H. The precipitate is dissolved in 10 N HCI and passed through an anionexchange column (AG1-XlO). The solution is heated to reduce the volume before precipitation of the hydroxide with NH40H. The precipitate is dissolved in minimal HCI, and the REE precipitated as fluorides at pH = 4 with freshly prepared 1 M NH4HF, and 3 to 4 drops of HF. The fluoride is dissolved in HN03/H3B03and precipitated as hydroxide with NH40H. The fluoride and hydroxide cycle is repeated at least twice to ensure radiochemical purity. The final precipitate is dissolved in minimal HNO, and used for counting. In order to achieve maximum detection sensitivity for the individual REE at very low concentration, Laul et al. used three count modes - a normal Ge(Li), a low-energy photon detector (LEPD), and a Ge(Li)-NaI(T1) coincidence/noncoincidencespectrometer. The typical spectra for REE standards counted 4 d after irradiation on each of the counting systems are seen in Figures 2 to 4. Based on their selected gamrna-ray energies, normal Ge(Li) counting is favored for I4'La, 170Tb,and 169Yb,LEPD for the low energies of 14'Nd, I5%m, 166Ho,and 169Yb,and noncoincidence counting for 141Ce, 143Ce,I4'Pr, 15?3m, 17'Er, and 175Yb. Rapid ion-exchange procedures and hydroxide precipitations were used also by Wandless and Morgan41in the group separation of REE. Figure 5 summarizes this separation scheme. However, they include the additional step in their procedure of anion-exchange purification from a 0.8 M KSCN-0.5 M HCl solution. In their REE group separation procedure, Bishop and Hughes39preferred to use a cation exchanger to purify and separate REE. Other separation procedures either added a solvent extraction step37,40,44 or substituted it for the ion-exchange procedure.37

70

Activation Analysis Sample "

carriers ( i n a c t ~ v e )

+

fuse N a z O 2 + NaOH Dissolve I n H20 and ac~dify

REE (OH),

1

Anlon exchange column

t REE

-----

-

I I Repeat two t~mes

I I I I

I

REE IOH13

P P ~

HI1 N H r HFz HF

I

RE€ (FLUORIDE1

H3 8 0 3 and HNOt NHrOH

'Irrnd~atedsample FIGURE 1. Radiochemical separation scheme of Lepal and Laul for REE. (From Lepal, E. A. and Laul, J. C., J. Radioanal. Nucl. Chem. Articles, 113, 275, 1987. With permission.)

'

2. Noble Metals

Because the noble metals are often present in rocks and ores at very low levels and are inhomogeneously distributed, large samples must be analyzed in order to obtain truly representative results. Preconcentration techniques are favored by most laboratories before final analysis by instrumental techniques including NAA. Recent procedures favor the nickel sulfide fire-assay technique of Hoffman et al.45based on a recommended method of Robert et al.46 Where ultra-trace concentrations of noble metals are present or more precise gold analyses are required, Shazali et aL4' recommend the addition of a tellurium co-precipitation step. This addition also permits the recovery and analysis of silver. However, during the

71

Volume 1 R E E group s p e c t r u m lsotopes o b s e r v e d 1 4 0 ~ 152 ~ E~ 171Er 161 Ce 1 5 2 m ~ ~

0

153

0

200

400

600

800

1000

Sm

166

Ho

1400

1200

1500

ENERGY ( k e V )

FIGURE 2. REE group gamma-ray spectrum by normal Ge(Li) detector. (From Laul, J. C., Lepal, E. A , , Weimer, W. C., and Wogman, N. A , , J . Radioanal. Chem., 69, 181, 1982. With permission.) R E E group spectrum Isotopes observed

0

60

80

120

160

200

26 0

280

320

360

-

ENERGY I k e V )

FIGURE 3. REE group X-ray and gamma-ray spectrum by LEPD detector. (From Laul, J. C., Lepal, E. A , , Weimer, W. C., and Wogman, N. A , , J. Radioanal. Chem., 69, 181, 1982. With permission.)

1970s, various radiochemical separation procedures were developed for noble metals although their use was limited to sample sizes of 500 mg and less. Notable among these was the procedure of Nadkarni and Morrison4' who used a selective chelating ion-exchange resin Srafion NMRR to isolate and determine the noble metals. A summary of radiochemical separation schemes developed for the determination of noble metals in geological samples during the 1980s is given in Table 4. With the exception of the radiochemical fire-assay technique of Parry et a1. ,54all procedures involve the analysis of sample sizes from 200 to 500 mg. Chemical carriers are added before'the separations, and chemical yield is determined by reactivation in each method. The procedure of Stockman49involved fusion of the irradiated rock samples with carriers and Na,O,/NaOH, dissolution of the fusion cake in dilute HCl and co-precipitating the noble metals and carriers as a group with Te using SnCI, as the reductant. Cocherie et al.51adopted a similar procedure for the analysis of silicates but co-precipitated noble metals with Se in addition to Te. When analyzing chromites, they added an additional co-precipitation step,

Activation Analysis REE group spectrum

soto opes o b s e r v e d

Non- Co~nc~dence

ENERGY IkeV FIGURE 4. REE group gamma-ray spectrum by Ge(Li)-NaI(T1) coincidencelnoncoincidence counting. (From Laul, J. C., Lepal, E. A., Weimer, W. C., and Wogman, N. A , , J. Radioanal. Chem., 69, 181, 1982. With permission.)

first using A1 and Fe as the hydroxide and then removing Cr as BaCr20, from the supernatant. Finally, the noble metals were co-precipitated with Se and Te from the solution of the hydroxide precipitate dissolved in HC1 combined with that left from the Cr,O,= precipitation step. Two radiochemical fire-assay procedures were developed recently for noble metal determination in geological samples. In the first, Millard52treated samples (500 mg) irradiated in an epithermal neutron facility with a minifire-assay technique. The Ir, Au, and Ag were collected in a 1-g lead button, and the primary contaminants As and Sb removed by heating the button with a mixture of Na20, and NaOH. The resulting 0.2-g lead bead was counted on a Ge(Li)-NaI(T1) coincidence/noncoincidencespectrometer. Detection limits for this technique varied from 0.01 ng g-' for Ir up to 10 ng g-' for Ag. Chemical yields were as low as 15% for Ir, with a maximum yield obtained for gold of 77%. In the second of the radiochemical fire-assay procedures, Parry et a1.54translated the nickel sulfide fire-assay technique into a radiochemical method in an effort to reduce the blank value introduced by the chemicals in the fusion mixture when analyzing for ng g-' amounts of noble metals. Suitable radiochemical handling facilities were required to handle the fire-assay procedure for 50 g of irradiated geological sample. Chai et a1.53.55developed two different radiochemical procedures for analyzing noble metals. These involved newly synthesized chelate ion exchangers (NANKAI-3926, BEI-5) and a long-chain primary amine solvent (N-1923). In their ion-exchange procedure, Chai et al. fused irradiated 200-mg samples with carriers and Nq02/NaOH which was dissolved in dilute HCl and adjusted to pH 1.5 with NH,OH before Os, Ru, Pt, Re, Au, and Mo were adsorbed onto the chelate resin. The resin phase was counted. Iridium does not adsorb on

Volume I

73

Flow c h a r t f o r REE Analysis Sample + R E E carrlers

A

Fuslon w ~ t hNaOH. N a 2 0 2 Leach w ~ t hH20

Supernate SI, Al, K. Rb, Cs

PPt REE NI, Fe, Mg, Zr, Sc, Hf NHc CI + NHrOH to pH9

Supernate Mg, Ca, Ba. Sr. NI

P P ~ S102

PP~ REE Fe. Cr. Zr. Sc, Hf

Supernate REE Fe. Cr, Zr, Sc, Hf 8M HCI

200- 400 mesh

REE Cr3, Zr, SC, H f Evaporate to dryness OEM K S C N - 0 5 M HCl 100 - 200 mesh

REE Kr31

I-

Supernate

8M NaOH P P ~ RE€

REE

FIGURE 5. Flow scheme of Wandless and Morgan for REE group separation. (From Wandless, G. A. and Morgan, J. W., J . Radioanal. Nucl. Chem. Articles, 92, 273, 1985. With permission.)

the resin, but if the eluate is heat treated in the presence of H,O,, Ir(1V) can be adsorbed on a further ion-exchange column and counted. An alternative solvent-extraction procedure, in which 80 to 100%of all noble metals are extracted by the primary arnine N-1923 compared with the negligible extraction of base metals, achieved a detection limit for Ir of 10-l2 g g-'. Among the many radiochemical separation procedures developed by Pietra et aLS6is one used to determine the quantities of noble metals and Hg in rocks, soils, sediments, coal,

and fly ash. The method involved the fusion of the sample in a Ni boat in a closed system with the simultaneous recovery of Hg. The fused mass was dissolved in H,SO,, and 0 s and

74

Activation Analysis

TABLE 4 Radiochemical Separation Schemes for Noble Metals in Geological Samples Matrix Rocks Rocks(Au) Rocks, chromites

Rocks Rocks

Rocks Rocks Rocks, soils, sediment, coal, fly ash

Mineralization Na202/NaOHfusion HF, HCVHNO, Silicates-HNOJHF, N%02/NaOH fusion; Chromites-Na,021NaOH fusion Na202 fusion Na,02/NaOH fusion

NiS fire-assay Na202/NaOH fusion Na,02/NaOH fusion (closed system)

Separation technique Te copptn Adsorption on activated carbon Te/Se copptn OH- pptn, Cr20,'- pptn Te/Se copptn PB fm assay (1) Chelate resins-Srafion NMRR, NANKAI-3926, BEI-5 (2) Amines - N-1923, N-235 HCI leach N-1923 primary m i n e extraction Volatilization, distillation, IONAC SR3

Ref. 49 50 51

52 53

54 55 56

Note: copptn, coprecipitation; pptn, precipitation.

Ru are distilled as the tetroxide. After concentration of the residual H,SO, solution and dilution with HCl, the precious metals Au, Ir, Pd, and Pt were separated on the ion exchanger IONAC SR3. The procedure can determine these down to ng g-I concentrations.

3. Other Trace Elements Recent development and applications of RNAA to the analysis of other trace elements in geological materials are briefly summarized in Table 5. Rarnmansee and Palme5' reported the development of a simple radiochemical technique for the determination of nonvolatile siderophile elements (Mo, W, Ir, Au, Re, Cu, Ni, Co, etc.) in geological samples and meteorites. They equilibrated a thermal-neutron irradiated sample with an excess of metallic iron at high temperatures (1573 K to 1853 K), separated metal and silicate phases manually, dissolved them in HN03/H3POdHF and HFIHCIO,, respectively, and counted them on a large Ge(Li) detector. van der Sloot et al.58 applied the technique of hydride generation in a sensitive radiochemical procedure for the determination of As, Se, and Sb in rocks, sediments, and residues from coal firing. After dissolution of the irradiated sample in a TeflonB bomb, it was evaporated to near dryness, dissolved in 6 M HCl, separated from the interfering matrices by hydride generation onto activated carbon grains and the latter counted. The limits of detection for As, Sb, and Se were 20, 40, and 2 ng g- I, respectively. Several procedures were developed for single-element determinations. Gavini et al.59 used a macroporous anion exchanger AG-MP-1 to separate U in 10 N HCl from dissolved, irradiated samples of air particulates, soils, sediments, and coal as well as biological material. For the determination of Mo in terrestrial, lunar, and meteorite basalts, Newsom and Palme60 combined a preconcentration technique of metal-silicate extraction with a radiochemical separation procedure involving sulfide precipitation. The precipitate is dissolved in aqua regia and counted. A precipitation technique formed the basis of a rapid radiochemical procedure for Se determination in soils and dust reported by Singh and S a ~ a n t Ethyl-a-isonitrosoacetate .~~ (HEINA) was used to reduce Se to its metal form in 6.5 M HC1 solution. Both yield determinations and counting in a well-type NaI(T1) detector were performed on the red Se precipitate. Drabaek et a1.61found distillation provided an excellent means for separating Hg, As, and Se from most other radioisotopes after wet decomposition of irradiated samples in their

75

Volume I

TABLE 5 Radiochemical Separation Schemes for Trace Elements in Geological Samples Matrix Rocks, meteorites

Rocks, sediment, fly ash, slag

Elements Cr, Fe, Co, Cu, Ga, As, Mo, Sb, W , Re, Ir, Au, 0 s As, Se, Sb

Soil, sediments, coal Earth and lunar rocks

Sediments, coal

Hg, As, Se

Mineralization

Hydride generation, adsorption on activated carbon AG-MP- 1IHC1 HN03/H20,, HCI Preirradiation-metallsili- S - pptn cate extraction Postirradiation-HCI Distillation, Hg electroHN03/H2S04(Bethge) deposition, Se pptn, As

HN031HFIHCI0, (Teflon" bomb)

Soil, rocks

HN03(TeflonB bomb)

Rocks, soil

HNO,(Teflona bomb)

Coals, fly ash

HN0,/H,S04, H,O,IHBr As, Cd, Cu, Cr, Sb, Se

Fly ash, rocks, sediments

As, Cd, Cr, Hg, Se

Ref.

Liquid metal extraction

Soil, dust Lunar rocks

Coals, river sediment, air particulates

Separation technique

Na202/NaOH fusion (closed system)

PPt" Se pptn1HEINA HAPIanion exchanger, l T A extraction, Cupferron extraction, HDEHP-RPC extraction AAO, TDO, CUS, CDO, HAP HAP, AG 50-X8, AG 1X8 Distillation, Dowex 1X8, TDO, CUS, Dowex 1-X8 HMD, Zn(DDC), and Bi(DDC), solvent extraction Volatilization, HMD, Zn(DDC), solvent extraction

Note: HEINA, ethyl-a-isonitrosoacetoacetate;HAP, hydrated antimony pentoxide; AAO, acid aluminum oxide;

TDO, tin dioxide; CUS, copper sulfide; CDO, cadmium dioxide; HMD, hydrated manganese dioxide; l T A , thenoyltrifluoroacetone;HDEHP-RPC, reversephase chromatography with di(2ethylhexyl) phosphoric acid; pptn, precipitation.

reported radiochemical procedure for these elements. Mercury was isolated from the distillate by electrodeposition on gold foil, while As and/or Se were precipitated by reduction to their elemental form with ammonium hypophosphate. The chemical yields for this method were 80 to 90% for Hg and 90 to 100% for As and Se. A total of 36 elements, including all 14 naturally occurring rare earths, were determined in lunar rocks by a radiochemical procedure developed by Sun et al.63 Their technique separated the elements into 12 groups by successive ion exchange, solvent extraction, and reversed-phase chromatography (RPC) techniques. Irradiated samples and carriers were decomposed with HNO,/HCLO,/HF, evaporated to dryness and dissolved in 9 N HCl. The solution was passed through a serial column of hydrated antimony pentoxide (HAP) and 717 anion exchanger. The anion exchanger was eluted with 0.5 N HCl leaving Groups 1 and 2 isolated on HAP and the anion-exchange resin and Group 4 in the 0.5 N HC1 solution. The combined 9 N HCl eluates from the columns were then subjected to consecutive thenoyltrifluoroacetone (TTA) and cupferron extraction to isolate a further group of elements. The remaining groups were separated by RPC on a further two solvent-loaded column using

76

Activation Analysis

a Kel-F support with 50 and 10% di-(2-ethylhexy1)phosphoric acid (HDEHP). The 50% HDEHP columns were eluted sequentially with 0.05 N HCl and 9 N HCl to isolate a further two groups of elements. The remainder were separated from the 10% HDEHP column by successive elutions with 0.15 M HCl, 0.21 M HNO,, 0.4 HNO,, 1.5 M HNO,, and 5 M HNO,. In a recent review of the radiochemical-separation procedures developed in their laboratories of the Joint Research Centre-Ispra Establishment in Ispra, Pietra et al.56listed 6 of their 22 procedures for use with geological materials. These are based on the use of inorganic ion retention media, such as aluminum oxide (AAO), hydrated antimony pentoxide (HAP), copper sulfide (CUS), copper chloride (CUC), tin dioxide (TDO), hydrated manganese dioxide (HMD), and cadmium oxide (CDO), the organic cation ion-exchange resin AG 50X8, the organic anion-exchanger AG 1-X8, and the chelating resin IONAC SR3, as well as complexing agents for solvent extraction, such as Zn- and Bi-diethyldithiocarbamate (DDC). Although they classified their inorganic ion retentions media as inorganic ion exchangers, I y e P questioned this classification, since sorption on these media in many cases cannot be described solely by ion-exchange mechanisms, often the process is irreversible, and there are discrepancies between batch and column experiments. Pietra et al.56based one of their procedures on the sequential use of five inorganic ionretention media: AAO, TDO, CUS, CDO, and HAP. This scheme, (see Figure 6) allows the determination of 39 elements separated into five fractions. Rock and soil samples were dissolved with HNO, in a TeflonB bomb, dried at 70°C, and dissolved in 0.1 M HNO,. They passed the solution through all five columns where Ga and Se distributes between TDO and AAO. Since 32Pis also removed on the AAO column, high detection limits were obtained for the Mo, Sn, W, and Th. The separation time for this simple procedure is 2 h. An alternative procedure reported by Pietra et al.56for the analysis of rocks and soil is based on the sequential use of three columns containing HAP, AG 50-X8, and AG 1-X8 in 6 M HF solutions. A total of 35 elements were determined in four different fractions. Like the previous scheme, it is suitable for automation and the separation time is 2 h after sample dissolution. The radiochemical separation scheme used for the m~ltielernental~~ analysis of coals and fly ash by Pietra et al.56 involved the separation from 3 M HCl on successive columns of TDS, CUS, and Dowex 1-X8 after distillation of volatile elements by H202/HBrtreatment. The distillate was further fractionated on a Dowex 1-X8 column. Longer separation times of 3 h after mineralization are required for this scheme. Solvent extraction has been employed in two radiochemical schemes of Pietra et a1.56 after prior passage of the dissolved irradiated sample through HMD. Where samples are wet ashed in HNO,/HClOdHF, then As, Cr, Sb, and Sc were sorbed on HMD from 1 M HNO, solution and Cd and Cu were extracted with chloroform solutions of Zn (DDC), and Bi (DDC),, respectively, after adjusting the solution to pH 1.5. In an alternative procedure, irradiated samples were rapidly mineralized by NqO,/NaOH fusion in a closed system allowing simultaneous destruction of the sample and quantitative separation of Hg. After dissolution of the fused mass in 3 M HNO,, the solution was cycled through an HMD column to separate As, Cd, Cr, and Se. Finally, Cd was isolated by solvent extraction as described above.

C. BIOLOGICAL MATERIALS The majority of radiochemical separation schemes have been developed for biological materials as evidenced by the listing in Table 6. Of the 66 procedures reviewed, 52% were devoted to the separation of one or two elements using a single-separation technique. Similarly, Pietra et a1.,56 in their reported development of radiochemical procedures at their Ispra laboratories, applied the majority (53.3%) to single element determinations as shown in Table 7.

Volume I

77

01 - 0.5 g sample

FIGURE 6 . Radiochemical separation scheme of Pietra et al. for the determination of 39 elements from 0.1 M HNO,. (From Pietra et al., J . Radioanal. Nucl. Chem. Articles, 102, 69, 1986. With permission.)

1. Single-Element Separation Single-element separation from all others in an irradiated sample enable determinations of quantities close to the interference-free limit of detection calculated by Guinn and Hoste. lZ8 Moreover, the radioisotopes of these elements can be determined with high precision on nonspecific p-sensitive counters or the high sensitivity, low resolution NaI(T1) detectors. Sensitive radiochemical procedures have been developed for platinum,83 mercury,'07

78

Activation Analysis

TABLE 6 Radiochemical Separation Schemes for Trace Elements in Biological Material Matrix Spinach, orchard leaves, pine needles, Bowens kale Infant food

Elements

Mineralization

Human plasma Kidney, bovine liver Human lung Blood serum

Cu As Pt, Au Se v , Cu Se, Zn, As, Sb, Fe, Cu

1. Dry ash at 700°C 2. H2S04/HN0, 3. Low temp. dry ash HNOJH202 H2S04/HN0JH202 H2S04/H202/HCI HNOJH2S04 HCI Mg(N03)2solution

Vegetation Fish Food

Pt, Ir As Zn

HNOJH202 H2S04/H202 HNOJHCl

Orchard leaves, bovine liver, fish, milk powder Plant, animal tissue Plants, muscle

H2S04/HN03

Vegetation, biological materials Bovine liver, Bowen's kale, pig kidney, milk powder Human milk, milk powder, bovine liver Human feces, urine, plasma Bowen's kale, bovine liver, wheat flour Oyster, fish, bovine liver

HNOJH202, HCI

Dog's blood, urine Orchard leaves, bovine liver, citrus leaves Hair, wheat flour, cereals, milk powder Orchard leaves, bovine liver, human organs, Bowen's kale Rice flour, bovine liver, milk Lung, liver, kidney

HClOJHNO, HNOJHCl

Ref. 65

extraction B . BPHA extraction HAP, SCN- pptn 1- pptn extraction Dibutylsulfide extraction Se pptn, MIBK extraction Cupferron extraction Chelate adsorption on activated carbon (AC), hydride generation, adsorption on AC Srafion NMRR resin As metal pptn HINAP substoich extraction I- extraction

66 67 68 69 70

71 72 73 74

Dowex 50-X8/HC104 HETAcAc substoich extraction AG-MP- l/HCI

75 76

Distillation, HAP, Dowex 2-X8/HCVHBr/HN03

77

S-pptn, solvent extraction

78

Fe, Zn, Se

APDC pptn

79

V, Mo, As

BPHA extraction, Iextraction Carbamide extraction

80

Se copptn Au PPh

82 83

Hg pptn, SCN pptn

84

Distillation, PAA, Dowex 2-X8, OH- pptn, FPPt" HMD, Ag glass wool

85

As, Cd, Co, Cr, Cu, Mo, Fe, Hg, Sb, Se, Sn, Zn As, Mn, Mo, Cu, Zn

H2S04/H202

Rare earths

Combustion furnace As, Cd, Hg

HNO, (autoclave)

Orchard leaves, animal bone, wheat flour Kidney, liver

Sc, Fe, Co, Zn, Cu, Se, Sat'd Mg(NO,), soln Sb, Hg, Au, REE Se, Hg, Fe, Co, Zn, Ag, HNOJH,02(reflux) Sn, Cr, Yb, Sb, Au, As Blood, bovine liver, milk Cu, Mn, Zn H2SOdHN0, powder, orchard leaves (Bethge) Human milk Fe, Co, Se HNOJH2S04 (Betbe) Fish

Separation technique

Mn, Zn, Cr, Ag

HNO,

59

81

86

Cu grains, Zn ferrocyanide, I- extraction Chelate adsorption on AC

87

S-/OH- pptn

89

Chelate adsorption C,, silica gel Distillation, OH- pptn, ether extraction, OHPPW Ag pptn, HAP, TBA substoich extraction

90

88

91

92

Volume 1

79

TABLE 6 (continued) Radiochemical Separation Schemes for Trace Elements in Biological Material Matrix Milk Orchard leaves, Bowen's kale, bovine liver, wheat and rice flour, spinach Rat liver

Elements

Mineralization

Separation technique

Ref.

Ag, Se, Co, Cr, Fe, Mo, H2S041HN03 Sb, Zn, Hg (Bethge) Cd, Cu, Mo, As H2S041HN0,

Distillation, deposition on Cu foil DDTC extraction, Adsorption on TDO

93

Se, Ag, Au, Sb, Pt, Hg, Co, Ni, Fe, Zn, Mo, Sn, Cr, Cd, Cu, As Hg, Mn La Cu

95

94

Hg

HNOJH2S0, (Bethge) HNOJHCl

Orchard leaves Human tissue

Sm, Eu, Tb, Yb Cu

HNOJHCI041H202 HNOJHCl

Animal muscle

Cd

HNO,, HCI

Oyster homogenate, wheat flour, blood, egg Cabbage, cracow, bovine hair, oyster tissue, hair Orchard leaves, bovine liver, mussel, pepperbush, Bowen's kale, capepod, fish, wheat flour, milk powder Animal blood and muscle, milk powder, human diet, horse kidney Milk powder

Se

HNO,, HCI

Cd, Co, Cu, Se, Zn

Sat'd Mg(NO,),

Ag

HNO, or HNOJ H,S041H202

HgIZn amalgam, ethyl acetate extraction, DMG PPt" HAP l T A substoich extraction LIX 70 extraction chromatography Distillation, Hg amalgam, AsISe metal pptn 2-HMBT substoich extraction TTA substoich extraction S-pptn, 2-HMBT substoich extraction S-pptn, 2-HMBT substoich extraction Se metal, 2-HMBT substoich extraction 4-NDP extraction, NaDDC extraction C1- pptn, OH- pptn, S PPtn

HNOJHCIO,

MnO, pptn

106

HMD, Bi(DDC), extraction, Zn(DDC), extraction Ni(DDC), extraction

107

Hair, guinea pig organs Orchard leaves, spinach Spinach, rye flour, animal muscle Pine needles, orchard leaves, oyster tissue Wheat and corn flour

Hg, As, Se

H,SO,IHNO, HNOJHCI04/H20Z HNOJHCIO,

Ag, As, Cr, Mo, Sb, Se, Cu, Cd, Hg, Sn HN03/H2S04(Teflon@bomb), H20,

Human tissue, plasma Hair, animal bone, orchard leaves, bovine liver Blood serum

Rare earths Rare earths

Foods

As, Cr, Mo, Sb, Se

Human tissue Orchard leaves Human tissue

Hg, Se Pt, Mo

v Pt

Blood Blood, milk Diet, feces Food Drugs

Cu, Mn Cu Se I, Pd, Ba

HN031H2S041HF HCVHNO, Nq0,lNaOH fusion

Distillation, I- extraction Dowex 50-X8lHC1 OH- pptn, AG1-X10, OH- pptn, F- pptn

HCl HNOJHC1041HF Preirradiation HNO, HNOJH,SO, Pt-HCUHNOJH,SO, Mo-H2S041H,0, HNOJH2S0,

Cupferron extraction HMDIHNO, Dowex 2- IXIHCI H2S041HBrextraction Au pptn, AAO BPHA extraction Dowex 50-X8/HC1, S PPt" HAP Neocuproin extraction Se pptdHEINA 1- extraction, DMG pptn SO4= pptn

HNOJH,S04/H,02 Preirradiation HNO, HNOJHCIO,

-

96 97 98 60 99 100 101 102 103 104 105

lo8 109

110 111 112 113 114 115 1 I6

117 62 118

80

Activation Analysis

TABLE 6 (continued) Radiochemical Separation Schemes for Trace Elements in Biological Material Matrix Foods Horse kidney

Blood, serum, milk products Food Human liver Foods Sorghum, soya, alfalfa flour Biological Horse kidney, hair, tomato leaves

Elements

Mineralization

Separation technique

As, Cd, Cr, Sc Hg, Se

? H2S04/HN0,/H202 (Bethge)

HMD Chelate adsorption C,, silica gel Chelate adsorption AC TBA extraction, CrO,=

Au, Cd, Mo, W, Cu, Mn Se, As, Mo, Ag, Sb, Cr, Sn As, Au, Cu, Fe, Hg, Mo, Sb, Se, Zn Se

HNOJHCIO, HNOJHClOJKMnO, HNO,/HCI

HMD, TBA extraction

?

As, Cd, Hg, Cu, Zn

Ref.

CuS copptn, ZnS, copptn

HNO,(Teflonm bomb) Adsorption APDC chelate on AC ? Adsorption on MgO H,SO JHNO,(Teflonm Chloroform extraction, bomb) Zn(DDC), extraction, 1extraction, MIBK extraction

Note: BPHA, N-benzoyl-N-phenylhydroxylamine; MIBK, methyl isobutyl ketone; substoich, substoichiometric; HETAcAc, ethyl thioacetoacetate; PAA, polyantimonic acid; APDC, ammonium pyrrolodinedithiocarbamate; TBA, tribenzylarnine; DMG, dimethylglyoxime; LIX 70, 2-hydroxy-3-chloro-5-nonylbenzophenone oxine; 2 HMBT, 2-mercaptobenzothiazole; 'ITA, thenoyltrifluoroacetone;Na-DDC, sodium diethyldithiocarbarnate; HAP, hydrated antimony pentoxide; ppn, precipitation; HINAP, isonitrosoacetophenone; 4NDP, 4-nitro-0-phenylenediamine.

tin,'" and ~hromiurn'~' in biological Standard Reference Materials and samples. The procedure for Pt utilizes the inherent sensitivity of the activation reaction '98Pt ( n , ~ '99Pt ) and counting of the daughter nuclide 199Au.Separation of this radionuclide is achieved by autoreduction of gold compounds to elemental gold in aqua regia at temperatures above 250°C. Platinum is determined down to concentrations of 5 X 10-l2 g g-'. Similarly, gold can be determined down to these levels. The accurate determination of Cr in biological materials continues to be one of the more difficult determinations in analytical chemistry. Because of the relatively low energy of the gamma ray from its activated rationuclide "Cr at 320 keV, background interferences limit its sensitive determination by NAA. Isolation by radiochemical procedures has been the preferred means of analysis, but low yields and cross-contamination have proved troublesome. Greenberg and Zeisler"' reported a procedure that eliminates these problems. The method involves the addition of potassium permanganate to oxidize and maintain Cr in the 6 state, two extractions with 5% tribenzylamine (TBA) in chloroform, and a back extraction in NaOH solutions to provide a stable climate. Typical recoveries of 99% were obtained. Where Cr concentrations were less than 1kg g- ', Cr was finally precipitated as barium chromate to improve counting geometry. Individual radiochemical procedures were developed by Greenberglo' for the determination of extremely low levels of Hg and Sn (G1 ng g-') in the Milk Powder Standard Reference Materials, 'SRM 1549. When analyzing for Hg, Greenberg dissolved the irradiated sample in a Teflonwbomb using HNO,/H,SO,. No loss of Hg occurred in the mineralization process when the temperature was maintained at 110 to 140°C for several hours and then cooled to - 15°C before opening. Mercury was extracted by nickel diethyldithiocarbamate (Ni (DDC),) in chloroform and counted. For Sn determinations, the sample was dissolved

+

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81

TABLE 7 Radiochemical Separation (RS) Schemes of Pietra et aLS6For the Analysis of Biological Materials

Element

Re-dissolution (starting media

Sequential

complete

dissolution

for RS)

steps of RS

RS

Typical applications

2d

Certified SR materials; diet samples, animal tissue: water

2h

1 M HC10,

HAP, AG 1-X8, CUS, TDO, AAO, AG 50-X8(on the residue); AG 1-X8, TDO (on the distillate) AAO, TDO, CUS, C W , HAP AAO, AG 50-X8

I1 MHC1

TDO, HAP

2h

Animal diets; human tissues Plant and animal certified SR materials Biological samples

6 M HCIO,

CUC, TDO

2h

HNO, (Teflonm bomb) HNO, (Teflona bomb) HN03-H202

1 (or 10) M HCI 2MHF

CUS, Dowex 1-X8

3h

Dowex I-X8, CUS

3h

6 M HCIO,

AG 50-X8

HNO, (Teflon" bomb)

8 M HCI

Dowex 1-X8 (elution by 1.5 M HCI)

0.5 M HNO,

HAP. AG 50-X8

HNO, (Teflonm bomb)

7 M HNO,

Ih Hg volatilization, condensation at - 20O0C, precipitation as HgS I, generation, absorp- 20 min tion on HMD, heating to 300°C, absorption on Ag coated quartz wool 1.5 h AA 0

HNO, (Teflon" bomb)

6MHF

AMD

HNO, (Teflon" bomb) HNO, (Teflona bomb)

1 M HCI

50

As, Co, Cu, Fe, Mo, Sb, Zn As, Cu, Sb

Cd, Se, Hg Cu, Mo, W Ca

Hg

HNO, (Teflone bomb) HN0,-H2S04 H,02-HBr HN0,-HCI04-HF

0.1 M HNO,

Combustiondistillation

Combustiondistillation

Rare earths

Time for

Procedure of mineralization

2 M HCI

3h

<30 min

3h

<30 min

2h

2h AG 1-X8, elution by 0.1 M thiourea

5h

Certified SR materials; animal tissues, body and fluids Hair samples Blood proteins of sheep Certified biological SR materials; enzymes Whole blood and serum from uremic and normal subjects Certified biological SR materials; plants; animal tissues; enzymes Certified biological SR materials (bovine liver; wheat flour; milk) Certified biological SR materials (milk)

Nucleic acids; molecular weight and chain length of polynucleotides Human lung biopsies and autopsies; bronchoalveolar lavages Human blood and urine Unexposed rats; animal diets

Note: HAP, hydrated antimony pentoxide; CUS, copper sulfide; TDO, tin dioxide; AAO, acid aluminum oxide; CDO, cadmium oxide; CUC, copper chloride; AMD, anhydrous manganese dioxide.

82

Activation Analysis

in HNO,/H,SO,/HF in a TeflonB vessel, heated to 200°C to expel Se, iodide added, and SnI, extracted into toluene. After back extraction with 5% EDTA/l% NaOH, the aqueous phase was counted. Substoichiometric radiochemical separations, developed by Ruzicka and StarylZ9in the 1960s, were used successfully to determine zinc,73 copper,76J01lanthanum,98mercury,99 cadmium,'02and seleniumlo3in various biological matrices. In conventional procedures, the same amount of carrier is added to the sample and standard. When separations are not quantitative, chemical yield of the carrier must be determined. However, Ruzicka and Stary avoided this step by adding the same substoichiometric quantity of reagent to sample and standard. This reagent must react quantitatively with the carrier element to form a compound readily separable from the excess carrier and from any interfering elements in the matrix. Although solvent extraction, ion exchange, and precipitations have been used for these separations, metal chelate extractions, and to a lesser extent, ion-association systems have been the most popular. All the cited separation procedures have used solvent extraction with isonitrisosacetophenone (HINAP), ethyl thioacetoacetate (HETAcAc), Zmercaptobenzothiazole (2-HMBT), or thenoyltrifluoroacetone (TTA). In developing separation procedures for a single element, care is often required to minimize losses during the dissolution of the sample in order to achieve the highest sensitivity for the method; thus, the care necessary in the above to dissolve milk powder samples in a Teflon@bomb for Hg analysis. Grimanis and Kanias7, preferred sample destruction with HNO,/H,SO, in open vessels for Hg determinations. They claim negligible losses in the presence of 150 kg chloride, but found an 8 and 60% loss in the presence of 1500 and 15,000 pg chloride, respectively. Polkowska-Motrenkoet a1.81investigated several sampledestruction techniques for Se determination. They preferred oxygen flask combustion for dry materials but acid wet ashing with HN03/H,S04/H,0z for fresh materials. Heydorn and Damsgaad'j5 reported that recovery of V varied with the decomposition methods used for different matrices. 2. Group Separation To determine one element at a time in a biological material is time-consuming and wasteful since many applications require results for a number of elements. Multielement separation procedures have been developed to overcome this problem, and for biological samples its procedures have usually been developed which separate the elements into groups. The number of elements in a group varies, but if maximum sensitivity is required, then these are limited to one or two elements, and the fraction counted on a NaI(T1) detector rather than a germanium system. Girardi and Pietra4 reported on more than 80 multielement separation schemes in their review written during the 1970s. Examination of Tables 6 and 7 reveals fewer than 20 schemes developed for biological samples over the last 7 years. Kucera and de G ~ e i j ~ ~ compared the use of NaI(T1) and Ge(Li) spectrometry for two separation schemes developed in their laboratory. Both schemes use distillation, passage through HAP, and anion-exchange chromatography to fractionate elements into groups. Figures 7 and 8 list details of these group-separation schemes and show that the scheme used for NaI(T1) counting separated the elements into more groups with only one to three elements in each group. Their studies showed this scheme was attractive due to its lower limits of detection and the slight dependence of these limits on matrix composition. The alternative system gave better precision of analysis and faster analysis times. Siripone et a1.88developed a group separation scheme based on the use of active carbon as a scavenger for chelated trace elements after wet open mineralization of dry biological materials with a saturated Mg(N03), solution. Full details of this procedure are shown in Table 8. The procedure can determine ten or more elements in less than 3-g aliquots of dry biological material and only Cr, Sc and Sb require their chemical yields to be determined.

83

Volume I

I

A 2Oml conc HBr

I

HCI g a s d ~ s t i l l a t ~ o n

I

B

r

h 201111 EM HCI

Na. K. P, Cu, Co, NI

FIGURE 7 . A group separation scheme by Kucera and de Goeij for NaI(T1) spectrometry. (From Kucera, J . and de Goeij, J . J . M . , J . Radioanal. Chem., 63, 23, 1981. With permission.)

C z a u d e r t ~ a reported ~ ~ . ~ ~ development of two radiochemical separation procedures for multielement determinations. The first involved the destruction of a sample in HN0,/H20, under reflux, followed by group separations using sulfide and hydroxide precipitation. This method permits the simple and rapid determination of Se, Hg, Fe, Co, Zn, Ag, Sn, Cr, Yb, Sb, Au, and As. In the second procedure, postirradiation separation was achieved chiefly by extraction with mercury or zinc amalgam. Gold, however, was separated by extraction with ethyl acetate or by precipitation with dimethylglyoxime (DMG). developed 4 separation schemes for the determination As Table 7 shows, Pietra et of between 4 and 50 elements in biological samples. An earlier scheme developed by the same researchers for geological samples and summarized in Figure 6 was used also for analysis of 39 elements in animal diets and human tissues. In another scheme (see Figure 9) f i r analysis of these samples and Standard Reference Materials, Pietra et al.56separated and determined up to 50 elements. Their procedure consisted of two main operations - the first dissolution and distillation from H2S0,/HN0, and then from H20,/HBr mixtures; in

84

Activation Analysis

I

8 M HCI

I

B--

As, Hq.Sb

Se, Sn

Na.

Co. Cu Mo, N i

FIGURE 8. A group separation scheme by Kucera and de Goeij for Ge(1i) spectrometry. (From Kucera, J. and de Goeij, J. J. M . , J . Radioanal. Chem., 63, 23, 1981. With permission.)

the second, they separated the remaining elements on ion-retention media and ion exchangers starting with 4 M HCl solutions. The distillate was fractionated on AG 1-X8 and TDO while the sequential use of HAP, AG 1-X8, CUS, TDO, AAO, and AG 50-X8 fractionated the residue from the distillation. They separated the elements into 20 groups for counting. Another separation scheme that Pietra et al.56used for analysis of plant and animal material was based on the sequential use of AAO and AG 50-X8 in addition to the quantitative recovery of the distillate during sample dissolution. This procedure was used to determine 30 elements subdivided into 5 groups. Greenberglo' used a single column of HMD to separate and determine Ag, As, Cr, Mo, Sb, and Se in milk powder. The irradiated sample was dissolved in HNO,/HClO, before passage through a HMD column, and the HMD fraction was counted twice. Interest has increased in determining the distribution of rare earths in biological materials as well as understanding their role in biochemistry, biology, and medicine. However, the

Volume 1

85

TABLE 8 Flow Scheme for the Analysis of Tra'ce Elements in Biological Materialss8 Preparation of samples: drying, weighing, and encapsulation Irradiation for 12 h at 5.1012n ~ m - ~ s - cooling ', 12 h. Destruction with a saturated Mg(NO,), solution. Dissolved in 6 N HCI and diluted to 30 to 150 ml. Separation of trace elements by adsorption on activated carbon using filtration through layers of 50 mg crurbon on a membrane filter. Dilute to == 250 ml. Filtrate adjusted to pH 2.5

+ APDC + oxine

Direct filtration through carbon. pH 0.2 no reagent Filtrate adjusted to pH 2.5 bic acid + oxine

+ APDC + ascor-

Filtrate adjusted to pH 5.0 + APDC bic acid + cupferron oxine

+

Filtrate adjusted to pH 7.5 volume = 600 ml

+ ascor-

+ APDC, Final

I I

SICr, WMo 187W 122Sb, 197Hg , 6 4 C ~ 9

9

1 9 8 A ~IlanAg, , 197Hg,T u , 2'3Pa(Th)

4 6 S ~15Se, , lZ2Sh,T u , 47S~(Ca), 239Np(U), 69mZn,Il5Cd, T o , 52C104-2,56Mn "Co, @'"Zn, 56Mn,58C~(Ni), 59Fe, 115Cd, Il3Sn, "Sc, 4 7 S ~"'Sm, , I"La, I4lCe, Is2Eu, 1 7 5 n 177LU, 147Nd, 1"Mn, lanthanides

Counting of the combined carbon concentrates on a Ge(Li) detector.

REE concentrations in these samples lie at or below ng g-' and there has been urgent need for analytical techniques to determine them. Tijoe et al.85reported a chemical separation scheme which consisted of sample dissolution in H,SO,/H,O,, distillation of the volatile elements, dissolution of the residue in 8 M HCI, and passage over a polyantimonic acid (PAA) and Dowex 2-X8 columns. The REE in the 8 M HCI eluate were finally purified by successive hydroxide fluoride precipitation. The fluoride precipitate was counted. Detection limits for this procedure were 0.3 ng g g l for La, 0.7 ng g-' for Ce, 2 ng g - ' for Nd, 0.01 ng g-I for Sm, 0.03 ng g Lfor Eu, 0.1 ng g-I for Tb, 0.05 ng g-I for Yb, and 0.01 ng g g ' for Lu. Lepal and Laullw used the same radiochemical procedure developed for the determination of REE in geological materials (see Section 1V.B. 1 and Figure 1) for REE analysis in biological materials. In this, they determined chemical yields by reactivation. Their procedure successfully analyzed REE concentrations in the range l o - @to 10-'I g g ' which are several orders lower than typical concentrations in geological material. For this concentration range typical errors were: 5% for La, Sm, Eu, Tb, and Lu; -+ 5 to 10% for Ce, Nd, Tb, and Ho; ? 10 to 20% for Pr, Gd, and Tm. Collechi et al. Ion reported using a simple cation-exchange technique to separate, purify, and determine La, Ce, Nd, Eu, Gd, Yb, and Lu in human tissue and body fluids. Irradiated samples were dissolved in HCVHNO,, evaporated to dryness, dissolved in 0.5 M HCI and REE, and most elements adsorbed on a Dowex 50-X8 column. Successive washes with 0.5 M HCl, 0.1 M H2C,0,/0.5 M HCl, and 2 M HNO, solutions removed most interferences from the column. Finally, the REE are removed with 6 M HCI and the elutriant counted. Chemical yields were determined by re-irradiation of REE carriers and results showed yields of 90 k 4%.

86

Activation Analysis 0.1 -1 g sample

bOcm3O.SM HCI

E c m 3 0.3M HCI

-E

"

E c m 3 3M HCI

+Sb

Hf, &,W. 2,I -

%wiq Carrier As

20cm3 HBr b6V0

-

1

TO0

*.

Re, Ru

w

Co, Cr, e,Mn -

FIGURE 9. Radiochemical separation scheme of Pietra et al. for the determination of 50 elements from 4 M HCI. (From Pietra et al., J . Radioanal. Nucl. Chem. Articles, 102, 69, 1986. With permission.)

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3. Ion-Retention Media The classification of materials as ion retention media was discussed briefly in Section IV.B.3. The term describes those products which can remove elements from solution but which give rise to more than one process. A number of insoluble inorganic compounds display this property. The best known of these is HAP which can remove sodium from highly acidic solutions; however, some difficulties occur when it is used in column operation because of the fine particle size of most commercially available material. Bilewiez et al.I3O recently solved this problem by incorporating HAP in a resin matrix. The development of radiochemical separation procedures in the 1980s has been characterized by the increasing use of ion-retention media to effect separations. Many of the procedures of Pietra et a1.56used one or more columns of inorganic compounds to separate elements into groups (Table 7). HMD has been incorporated into the separation schemes of Greenberglo' for analysis of milk powder, Cunningham"' for analysis of food, and of ShelenzH9for the characterization of diets as a standard reference material. Brandone et al.87used a column of zinc ferrocyanide (ZFC) for quantitative sorption of Cd, while Yeh et a1.Iz6recommended the use of magnesium oxide for the separation of some elements. Ion retention media are not only insoluble inorganic compounds. Activated carbon has been widely used in radiochemical procedure^.^^^^^^'^^ Successive addition of chelating agents enables up to 25 elements to be sorbed onto separate fractions of activated carbon. Most procedures involved either adding small portions of activated carbon to the solution and later separation on a filter or using filtration through a 50-mg layer on a membrane filter to achieve sorption on activated carbon. Still another ion-retention medium CIS-bondedsilica gel, was introduced recently into radiochemical separation schemes by Fardy et al.90and Fardy.120.132 These procedures were based on the selective complexation of metal ions and their subsequent removal on a small column of CIS-bonded silica gel, using simple equipment and having the advantage that columns of this material are commercially available. Separations are extremely rapid, with column flow rates of 10 to 20 ml min- ' . Recent comparisons with activated-carbon fillings in columns of similar d i r n e n s i o n ~show ' ~ ~ little difference between the sorption property of both materials, but some elements become irreversibly sorbed onto activated carbon. Since columns of activated carbon are not commercially available, the use of CIS-bonded silica gel is generally preferred. D. AUTOMATED RADIOCHEMICAL SEPARATIONS Since many multielement radiochemical separation schemes involve a large number of steps, automation of such operations as sample dissolution, distillation, sorption, and elution have obvious advantages especially for analysis of a large number of samples of the same type. In their review of radiochemical separation schemes, Girardi and Pietra4 summarized the advantages of automated separations in terms of time saved, increased precision, and elimination of operator errors. Das et al.' examined the main features of automated systems used for biological materials (see Table 9). They estimated the time spent in minutes of effort during the separation for each element. This is included in the last column of the table where the lowest value is approximately 5 min. The time spent for separation was between 4 to 10 h while the turnaround time for the whole analysis was approximately between 2 and 4 weeks. Therefore, they concluded the application of such schemes to routine analysis was severely limited. Only a few specialized laboratories can perform automated radiochemical separations since construction and operation of such complex systems exceeds the skills available in most laboratories. Interest in these systems has, therefore, waned and no new developments in automation have been published in the last 7 years. However, Pietra et aLS6reported that 3 of the 22 radiochemical procedures developed in their laboratory are suitable for automation.

Reagent supply Solvent extraction Phase separation Reagent supply Ion exchange Fraction collection

Reagent supply Ion exchange

Reagent supply Extraction Phase separation Fraction collection Reagent supply Ion exchange Fraction collection Reagent supply Mineralization Ion exchange

Extraction

Automated operations

Tie

1d

Na, P, K, Sc, Cr, Mn, Fe, Co, Cu, Zn, Ga, As, Se, Be, Rb, Mo, Ag, Cd, In, Sh, Cs, lanthanides, W, Au, Hg Na, P, K, Ca, Sc, Cr, Mn, Fe, Co, Zn, Cu, Ga, As, Se, Sr, Mo, Pd, Cd, Sb, Cs, Ba, W, Au, Hg Na, Ca, Sc, Cr, Fe, Cu, Rb, Sr, Zr, In, Sb, Cs, Ba, lanthanides, Hf, Ta, Th Na, P, C1, K, Ca, Cs, Cr, Mn, Fe, Co, Cu, Zn, Ga, As, Se, Br, Rb, Sr, Mo, Ag, Cd, In, Sn, Sb, Cs, lanthanides, Hf, W, Au, Hg 1 Week

Not specified

Not specified a1d

Not specified 21d

Not specified 21d

Not specified 31d

needed

Na, P, K, Ca, Sc, Mn, Rb, Sr, C, Ba, lanthanides

Na, Mg, Al, P, K, Ca, V, Cr, Mn, Cu, Zn, As, Se, Sr, Sb, Ba, La, Hg, U Cu, As, Ag, Sb, Hg

Elements determined or separated

samples processed simultaneously

Number of

TABLE 9 Summary of Automatkd Separation Systems Time per element in minutes of effort Ref.

Na, P, K, Cr, Fe, Co, Cu, Zn, As, Se, Br, Mo, Ag, Cd, Sb, Au, Hg

Na, C1, K, Sc, Cr, Mn, Fe, Co, Cu, Zn, Ga, Ge, As, Se, Br, Rb, Mo, Ag, Cd, In, Sn, Sb, I, Cs, lanthanides, Hf, W, Re, 0 s . Ir, Au, Hg, Th Na, P, K, Ca, Cr, Fe, Co, Cu, Zn, Ga, As, Se, Br, Rb, Sr, Mo, Cd, Sb, Cs, Ba, Cs, lanthanides, Ta, W, Hg, Th, U P, C1, Sc, Cr, Mn, Fe, Co, Cu, Zn, As, Se, Br, Mo, Ag, Cd, Sn, Sb, Te, I, lanthanides, W, Au, Hg

Sc, Fe, Co, Cu, Zn, Ga, As, Se, Br, Mo, Pd, Ag, Cd, Sb, Hf, Ta, W, Au, Hg Sc, Cr, Mn, Fe, Co, Cu, Zn, As, Se, Rb, Mo, Ag, Sn, Sb, Cs, La, Hg As, Se, Sn, Sb, Te

48 samples per week for 3 to 4 analysts

4d

3 to 4 days [Ion-exchange in 5 min]

5 h for the separation only 2 to 5 h for the separation only

Not specified 2 1d

6 1 2

Adapted from Das et al., Eds., Environmental Radioanalysis, Elsevier Science, Amsterdam, 1983. With permission.

Dissolution Reagent supply Ion exchange Adjustment of pH Reagent supply Mineralization Distillation Ion exchange Fraction collection

Reagent supply Distillation Ion exchange

Reagent supply Solvent extraction Column chromatogr. Reagent supply Ion exchange Column chromatogr. Reagent supply Mineralization Distillation Ion exchange Reagent supply Mineralization Distillation Ion exchange

90

Activation Analysis

E. FUTURE PROSPECTS The development of radiochemical separation procedures for NAA peaked in the mid1970s, but since then, the number of methods published has declined markedly. This decline is attributed to the demand for faster instrumental methods in addition to a world-wide shortage of experienced radiochemists available to develop suitable procedures. Recent developments in INAA, through more extensive use of the short-lived induced radionuclides, has hastened the decline. This is well illustrated with selenium determinations, where radiochemical and instrumental NAA via the long-lived 75Se(120 d) has now been replaced by the more rapid instrumental technique using the short-lived 77mSe(17.5 s) for many applications.133.'34 Most of the recent developments in NAA towards more rapid and sensitive trace element analysis were made in response to the analytical capabilities of nonnuclear techniques of analysis, especially inductively coupled plasma atomic emission spectroscopy (ICP). Although ICP has multielement capabilities for a wider range of elements, its optical emission spectra are very complex. NAA has successfully coexisted with ICP and has seen little reduction in its use in the determination of trace elements. However, the future of NAA is now threatened by the recent commercial availability of ICP-mass spectrometry (ICP-MS). ICP-MS is a technique in which the ICP provides ions for resolution by a quadropole mass spectrometer. The simplicity of the predominantly singly ionized spectra make interpretation straightfornard. The sensitivity is usually better than ICP, and more than 90% of the elements are accessible to the method, with detection limits for most elements between 0.01 and 1 pg I-' in aqueous solutions. Analysis times of 2 to 3 min are usual and ensure high throughput of dissolved samples. Can NAA, especially the radiochemical version, continue to compete with this latest technique? The answer depends on the limitations of ICP-MS: first, it cannot handle solid samples, so they must be dissolved before analysis, thus significantly extending analysis times; second, the detection levels achievable for the dissolved samples are then limited by the blank values of the reagents used; third, although this technique markedly reduces spectral interferences, some do occur largely because of the argon plasma or chloride either already in the matrix or added to it during the dissolution process. The presence of these substances results in higher limits of detection for As, Sb, and Se. Radiochemical NAA developments and applications have diminished as more and more fast instrumental techniques, including instrumental NAA, have arisen. Nevertheless, because it has no blank problems after irradiation of the sample (in contrast to nonnuclear analytical techniques including ICP-MS), this technique becomes the obvious choice where high sensitivity and low limit of detection are required for trace elements in solid samples. Future developments will rely mainly on rapid, simple separation techniques whose detection limits are economically competitive. More sophisticated and complex procedures must be restricted to separation of those elements whose presence in materials cannot be detected by conventional techniques. Possible future developments in ICP-MS, such as the use of helium plasma or more significantly the capability for direct analysis of solids, could see the demise of most alternative instrumental methods for trace element analysis including NAA.

ACKNOWLEDGMENTS The author is grateful for the assistance of Tan Mingguang, IAEA Research Fellow from the Shanghai Institute of Nuclear Research, and Yvonne Farrar, Senior Technical Officer with CSIRO, for their assistance in compiling relevant publications for this review.

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REFERENCES 1. Das, H. A., Faanhof, A. and van der Sloot, H. A., F'reconcentration and Decontamination in Radioanalysis, in Environmental Radioanalysis, Das, H . A., Faanhof, A., and van der Sloot, H. A,, Eds., Elsevier, Amsterdam, 1983, 83. 2. Kruger, P., Principles of Activation Analysis. Wiley-Interscience, New York, 1971, chap. 7. 3. de Soete, D., Gijbels, R., and Hoste, J., Neutron Activation Analysis, 1972, Wiley-Interscience, 1972, chap. 8 4. Girardi, F. and Pietra, R., Multielement and automated radiochemical separation procedures for activation analysis, At. Energy Rev., 14, 521, 1976. 5. Laul, J. C., Neutron activation analysis of geological materials, At. Energy Rev., 17, 603, 1979. 6. Bowen, H. J. M., Application of activation techniques to biological analysis, CRC Crit. Rev. Anal. Chem., 10, 127, 1980. 7. Iyengar, G. V. and Sansoni, B., Sample preparation of biological material, in Elemental Analysis of Biological Materials, IAEA-197, International Atomic Energy Agency, Vienna, 1980, 73. 8. Iyengar, G. V., Preservation and preparation of biological materials for trace element analysis, quality assurance considerations, in Quality Assurance in Biomedical Neutron Activation Analysis, IAEA-TECDOC-323, International Atomic Energy Agency, Vienna, 1984, 83. 9. International Atomic Energy Agency Advisory Group, Quality assurance in biomedical neutron activation analyses, Anal. Chim. Acta, 165, 1, 1984. 10. Bock, R., A Handbook of Decomposition Methods in Analytical Chemistry, (trans. from German and rev. by I. L. Marr), International Textbook, Glasgow, 1979. 1 1 . Iyengar, G. V. and Kasperek, K., Application of brittle fracture technique (BFT) to homogenize biological samples and some observations regarding the distribution behaviour of the trace elements at different concentration levels in a biological matrix, J . Radioanal. Chem., 39, 301, 1977. 12. Nicholas, J. A. and Hageman, L. R., Noncontaminating, representative sampling by shattering of cold, brittle, biological tissues, Anal. Chem., 51, 1591, 1979. 13. de Boer, J. C. M. and Maesson, F. J. M. J., Optimum experimental conditions of the brittle fracture technique for homogenization of biological materials, Anal. Chim. Acra, 117, 371, 1980. 14. Zeisler, R., Harrison, S. H., and Wise, S. A., Analyses of Human Liver Samples in the U.S. Pilot National Environmental Specimen Bank Program, presented at International Workshop on Environmental Specimen Banking and Monitoring, Saarbrucken, Austria, 1982. 15. Heydorn, K., Neutron Activation Analysis for Clinical Trace Element Research, Vol. 1, CRC Press, Boca Raton, FL, 1984, 113. 16. de Goeij, J. J. M., Error sources involved in radiochemical processing in Quality Assurance in Biomedical Neutron Activation Analysis, IAEA-TECDOC-323, International Atomic Energy Agency, Vienna, 1984, 153. 17. Tijoe, P. S., de Goeij, J. J. M., and Houtman, J. P. W., Extended automated separation techniques in destructive neutron activation analysis; application to various biological materials including human tissues and blood, J. Radioanal. Chem., 37, 51 1, 1977. 18. Verlinden, M., On the acid decomposition of human blood and plasma for the determination of selenium, Talanta, 29, 875, 1982. 19. Robberecht, H. J., Van Griekan, R. E., Van Den Bosch, P. A., Diestra, H., and Vanden Berghe, D., Losses of metabolically incorporated selenium in common digestion procedures for biological materials, Talanta, 29, 1025, 1982. 20. Reamer, D. C. and Veillon, C., Preparation of biological materials for the determination of selenium by hydride generation - atomic absorption spectrometry, Anal. Chem., 53, 1192, 1981. 21. Haas, H. F. and Krivan, V., Open wet ashing of some types of biological materials for the determination of mercury and other toxic elements, Talanta, 31, 307, 1984. 22. Van Der Veen, N. G., Keubens, H. J., and Vos, G., Comparison of ten digestion procedures for the determination of arsenic in soils by hydride-generation atomic absorption spectrophotometry, Anal. Chim. Acta, 171, 285, 1985. 23. Adeloju, S. B., Bond, A. M., and Briggs, M. H., Critical evaluation of some wet digestion methods for the stripping voltammetric determination of selenium in biological materials, Anal. Chem., 56, 2397, 1984. 24. Krynitsky, A. J., Preparation of biological tissue for determination of arsenic and selenium by graphite furnace atomic absorption spectrometry, Anal. Chem., 59, 1884, 1987. 25. Hoede, D. and van der Sloot, H. A., Application of hydride generation for the determination of antimony and arsenic in biological materials by neutron activation analysis, Anal. Chim. Acta, 111, 321, 1979. 26. Siu, K. W. M. and Berman, S. S., Comparison of two digestion methods used in the determination of selenium on marine biological tissues by gas chromatography with electron-capture detection, Talanta, 31, 1010, 1984.

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27. Nadkami, R. A., Application of microwave oven sample dissolution in analysis, Anal. Chem., 56, 2233, 1984. 28. Lamothe, P. J., Fries, T. L., and Consul, J. J., Evaluation of a microwave oven system for the dissolution of geological samples, Anal. Chem., 58, 1881, 1986. 29. Kingston, H. M. and Ja.s.de, L. B., Microwave energy for acid decomposition at elevated temperatures and pressures using biological and botanical samples, Anal. Chem., 58, 2534, 1986. 30. Desad, H. B., Kayasth, R., Parthasarathy, R., and Sankar Das, M., Loss of elements in the oxygen flask decomposition of biological materials. A study by neutron activation analysis, J . Radioanal. Nucl. Chem. Articles, 84, 123, 1984. 31. Narasaki, H., Determination of arsenic and selenium in fat materials and petroleum products by oxygen bomb combustion and automated atomic absorption spectrophotometly with hydride generation, Anal. Chem., 57, 2481, 1985. 32. Uhrberg, R., Acid digestion bomb for biological samples, Anal. Chem., 54, 1906, 1982. 33. Okamoto, K. and Fuwa, K., Low-contamination digestion bomb method using a Teflon" double vessel for biological materials, Anal. Chem., 56, 1758, 1984. 34. Faanhof, A. and Das, M. A., The wet destruction of dry organic material in a closed quartz tube, Radiochem. Radioanal. Lett., 30, 405, 1977. 35. Mizuike, A., Separations and Preconcentrations in Trace Analysis. Physical Methods, Morrison, G . H . , Ed., Interscience, New York, 1965, 103. 36. Laul, J. C., Lepal, E. A., Weimer, W. C., and Wogman, N. A., Precise trace rare earth analysis by radiochemical neutron activation, J. Radioanal. Chem.. 69, 181, 1982. 37. Zilliacus, R., Kaistila, M., and Rosenberg, R. J., Radiochemical neutron activation analysis of small lanthanoid concentrations, J . Radioanal. Chem., 71, 323, 1982. 38. Meloni, S., Oddone, M., Cecchi, A., and Poli, G., Destructive neutron activation analysis of rare earths in geological samples: a comparison between two methods, J . Radioanal. Chem., 71, 429, 1982. 39. Bishop, J. G. and Hughes, T. C., Radiochemical neutron activation analysis determination of rare earth elements in geological materials with ppb sensitivity, J . Radioanal. Nucl. Chem. Articles, 84, 213, 1984. 40. Joron, J. L. and Ottonello, G., Radiochemical neutron activation analysis of rare earth elements in peridotitic rocks, J . Radioanal. Nucl. Chem. Articles, 88, 259, 1985. 41. Wandless, G. A. and Morgan, J. W., Analysis of low levels of rare earths by radiochemical neutron activation analysis, J . Radioanal. Nucl. Chem. Articles, 92, 273, 1985. 42. Parthasarathy, P., Desai, H. B., and Kayasth, S. R., Radiocheinical neutron activation analysis of individual rare earth elements in monazites from different geological environments, J. Radioanal. Nucl. Chem. Lett., 105, 277, 1986. 43. Stosch, H. G., Herpers, U., and Kotz, J., Determination of the rare earth elements in rocks from the earth's upper mantle and deep crust by neutron activation analysis, J . Radioanal. Nucl. Chem. Articles. 112, 545, 1987. 44. Koeberi, C., Kluger, F., and Kiesl, W., Rare earth element determinations at ultratrace abundance levels in biological materials, J . Radioanal. Nucl. Chem. Articles. 112, 481, 1987. 45. Hoffman, E. L., Naldrett, A. J., Van Loon, J. C., Hancock, R. G. V., and Manson, A., The determination of all the platinum group elements and gold in rocks and ores by neutron activation analysis after preconcentration by nickel sulphide fire-assay technique on large samples, Anal. Chim. Acta, 102, 157, 1978. 46. Robert, R. V. D., van Wyk, E., and Palmer, R., Concentration of the noble metals by a fire-assay technique using nickel sulphide as the collector, N.I.M. Report 1371, National Institute for Metallurgy, Johannesburg, 1971. 47. Shazali, I., Van't Dack, L., and Gijbels, R., Determination of precious metals in ores and rocks by thermal neutron activationly-spectrometry after preconcentration by nickel sulphide fire assay and coprecipitation with tellurium, Anal. Chim. Acta, 196, 49, 1987. 48. Nadkami, R. A. and Morrison, G. H., Determination of the noble metals in geological materials by neutron activation analysis, Anal. Chem., 46, 232, 1974. 49. Stockman, H. W., Neutron activation determination of noble metals in rocks: a rapid radiochemical separation based on tellurium coprecipitation, J. Radioanal. Chem., 78, 307, 1983. 50. Ziiacus, R., Radiochemical neutron activation analysis of gold in geochemical samples, Radiochem. Radioanal. Lett., 57, 137, 1983. 51. Cocherie, A., Volfinger, M., and Meyer, G., Determination of the noble metals in chromites and other geological materials by radiochemical neutron activation analysis, J . Radioanal. Nucl. Chem. Articles, 113, 133, 1987. 52. Millard, H. T., Neutron activation determination of iridium, gold, platinum, and silver in geological samples, J. Radioanal. Nucl. Chem. Articles, 113, 125, 1987. 53. Chai, C. F., Ma, S. L., Mao, X. Y., Liao, K. N., and Liu, W. C., On the methodology of radiochemical neutron activation analysis of noble metals, J. Radioanal. Nucl. Chem. Articles., 114, 281, 1987.

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54. Parry, S. J., Asif, M., and Sinclair, I. W., Radiochemical Fire-Assay for the Determination of the Platinum Group Elements, J. Radioanal. Nucl. Chem. Articles, 123, 593, 1988. 55. Chai, C., Ma, S., Mao, X., Liao, K., Liu, W., and Li, Y., A New and Rapid Radiochemical Separation Procedure for NAA of Os, Ir, Ru, Re, Pt, Au and Mo in Geological Materials, presented at Int. Conf. Methods and Applications of Radioanalytical Chemistry, Hawaii, April 5 to 10, 1987, Log No. 33. 56. Pietra, R., Sabionni, E., Gallorini, M. and Orvini, E., Environmental, toxicological and biomedical research on trace metals: radiochemical separations for neutron activation analysis, J. Radioanal. Nucl. Chem. Articles, 102, 69, 1986. 57. Rammansee, W. and Palme, H., Metal-silicate extraction technique for the analysis of geological and meteoritic samples, J. Radioanal. Chem., 71, 401, 1982. 58. van der Sloot, H. A., Hoede, D., Klinkers, Th. J. L., and Das, H. A., The determination of arsenic, selenium and antimony in rocks, sediments, fly ash and slag, J. Radioanal. Chem., 71, 463, 1982. 59. Gavini, M. B., Rocco, F. G., and Kim, S. M., A new radiochemical procedure for uranium assay in environmental samples, J. Radioanal. Chem.. 67, 437, 1981. 60. Newsom, H. E. and Palme, H., The determination of molybdenum in geological samples by neutron activation analysis, J. Radioanal. Nucl. Chem. Lett., 87, 273, 1984. 61. Drabaek, I., Carlsen, V., and Just, L., Routine determination of mercury, arsenic and selenium by radiochemical neutron activation analysis, J. Radioanal. Nucl. Chem. Len., 103, 249, 1986. 62. Singh, M. and Sawant, A.D., Neutron activation and radiochemical separation of selenium from environmental and food samples from and around Bombay using ethyl-a-isonitrosoacetoacetate,J. Radioanal. Nucl. Chem. Articles, 114, 83, 1987. 63. Sun, Y., Tian, W., Xiao, J., and Zhou, Y., INAA and RNAA of an Apollo-17 Lunar Mare Basalt Sample for 36 Elements, presented at Int. Conf. Modem Trends in Activation Analysis, Copenhagen, June 23 to 27, 1986, 1273. 64. Iyer, R. K., Radiochemical Separations and Preconcentration Methods for Neutron Activation Analysis, Report B.A.R.C.lI-842, Bhabha Atomic Research Centre, Bombay, 1985, 131. 65. Heydorn, K. and Damsgaard, E., Evaluation of botanical reference materials for the determination of vanadium in biological samples, J. Radioanal. Chem., 69, 131, 1982. 66. Cortes, E., Gras, N., Munoz, L., and Cassorla, V., A study of some trace elements in infant foods, J. Radioanal. Chem., 69, 401, 1982. 67. Sykes, T. R., Stephens-Newsham, L. G., Apps, M. J., and Noujaim, A. A., Assay of platinum and gold in biological material by neutron activation analysis, J. Radioanal. Chem., 69, 441, 1982. 68. Damsgaard, E., Ostergaard, K., and Heydorn, K., Concentrations of selenium and zinc in human kidneys, J. Radioanal. Chem., 70, 67, 1982. 69. Vanoeteren, C., Cornelis, R., Versieck, J., Hoste, J., and De Rome, J., Trace element patterns in human lung tissues, J. Radioanal. Chem., 70, 219, 1982. 70. Woittiez, J. R. W., van Kemp, G. J., and Das, H. A., Neutron activation analysis of human serum proteins, J. Radioanal. Chem., 70, 239, 1982. 71. Valente, I. M., Minski, M. J., and Peterson, P. J., Neutron activation analysis of noble metals in vegetation, J. Radioanal. Chem., 71, 115, 1982. 72. Comparetto, G. M., Jester, W. A., and Skinner, W. F., Arsenic activation analysis of freshwater fish through the precipitation of elemental arsenic, J. Radioanal. Chem., 72, 479, 1982. 73. Shah, P. K. and Haldar, B. C., Substoichiometric determination of zinc by neutron activation analysis, J . Radioanal. Chem., 72, 519, 1982. 74. Grimanis, A. P. and Kanias, G. D., Rapid determination of mercury in biological materials by radiochemical neutron activation analysis, J. Radioanal. Chem., 72, 587, 1982. 75. Hatzistelios, I. and Papadopoulou, C., Radiochemical determination of molybdenum in biological tissues by ion exchange, J. Radioanal. Chem., 72, 597, 1982. 76. Khan, S. Z., Turel, Z. R., and Haldar, B. C., Estimation of copper by substoichiometric neutron activation analysis, Radiochem. Radioanal. Lett., 48, 109, 1981. 77. Kucera, J. and de Goeij, J. J. M., A comparison of two separation techniques using Nal(T1) and Ge(Li) spectrometry for trace element determination in biological materials by neutron activation analysis, J. Radioanal. Chem., 63, 23, 1981. 78. Dang, H. S., Jaiswar, D. D., DaCosta, H., and Somasundaram, S., Determination of trace elements in human milk and commercial milk formulae using neutron activation and radiochemical separation, J. Radioanal. Chem., 70, 163, 1982. 79. Ting, B. T. G., Pagounes, J., Janghorbani, M., and Young, V. R., Radiochemical neutron activation analysis of stable isotopes in relation to human mineral nutrition, J . Radioanal. Chem., 70, 133, 1982. 80. Byrne, A. R., Simultaneous radiochemical neutron activation analysis of vanadium, molybdenum and arsenic in biological samples, Radiochem. Radioanal. Lett., 52, 99, 1982.

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81. Polkowska-Motrenko, H., Dermelj, M., Byrne, A. R., Fajgelj, A., Stengar, P., and Kosta, L., Radiochemical neutron activation analysis of selenium using carbamate extraction, Radiochem. Radioanal. Len., 53, 319, 1982. 82. Kucera, J. and Drobnik, J., Determination of platinum in urine and serum after the administration of cisplatin by neutron activation analysis, J . Radioanal. Chem., 75, 71, 1982. 83. Zeisler, R. and Greenberg, R. R., Ultratrace determination of platinum in biological materials via neutron activation and radiochemical separation, J . Radioanal. Chem., 75, 27, 1982. 84. Biso, J. N., Cohen, I. M., and Resnisky, S. M., A method for radiochemical neutron activation analysis of mercury in biological materials, Radiochem. Radioanal. Lett., 58, 175, 1983. 85. Tioe, P. S., Volkers, K. J., Kroon, J. J., and de Goeij, J. J. M., Distribution patterns of rareearth elements in biological materials evaluated by radiochemical neutron activation analysis, J. Radioanal. Chem.. 80, 129, 1983. 86. Allegrini, M., Delfanti, R., DiCasa, M., and Orvini, E., Determination of iodine in biological matrices using a fast radiochemical separation, Radiochem. Radioanal. Lett., 59, 163, 1983. 87. Brandone, A., Borroni, P. A., and Genova, M., Determination of arsenic, cadmium and mercury in biological samples by neutron activation analysis, Radiochem. Radioanal. Len., 57, 83, 1983. 88. Siripone, C., Wals, G. D., and Das, H. A., Neutron activation analysis of dry biological materials using mineralization with a saturated Mg(NO,), solution and scavenging by activated carbon, J . Radioanal. Chem., 79, 35, 1983. 89. Czauderna, M., Simultaneous determination of some trace elements in biological materials by neutron activation analysis, In?. J. Appl. Radiat. Isot., 35, 681, 1984. 90. Fardy, J. J., McOrist, G. D., and Florence, T. M., Rapid radiochemical separation in neutron activation analysis Part 1. The use of C,,-bonded silica gel and selective complexation for determination of manganese, copper and zinc in biological materials, Anal. Chim. Acta, 159, 199, 1984. 91. Dang, H. S., Desai, H. B., Kayasth, S. R., Jaiswal, D. D., Wadhwani, C. N., and Somasundaram, S., Daily requirements of Fe, Co and Se during infancy, J . Radioanal. Nucl. Chem. Articles, 84, 177, 1984. 92. Turel, Z. R. and Haldar, B. C., Radiochemical thermal neutron activation analysis of fish solubles, J. Radioanal. Nucl. Chem. Articles, 84, 109, 1984. 93. Gharib, A., Rahiii, H., Pyrovan, H., Raoffi, N. J. and Takerpoor, H., Study of trace elements in milk by nuclear analytical techniques, J . Radioanal. Nucl. Chem. Articles, 89, 31, 1985. 94. Tian, W. and Ebmann, W. D., Radiochemical neutron activation analysis for arsenic, cadmium, copper and molybdenum in biological matrices, J. Radioanal. Nucl. Chem., 89, 109, 1985. 95. Czauderna, M., Multielement neutron activation analysis of biological materials using chemical group separation and Ge(Li) gamma spectrometry, J . Radioanal. Nucl. Chem. Articles, 89, 13, 1985. 96. Ohmori, S. and Hashimoto, K., Neutron activation analysis of trace metals in the hair and organs of small animals treated chronically with Hg and Mn, J . Radioanal. Nucl. Chem. Articles, 89, 277, 1985. 97. Katoh, M. and Kudo, K., Study on the comparator method using substoichiometry I. Principle, J . Radioanal. Nucl. Chem. Lett., 95, 55, 1985. 98. Dybczynski, R., Maleszewska, H. and Wasek, M., An accurate method for the determination of copper in biological materials by neutron activation analysis and extraction chromatography, J. Radioanal. Nucl. Chem. Lett., 96, 187, 1985. 99. Subramanian, S. and Turel, Z. R., Substoichiometric determination of Hg by radiochemical neutron activation analysis, J . Radioanal. Nucl. Chem. Len., 105, 317, 1986. 100. Katoh, M. and Shigematsu, T., Simultaneous multielement determination of rare earth elements by substoichiometric activation analysis, J . Radioanal. Nucl. Chem. Lett., 106, 37, 1986. 101. Rajadhyaksha, M. and Turel, Z. R., Determination of copper in biological samples by substoichiometric neutron activation analysis, J. Radiochem. Nucl. Chem. Len., 106, 99, 1986. 102. Itawi, R. K. and Turel, Z. R., Determination of cadmium by substoichiometric thermal neutron activation analysis, J. Radioanal. Nucl. Chem. Lett., 106, 71, 1986. 103. Itawi, R. K. and Turel, Z. R., Determination of selenium by substoichiometric thermal neutron activation analysis, J . Radioanal. Nucl. Chem. Len., 106, 81, 1986. 104. Dermelj, M., Byrne, A. R., Franko, M., Smodis, B. and Stegnar, P., The use of 4-nitro-o-phenylene diamine (4-NDP) and sodium diethyldithiocarbamate (Na-DDTC) in the radiochemical separation of Cd, Co, Cu, Se and Zn from different biological samples, J . Radioanal. Nucl. Chem. Lett., 106, 91, 1986. 105. Bowen, H. J. M. and Sujari, A. N. A., Testing the accuracy of neutron activation analysis of silver in biological materials, J . Radioanal. Nucl. Chem. Lett.. 106, 193, 1986. 106. Kucera, J., Soukal, L., and Faltejsek, J., Low level determination of manganese in biological reference materials by neutron activation analysis, J. Radioanal. Nucl. Chem. Lett., 107, 361, 1986. 107. Greenberg, R. R., Elemental characterization of the National Bureau of Standards Milk Powder Standard Reference Material by instrumental and radiochemical neutron activation analysis, Anal. Chem., 58, 251 1, 1986. - -

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108. Collechi, P., Esposito, M., Meloni, S., and Oddone, M., Rare earth element abundance in tissues and plasma of healthy subjects and patients by neutron activation analysis, J. Radioanal. Nucl. Chem. Articles, 112, 473, 1987. 109. Lepal, E. A. and Laul, J. C., Trace rare earth element analysis of IAEA Hair (HH-l), Animal Bone (H5) and other biological standards by radiochemical neutron activation, J. Radioanal. Nucl. Chem. Articles, 113, 275, 1987. 110. Simonoff, M., Llabador, Y., Hamon, C., Berdeu, B., Simonoff, G., Conri, C., Fluery, B., Couzigou, P., and Lucena, A., Vanadium in depression and cirrhosis, J . Radioanal. Nucl. Chem. Articles, 113, 107, 1987. 111. Cunningham, W. C., Radiochemical determination of As, Cr, Mo, Sb, Se in foods, J . Radioanal. Nucl. Chem. Articles, 113, 423, 1987. 112. Esposito, M., Collechi, P., Oddone, M., and Meloni, S., Platinum assay by neutron activation analysis and atomic absorption spectroscopy in cisplatin treated pregnant mice, J . Radioanal. Nucl. Chem. Articles, 113, 437, 1987. 113. Grimanis, A. P. and Pertesws-Keis, M., Simultaneous determination of mercury and selenium in biological materials by radiochemical neutron activation analysis, J. Radioanal. Nucl. Chem. Articles, 113, 445, 1987. 114. Haeberlin, S. T., Lux, F., Karl, J., Spruss, T., and Schonenberger, H., Determination of platinum and biologically important trace elements in structure-activity relationship studies on platinum-containing anti-cancer drugs. Special procedures for removing 32Pas well as for the estimation of %Mo and '%Au, J. Radioanal. Nucl. Chem. Articles, 113, 461, 1987. 115. Irigaray, J. L., Elmir, H., Pepin, D., and Communal, P. Y., Study of arsenic reabsorption in the body by neutron activation analysis after thermal spring treatment, J . Radioanal. Nucl. Chem. Articles, 113, 469, 1987. 116. Lavi, N., Lux, F., and Alfassi, Z. B., Determination of Mg, Al, P, Cu and Mn in Biological Fluids by Neutron Activation Analysis, presented at Int. Conf. Modem Trends in Activation Analysis, Copenhagen, June 22 to 27, 1986, 901. 117. Whitley, J. E., Stack, T., Miller, C., Aggett, P. J., and Lloyd, D. J., Determination of 58Feand TU enriched stable isotopic tracers in studies of mineral metabolism of babies, J . Radioanal. Nucl. Chem. Articles, 113, 527, 1987. 118. Kishi, T., Forensic neutron activation analysis - the Japanese scene, J . Radioanal. Nucl. Chem. Articles, 114, 275, 1987. 119. Shelenz, R., Determination of Trace Elements in Dietary Reference Materials by Nuclear and Other techniques at lAEA Seibersdorf Laboratory, presented at Int. Conf. on Methods and Applications of Radioanalytical Chemistry, Hawaii, April 5 to 10, 1987, Log No. 94. 120. Fardy, J. J. and Tan, M., Rapid Radiochemical Separation in NAA Using Ion Retention Media, J . Radioanal. Nucl. Chem. Articles. 123, 573, 1988. 121. Greenberg, R. R. and Zeiler, R., A Radiochemical Procedure for Ultra Trace Determination of Chromium in Biological Materials, J. Radioanal. Nucl. Chem. Articles, 124, 5 , 1988. 122. Iyengar, V., Radiochemical Separations for Trace Elements in Total Diets, presented at Int. Conf. Methods and Applications of Radioanalytical Chemistry, Hawaii, April 5 to 10, 1987, Log No. 121. 123. Zeisler, R., Greenberg, R. R., and Stone, S. F., Radiochemical and Instrumental Neutron Activation Procedures for the Determination of Low Level Trace Elements in Human Livers, J . Radioanal. Nucl. Chem. Articles, 124, 47, 1988. 124. Chatt, A., Dang, H. S., Fong, B. B., Jayawickreme, C. K., McDowell, L. S. and Pegg, D. L., Determination of Trace Elements in Food by Neutron Activation Analysis, J. Radioanal. Nucl. Chem. Articles, 124, 65, 1988. 125. Navarrete, M. T., Activation Analysis of Se Through a Radiochemical Separation, presented at Int. Conf. Methods and Applications of Radioanalytical Chemistry, Hawaii, April 5 to 10, 1987, Log No. 20. 126. Yeh, S. J., Lo, J. M. and Tseng, C. L., Radiochemical Separation by Magnesium Oxide Adsorption and Its Application, J . Radioanal. Nucl. Chem. Articles, 124, 157, 1988. 127. Zhuang, G. S., Wang, Y. S., Zh, M., Yin, J. H. and Cheng, Y. D., Determination of As, Cd, Hg, Cu and Zn in Biological Samples by Neutron Activation Analysis, presented at Int. Conf. Methods and Applications of Radioanalytical Chemistry, Hawaii, April 5 to 10, 1987, Log No. 217. 128. Guinn, V. P. and Hoste, J., Neutron Activation Analysis, in Elemental Analysis of Biological Materials, IAEA Tech. Rep. Ser. No. 197, International Atomic Energy Agency, Vienna, 1963, 123. 129. Ruzicka, J. and Stary, J., A new principle of activation analysis separations. I. Theory of substoichiometric determinations, Talanta, 10, 287, 1963. 130. Bilewiez, A., Bartos, B., Narhutt, J., and Polkowska-Motrenko, H., Composite ion exchange for the removal of sodium-24 in neutron activation analysis of biological materials, Anal. Chem., 59, 1737, 1987.

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131. van der Sloot, H. A., Wals, G. D., Weers, C. A., and Das, H. A., Simultaneous elimination of sodium24, potassium-42, bromine-B2 and phosphorus-32 in the determination of trace elements in biological materials by neutron activation analysis, Anal. Chem., 52, 112, 1980. 132. Fardy, J. J., Rapid radiochemical separations in neutron activation analysis, in Chemical Aspects of Nuclear Methods of Analysis, IAEA-TECDOC-350, International Atomic Energy Agency, Vienna, 1985, 7. 133. Damyanova, A. A., Akanie, 0. A., and Spyrou, N. M., Changes in the elemental content of blood components by selenium supplements in humans, J. Radioanal. Nucl. Chem. Articles. 113, 431, 1987. 134. McDowell, L. S., Griffin, P. R. and Chatt, A., Determination of selenium in individual food items using the short-lived nuclide 77mSe,J. Radioanal. Nucl. Chem. Articles, 110, 519, 1987. 135. Damsgaard, E., Heydorn, K., Larsen, N. A., and Nielsen, B., Simultaneous determination of arsenic, manganese and selenium in human serum by neutron activation analysis, RISO-271, Risoe Research Establishment, Denmark, 1973. 136. Elek, A., Use and automation of multielemental substoichiometric extraction of metal chelates in activation analysis, J . Radioanal. Chem., 16, 165, 1973. 137. Fourcy, A., Neuberger, M., Garrec, C., Fer, A., and Garrec J. P., Activation analysis in plant biology. Radioecological applications using simple or automated radiochemical separation techniques, in Modem Trends in Activation Analysis, Vol. I, Special Publication 3 12, De Voe, J. R. and La Fleur, P. D., Eds., National Bureau of Standards, Washington, D. C., 1969. 138. Gaudry, A,, Mazierc, B., and Comar, D., Multi-element analysis of biological samples after intense neutron irradiation and fast chemical separation, J. Radioanal. Chem., 29, 77, 1976. 139. Guzzi, G., Pietra, R., and Sabbioni, E., Determination of 25 elements in biological standard reference materials by neutron activation analysis, EUR-5282e, Commission of the European Communities, 1974. 140. Goode, G. C., Baker, C. W., and Brooke, N. M., An automated solvent extraction technique for radiochemical group separations: application to the analysis of glass fragments by thermal-neutron activation, Analyst, 94, 728, 1969. 141. Iyengar, G. V., Procedural and development aspects of multi-element automatic radiochemical machine, applied to neutron irradiated biomedical samples, JUL-1308, Kernforschungsanlange, Julich, 1976. 142. Kucera, J., Solvent extraction group separation scheme for NAA of trace elements in biological materials, Radiochem. Radioanal. Lett., 24, 215, 1976. 143. Lievens, P., Cornelis, R., and Hoste, J., A separation scheme for the determination of trace elements in biological materials by neutron activation analysis, Anal. Chim. Acta, 80, 97, 1975. 144. Nagy, L. G., Totok, G., and Kollar, J., Elaboration of a programmed, semi-automated radiochemical processing system for neutron activation analysis of some volatile elements, J. Radioanal. Chem., 34, 261, 1976. 145. Samsahl, K., High-speed, automatic radiochemical separations for activation analysis in the biological and medical research laboratory, Sci. Total Environ., 1, 65, 1972. 146. Schelenz, R. and Fischer, E., Development of methods for the application of neutron activation analysis to the determination of trace elements in food, BFE-197614, Bundesforschungsanstact fuer Ernae Rrung, Karlsruhe, W. Germany, 1976. 147. Schumacher, J., Maier-Borst, W., and Hauser, H., A half automated, non time consuming radiochemical separation scheme for determination of 25 trace elements in biological specimens, in Modern Trends in Activation Analysis, Henkelmann, R., Kim, J. I., Lux, F., Stark, H., and Zeisler, R., Eds., Bayern University, Munich, 1976. 148. Tioe, P. S., de Goeij, J. J. M., and Houtman, J. P. W., Extended automated separation techniques in destructive neutron activation analysis; application to various biological materials, including human tissues and blood, IRI-133-76-1I, Interuniversitair Reactor Instituut, Delft, 1976.

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Chapter 6

USE OF DELAYED NEUTRONS IN ACTIVATION ANALYSIS Zeev B. Alfassi

TABLE OF CONTENTS I.

Introduction.. .................................................................... 98

11.

Determination of Fissile Elements ............................................... 98

111.

Cyclic Activation with a Neutron Isotopic Source ..............................103

IV.

Determination of Nonfissionable Elements. .....................................107

V.

The Experimental System.. ..................................................... l o 7

References.. ............................................................................ 108

98

Activation Analysis

I. INTRODUCTION Most of the radionuclides formed by activation decay by beta and gamma emission. However, in a few cases, neutron emitters are formed. Usually in nuclear reaction, the neutrons are emitted promptly, but in these few cases, the neutron-emitting nuclides are formed not by the direct activation but rather from beta decay of a directly formed radionuclide and hence the neutron emission is delayed by the beta emission; consequently the neutrons are called delayed neutrons. Due to the scarcity of neutron emission and the possibility of neutron detection without interferences from other emissions, this method is a very selective one and consequently may be also a sensitive one but can be used only in few cases, i.e., only for few elements. Up to now the activation analysis followed by delayed-neutron detection was used mainly and almost only to determine fissionable material, i.e., U, Th, and Pu and mainly by neutron activation. However, in principle also other elements can be determined in this method, e.g., neutron activation can be used to determine 9Be and I7O since their reactions with fast neutrons lead to the delayed neutron precursors 9Li and I7N by the reactions 9Be(n,p)9Liand '70(n,p)17N.For 9Li, the half-life is quite short-178.3 ms, but still it can be determined with fast pneumatic transfer systems. Lithium can also be detected by using the double reaction 6Li(n,a)T, "0(t,a)17N and detecting the delayed neutrons due to I7N. These two reactions can be used also for the determination of ''0 adding a known amount of 6Li.

11. DETERMINATION OF FISSILE ELEMENTS In the process of fission of heavy elements, induced by either thermal neutrons, fast neutrons, high energy gamma rays, or charged particles, several delayed neutrons precursors are formed.' Echo and Turk2 demonstrated the feasibility of measuring 235Uby neutron activation followed by delayed-neutron counting (NADNC). Amie13 and Dyer et al.4 laid the foundation for the extensive use of NADNC for the determinationof uranium and thorium, investigating possible interferences and suggesting the optimal experimental conditions to be 60-s irradiation, 20-s decay, and 60-s count. The optimal time for the delay was determined by the interferences and the optimal time for irradiation and counting by the half-lives of the delayed-neutron precursors (DNP). The decay of neutron emission is usually divided for convenience into six groups with different half-lives, although there is more than one radionuclide in most of the groups. The experimental half-lives of the six groups are slightly different for the fission of the various fissile isotopes, due to the variation in the yields of the various DNPs belonging to the same group.5-7 Table 1 lists the main DNPs divided into six groups in accordance with their half-life. This division is somewhat arbitrary concerning the limits of the half-lives. However, the half-life of the group as a whole is determined e~perimentally.~ There are small differences between the groups' measured and calculated half-lives.7 Table 2 summarizes the delayedneutron groups' half-lives and yields for several cases of fission (slow or fast neutrons, 235U, 239Pu,or 232Th).The yield of the delayed neutrons is expressed in fractions, i.e., the number of neutrons emitted per lo4 fissions. As can be seen from Table 2, the main yield is due to the second group of 2.2 + 0.1 s. However, the neutrons from this group are not usually used for analytical determination due to two reasons: (1) in that short time, there are interferences from other neutron emitters, (2) a small error in the transfer time of the sample from the irradiation station to the counting station of about 0.1 to 0.2 s will lead to a large error in the fissile nuclides concentration. The main interferences due to nonfissile elements are due to "N (4.175 S) produced by the l70(n,p)I7Nand the I80(n,d)l7Nreactions. These reactions occur mainly with fast neu-

Half-life (s) 55.6

Group 1 isotope

"Br

"Br 134Sb 136Te 1371

10-30 s Group 2 isotopes 16.0 10.4 17.5 24.5

Half-Lie (s)

147La

1381

"Br 93Rb

4-10 s Group 3 isotopes

5.60 4.38 5.85 6.53 4.4

Half-life (s)

I43Cs

1391

135Sb 137Te lz8Te

BY 1271n

9Smy

%a 85As 9 e wBr "Rb

Group 4 isotopes

1 . L . O s

1.66 2.03 1.52 1.92 2.76 2.0 1.4 3.76 1.71 2.8 1.4 2.29 1.78

Half-life (s)

142Xe lMCs 145Cs I4'Ba 148Ba

81Ga "Ga %As 87As 89Se 91Br 93Kr 97Sr 98Sr '%r 1291n lZoln 134Sn I3%b L401 1411

0.61.4s Group 5 isotopes

TABLE 1 List of Delayed-Neutron Precursors in Fission Half-life (s)

c0.a s Group 6 isotopes

Half-life (s)

100

Activation Analysis

TABLE 2 Delayed-Neutron Properties for Fission by Group Group

Case Thermal neutrons 235U Thermal neutrons 239h

Fast neutrons 232n

Fast neutrons ZSSU Fast neutrons 239pU

Parameter

1

Group 2

Group 3

Group

Group

4

5

Group 6

Half-life (s) Fraction ( X 104) Half-life (s) Fraction ( X 104) Half-life (s) Fraction ( X 104) Half-life (s) Fraction ( X 104) Half-life (s) Fraction ( X 104)

trons (threshold neutron energies 8.2 and 10.5 MeV), and for well-moderated neutrons, their interference is quite lo^.^,^ In matrices containing 6Li, 17Ncan be also produced by double nuclear reaction with ''0 and 15N;6Li ( n p ) t; ''0 (t,a)17N and "N(t,p)17N. Amie13 studied the interferences due to 17Nin the presence and without 6Li, however, the effect of lithium as self shielding is much higher than its production of extra 17Nunless the concentration of the uranium is very low. The interference from 'Be(n,~)~Li is negligible in all systems used until now since the irradiation and decay times are considerably longer than 9Li half-life. Another interference can be caused by nuclides emitting high-energy gamma rays. The main problem is due to I6N(7.11 s) formed by 160(n,p)16Nwith its high-energy gamma rays of 6.13 and 7.11 MeV. These gamma rays can lead to neutron emission by the (y,n) reaction, mainly from the deuterium and beryllium in sample or in the counting system. Another problem, usually a more severe one when dealing with I7N, is that the high-energy gamma rays produce pulses with heights above the discrimination level set to eliminate the usually small pulses induced by gamma while passing the larger neutron-induced pulses. MeasureThe interment with 24Na(Ey = 2.75 MeV) did not show any rise in the ference due to "N and 16Nis none for thermal neutrons in the absence of 6Li and very small if nonthermal neutrons are also present. To eliminate this small interference, a delay time of 20 s is usually used. This reduces the measured activity of the delayed neutrons, but, when nuclear reactors are used for activation, the flux of activating neutrons is high enough for the determination of uranium in most samples even when not using the most abundant DNP. For example, Binney and Scherpelzg found that at reactor power of lOkW, 100 pg of natural uranium can be determined with accuracy of 5% in samples of 0.5 to 1.0 g. For activating sources, such as Californium-252 (2s2Cf), shorter delay times are used.8 The NADNC method is used mainly to determine uranium content in geological samples. The main activating system is with reactor neutrons; Binney and Scherpelzloreviewed the systems known up until 1977 and reported on 21 systems using reactor neutrons and 3 systems which used neutrons from 252Cfsources. Other systems use neutron generators," and, in principle, both photofission and fission by accelerated charged particles can be used although in the last case, only thin samples can be used. In most cases, this method is used for geological samples or aqueous samples and the samples are quite large, about -1 g. Binney and Scherpelz9used this method for rapid analysis of uranium in porcelain dentures; the uranium is due to the addition of uranium to dental porcelain to imitate the fluorescence of natural teeth. The most common procedure for NADNC by reactor neutrons is 60-s irradiation time, 20-s delay, and 60-s counting. In some cases, the 20-s delay time is used also to remove the sample from the irradiation capsule in order to reduce the activity due to the rabbit.3.9

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101

However, in many cases, the count is done inside the irradiating c a p ~ u l e . In ~ ~the ~ ~case ' of nuclear reactors, 1.5 to 2 min are sufficient for the measurement of one sample as a sample can be irradiated while the previous sample is being counted. Automatic pneumatic transfer systems attached to a nuclear reactor can analyse daily more than 500 samples of solid ore samples or liquid solutions of extracted uranium.14In some cases, different times were used, e.g., Minor et al.13 used 20-s irradiation, 11-s decay, and 30-s count, while Schlechte15 using an ultrafast conveyor system irradiates for 11 s (leading to 50% saturation activity of delayed neutrons), allows 0.1-s delay, and counts for 30 s. In many cases, the delay time, if not used for transfer, is not used at all. Millard and KeatenI6 used this delay time to measure the activity due to I7N in order to determine the concentration of the oxygen by the L70(n,p)17N. Their procedure consists of 60-s irradiation followed by 5-s decay time and then 5-s count. After an extra 20-s decay, the samples are counted again for 60 s. If the decay time is not used for measurement, it is preferable not to let the samples decay in the counting station since the emitted neutrons consume the I0B or 3He in the detectors. Thus, it is preferable to also build into the pneumatic system a delay station." Shenberg et al." used this delay time in order to measure some short-lived gamma-emitting radionuclide formed in the neutron activation. They measured the concentration of F, Al, Ca, and V besides the uranium in phosphate rocks. U was determined by the delayed neutrons while F, Al, Ca, and V were measured by the 1634-keV, 1779-keV, 3084-keV, and 1434-keV gamma rays of 20F (1 1.0 s), 28A1(2.25m), 49Ca (8.72 m), and 52V (3.75m), respectively. In order to reduce the background due to 28A1in the measurement of 20F, the irradiation was done for only 20 s and in order to increase the activity due to 49Caand 52V, the delay (gamma-rays measurement) was increased to 60 s, so from the point of view of U determination by delayed neutrons, the process was 20-60-60 instead of the usual 60-2060 system. This reduces the number.of counts of delayed neutrons by a factor of 5.37 (instead of 532 countslppm to 99 countslppm for 500 mg samples in a neutron flux of 5 X 1012 n cm-' s-I) which still give a good accuracy for U determination in phosphate rock. The accuracy obtained for A1 and F is quite good ( 52 to 3%), while the accuracy of vanadium and calcium is worse ( 25 to 10%). In the case of vanadium, it is due to the lower concentration of vanadium, while in the case of calcium, it is due to the low abundance of 48Ca (0.19%) and the low efficiency for the high energy y ray (3084 keV). This can be overcome by a simultaneous use of Ge(Li) and NaI(T1) detectors. Since in the high-energy region there are very few peaks, the high resolution of a Ge(Li) detector is not needed and NaI(T1) detector is sufficient for 49Ca determination. The use of NaI(T1) detector together with absorbers to reduce the activity of 28A1can increase the counts due to 49Caby more than a factor of ten, which increases considerably the accuracy of Ca determination. Another way to increase the measured activity of vanadium and calcium is to measure the gamma rays not only in the delay station but also simultaneously with the counting of the neutrons. The main gamma activity of the rocks is due to 28A1and since its half-life is shorter, the signalto-noise ratio for V and Ca will be larger in this measurement. The fissioning of the uranium by thermal neutrons is due practically only to fission of the 235Uisotope since the major isotope, 238U,is not fissioned with thermal neutrons. Thus, this method is applicable for the determination of total uranium only if the percentage of 235Uis known quite accurately. This condition is fulfilled for most cases of geological or hydrological samples. However, this is not always the case for industrial or commercial samples since for many purposes depleted uranium, i.e., uranium in which part of the 235U has been removed for the purpose of using it in nuclear reactors, is used. An error of this . ~ ~ standards of uranyl type was made by Smith et a1.I8 as was pointed by P a p a d o p o u l ~ sThe nitrate used by Smith et all8 contained depleted instead of natural uranium which lead to 40%19 error in the uranium determination.

-

.

102

Activation Analysis

Many studies used the fission delayed-neutron methods to determine the concentration of fissile/fertile isotope mixture^^.^^.^^ in one element, e.g., 235Uand 238Uin uranium, or mixtures of two fissile elements with different proportions of fissilelfertile isotopes, e.g., uranium and plutonium.22The determination is done by double irradiation, once with the whole spectrum of reactor neutrons and the second irradiation with only epithermal and fast neutrons by elimination of the thermal neutrons with a cadmium absorber which absorbs all the thermal neutrons. The method is based on the different effect of the energy of the fissioning neutrons on the cross-section of the fission process and hence on the formation of the DNPs. Fissile nuclides have high cross-section for fission with thermal neutrons and the cross-section decreases with increasing energy. For fertile nuclides, the energy dependence of the cross-section is the opposite, and the cross-section is an increasing function of the energy of the neutrons. Assigning the delayed-neutron count rate for the whole reactor neutrons spectrum by N, and for the fast and epithermal irradiation by N, leads to the two simultaneous equations

where m, and m, are the masses of the fertile and fissile nuclides, respectively, and the a's are calibration factors which depend on the flux, the cross-section, the number of delayed neutrons emitted, the efficiency of the counting system, the timing schedule, etc. The solution of these two equations yields m,

=

(N,

.a:

- N,

aVD

and

m,

=

-

.

( N , aR, - N, ag)/D

This method of two irradiations is sometimes a must in geological surveys of uranium in order to discriminate between uranium and thorium which is more abundant in the Earth's crust. Amie13 found that the specific activity obtained from thorium is about 0.01 1 of that of natural uranium for reactor neutrons and 0.23 for epithermal and fast neutrons. Thus, double irradiation is required unless it is known that the concentration of Th is not more than three to five times that of U, which limits the error due to Th contribution to less than 5%. The double irradiation allows the measurements of both thorium and uranium. While measurement of U in the presence of Th is possible by one irradiation (reactor neutrons) if ThIU < 3 to 5, even small concentration of U (UITh 0.01) interfere with the determination of Th by one irradiation, and if Th concentration is required, both irradiations, with reactor neutrons and epithermal neutrons, has to be done. Amie13used this method also to measure the 235Ucontent (fissile nuclide) in samples of uranium. Hoffman and Ernst stated23that although Th may be determined by this method, it is far simpler to determine thorium for commercial purposes by instrumental neutron activation analysis followed by emitted-photon measurement. Instead of using cadmium for absorbing the thermal neutrons, boron can also be used for this purpo~e.~' Rosenberg20 suggested that instead of using cadmium cover (e.g., References 3 and 16) or boron lining,2' the sample can be mixed with B4C powder. This is done in case there is no place in the reactor for a fixed-irradiation-positionshield either with Cd or B or in order to cut the expenses of either extra construction or increased fuel consumption. This autho9O is against cadmium rabbits due to their weight and activity after irradiation and against boron cover due to the difficulty in producing boron capsules. However, our experience shows that it is not so problematic to produce boronimd capsules and the use of a cadmium cover is also not a problem. The mixing of the sample with B4C powder prevents other studies on the sample and also reduces the thermal flux less efficiently.

-

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Schlechte and Bornz2used this method of double irradiation to measure the content of uranium and plutonium in samples containing mixture of both elements using the fact that the ratio of fissile to fertile isotopes is much higher in the case of plutonium (79% 2 3 9 P ~ and 2% 241Pu,the two fissile isotopes, compared to the 0.7% 235U,the fissile isotope of uranium) and assuming that the isotopic composition of each element is constant. They used either irradiation at steady-state reactor power or during pulse cycle each without and with Cd shielding. The main drawback of this method is that it is applicable only to samples with constant isotopic mixture. Each isotopic composition requires calibration with standards of the same isotopic composition. There are several sources of error concerned with the determination of U and Th, as for example, error in sampling and in sample weight, errors in irradiation and counting, and errors in data reduction. Both Rosenberg et al.24and Millard and Keaten16 found that the primary source of error is the counting statistics. The detection limit for uranium determination varies according to the neutron flux, irradiation and count procedure, and the detection system efficiency. RosenbergZ0used 1 to 5-g samples and the detection limit for uranium was 0.07 ppm. Minor et a1. ,I3 in a geological survey of enormous size, found for 1-g samples a detection limit of 0.01 ppm.

111. CYCLIC ACTIVATION WITH A NEUTRON ISOTOPIC SOURCE With the use of nuclear reactors which have high neutron flux, very small concentrations of uranium can be determined. However, when nuclear reactors are not available and the neutron source is an isotopic source, e.g., Am-Be or 252Cf,the detection sensitivity of U is not good and low concentration of U cannot be measured. One way to lower the detection limit and measure smaller amounts in activation analysis is by cyclic a c t i ~ a t i o n .In ~ ~this -~~ method, the sequence (cycle) of irradiation, delay, and counting is repeated several times and the count is accumulated in each cycle to give rise to a larger total activity. The usual case of cyclic activation with nuclear reactors is treated in another chapter of this book, however, cyclic activation with measurement of the delayed neutrons is different from the usual cyclic activation analysis in that there is not a single DNP but rather several with different half-lives. The mathematical treatment for cyclic activation analysis of fissile materials by delayed neutron counting was developed by MacMurdo and B o ~ m a nBinney ,~ and Scherpelz,lo and Spyrou et a1.28The number of counts due to one cycle from one DNP is given by the usual activation formula

where E is the efficiency of the counter, u the number of delayed neutrons emitted per one fission, p the fraction of delayed neutron belonging to the given group, uf the cross-section for fission, N, the number of fissionable nuclei, and
=

saturated measured activity

=

~vpu,N& = PR

104

Activation Analysis

neglecting the background this gives

Since there are six groups of DNPs, the final formula for one cycle counts is

where i stands for the various groups of the DNPs and k stands for the different operation, e.g., T,,, is equal to 1 -exp(- Alt3)where A, is the decay constant for the first DNP group. When more than one cycle is performed, another time is important, the time of transfer from the counting post back to the irradiation position - t,. The period of the cycle is given by P

The counts accumulated in the first cycle are given by Equation 2 as

For the second count, this activity decays by the factor T4 = e-" where A has its value for each DNP group. In addition to this remained activity, there is the activity due to the second irradiation which equals to C, (since the burnup is negligible). Thus the counts in the second cycle, C,, are

where

Similarly for the third cycle

and for the n-th cycle

The total accumulated counts C,, are given by the equation

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105

TABLE 3 Optimal Number of Cycles for Cyclic Activation Analysis of Natural Uranium as a Function of the Transit Times for Total Analysis Time of 600 s Transit time (s) Number of cycles Irradiation time = counting time (s)

0.25 83

3.36

0.50 57 4.76

0.75 45 5.92

1.25

1.00 38

33

6.89

1.50

1.75 27 9.36

30

7.84

8.25

2.0

2.25

2.5

24

22

20

10.5

11.4

12.5

TABLE 4 Optimum Condition for Analysis of Main Fissile Nuclides for 1-s Transit Time and Total Analysis Time of 600 s Nuclide Number of cycles

233U

23511

239pu

24IpU

33

38

35

41

For a fixed transit time (t,), there are two parameters to vary in order to obtain maximum total counts for a constant analysis time. These are (1) the number of cycles and (2) the ratio between irradiation and counting time. Although for irradiation with high fluxes, t, is not set to its minimal value, in order to suppress the interferences due to I6N and 17N and to reduce the error in the exact value of t,, in many systems with a low flux of neutrons where the error due to counting statistics is quite large, t, is set equal to its possible minimal value, the transit time, and hence t, = t4. For a known number of cycles, the maximum of C, (Equation 5) will be when Z S, has its maximum. For a single activation product, differentiation of Equation 5 leads to maximum when t, = t,, i.e., when the irradiation and counting times are equal. In order to obtain the optimal number of cycles for a given analysis time (i.e., the number of cycles which will lead to maximal number of net counts), MacMurdo and Bowman8 calculated the effect of the transit time on the total number of counts of delayed neutron as a function of the number of cycles. For each transit time, there is a different optimal number of counts and these results are given in Table 3. Plotting the results, it can be seen (Figure 1) that the optimal number of cycles is approximately a logarithmic function of the transit time, with the equation

The number of cycles depends also on the total analysis time, e.g., Spyrou et a1.28 obtained for 300-s total time and 0 . 2 5 s transit time the optimal number of cycles equal to 56, i.e., irradiation and counting time of 2.43 s as compared to 3.36 found by MacMurdo and Bowman.' With those short decay times, the interferences due to the neutron emitter 17N(4.16 s) and the high-energy gamma emitter I6N(7.11 s) do not decay. MacMurdo and Bowmans studied the interference from I70(n,p) l7N in low concentration aqueous solution where the ratio OIU is very high. They measured the activity of "0-enriched water and from it

Activation Analysis O p t i m a l number of c y c l e s

FIGURE 1 . The optimal number of cycles for determination of 235U as a function of the transit time for a total analysis time of = 600 s.

calculated the contribution due to I7O and found it to be in natural water much lower than the normal background counts. H s u ~suggested ~ the use of the 871-keV gamma ray of 1 7 0 to measure simultaneously the 1 7 0 by gamma-ray spectrometry and the 235U by delayed neutrons; however, for most practical samples, this line will be obscured by the Compton of 28A1and other interfering elements. Spyrou et a1." compared one-shot irradiation and counting with cycle activation and found that for transit time of 0.25 s, the cyclic activation is preferable if the total analysis time 2 2 min. The treatment of cyclic activation analysis should treat not only the maximization of the net counts for total analysis time but also the maximization of the signal-to-noise ratio, the noise being due to background count. The background is proportional to the total counting time = n t, and since t, = t, and t, = t, = Lit

(-

1 T Total count time = n - t-*,) 2 n

.

=

T n 5 - .

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where T is the total analysis time, n the number of cycles, and Lmi,is the transit time. As Equation 9 shows, the background decreases linearly with the number of cycles, and thus the maximization of the signal-to-noise ratio will lead to larger number of cycles than considering only the net counts, yet the difference is usually not large. For equal irradiation and counting times, differentiation of the ratio of the net counts from any delayed neutron to the background counts with respect to the irradiation (or counting time) indicates that the ratio undergoes a maximum for a irradiation and counting times equal to 1.81 times the half-life for that delayed neutron group.1° For short transit time, the main contribution is due to the most abundant group, Group 4 of 2.1 2 0.2-s half-life while for the usual 20-s delay time, Group 2 (of half-life of approximately 20 s) is the dominant one.

IV. DETERMINATION OF NONFISSIONABLE ELEMENTS Very few elements other than the fissile and fertile elements were determined by the emission of delayed neutrons. Millard and KeatenI6 measured by delayed-neutron emission the concentration of oxygen in addition to those of uranium and thorium. They used two irradiations, once inside a Cd-lined irradiation position and the second one in a bare terminus, each for 1 min, and 3 countings, 2 after the first irradiation and 1 after the second one. After the first irradiation, they counted for 5 s after a delay of 5 s and for 20 s after a delay of 20 s. In the second irradiation, they counted for 60 s after 20-s delay. The results of the three counts are used to solve the three equations with the three unknown concentrations of 0 , Th, and U. The parameters of fluxes and cross-sections etc. are determined by the use of known standards which are irradiated and counted in the same procedure. Amiel and Welwart30 studied the determination of lithium by the formation of I7N via the following I8O(t,a)l7N or 15N(t,p)17N.Since this method determined only 6Li, sequence: 6Li (n,a)t the total concentration of lithium can be measured only in samples of known isotopic composition (mainly natural). Care should be taken that the neutron flux is only thermal in order to avoid 17Nformation by "0(n,p)I7N. If the neutron flux is not completely thermalized, two irradiations should be performed, once without a cover and once with a Cd cover. Gilat and Gurfinke13' calculated the self shielding of high concentration of lithium. Interference due to uranium or other fissile nuclides can be found by the measurement of the decay curve of the neutrons. Measurement for two periods of times, e.g., 2 to 10 s and 10 to 50 s after the end of irradiation, can give the concentration of both lithium and uranium. If the total amount of lithium is known by other methods, this method can be used to determine the isotopic concentration of 6Li. Since determination requires high concentration of oxygen, it is done only in aqueous solution and not for untreated geological samples. Higher sensitivities can be obtained by using "0-enriched water for dissolution. A solution containing 1 mg of 6Li irradiated to saturation at a thermal neutron flux of 10% cm-2sec-1 yields 4. lo4 neutrons.32The same way as "0 is used for determination of 6Li, 6Li can be used for the determination of

+

.

V. THE EXPERIMENTAL SYSTEM The experimental system consists of an irradiation source and a detection system. Most of the experimental systems also include a sample transfer system which is essential for an automatic system if a large number of samples have to be analyzed economically; however, some laboratories transfer the sample manually. In those cases, the samples are introduced to the reactor pneumatically, but the transfer from the irradiation position to the detection system is done manually. The detection system consists of moderators (to thermalize the neutrons and hence increase the efficiency of their detection), shielding (from gamma rays

108

Activation Analysis

from both the outside and the inside and from neutrons from the outside), and of neutron detectors. The neutron detectors are mainly of the enriched 'OB-BF, type, due to their lower cost and lower sensitivity to gamma radiation yet they still have quite good sensitivity for neutrons. In some cases, ,He detectors are used due to their higher neutron sensitivity, although they are more expensive and have larger relative sensitivity to gamma radiation. The detectors consist of an array of several (4 to 20) long tubes (10 to 70 cm) filled with either ,He or BF, embedded annularly in a cylinder (radius of 8 to 5 cm) filled with the moderator. The tubes are kept not in the periphery of the cylinder but 1 to 2 cm from the edge in order that the shielding material will not be too close to the detector and hence degrade considerably the thermal neutron flux at the detector location. The most common moderator materials are paraffin, polyethylene, and water. The sample chamber is located in the center of the cylinder and is surrounded by a gamma-ray shield to protect the counters from the gamma rays of the sample, usually 3- to 6-cm thick lead. The cylinder is shielded in its outside part from external neutrons by borated paraffin or a cadmium sheet, and from external gammas by a lead sleeve. As both ,He and BF, neutron detectors are also sensitive (although the sensitivity is low) to gammas and since the lead shield does not stop all the gammas, a discriminator (or a single-channel analyzer) must be used to reject all the small pulses induced by the gammas. Usually the setting of the discriminator is done partly into the neutron peak, measured usually with a Am-Be source. The efficiency of the counting system for neutrons is 10 to 30%.

REFERENCES 1. Friedlander, G., Kennedy, J. W., Macias, E. S., and Miller, J. M., Nuclear and Radiochemistry, 3rd ed., John Wiley & Sons, New York, 1981, 166. 2. Erho, M. W. and Turk, E. H., Quantitative determination of U-235 by delayed neutron counting, U.S. AEC Repon PTR-143, January 28, 1957; and report TID 7531 (Pt 1) p. 153, A conference: Modem Approaches to Isotopic Analysis of Uranium, Chicago, February 5 to 7, 1957. 3. Amiel, S., (Israel AEC Report IA-621 1961), Analytical applications of delayed neutron emission in fissionable elements, Anal. Chem., 34, 1683, 1963. 4. Dyer, F. F., Emmery, J. F., and Leddicotte, G. W., A comprehensive study of the neutron activation analysis by delayed neutron counting, Report of the Oak Ridge National Laboratory, Oak Ridge, TN, 1962, 3342. 5. Keepin, G. R., Physics of Nuclear Kinetics, Addison-Wesley, Reading, MA., 1965, 82. 6. Rudstam, G., Status of delayed neutron data, in Fission Product Nuclear Data - 1977. IAEA Rep. 213, Vol. 11, International Atomic Energy Agency, Vienna, 1978, 567. 7. Rudstam, G., Six-group representation of the energy spectra of delayed neutrons from fission, Nucl. Sci. Eng., 80, 238, 1982. 8. MacMurdo, K. W. and Bowman, W. W., Assay of fissile materials by a cyclic method of neutron activation and delayed-neutron counting, Nucl. Instrum. Methods, 141, 299, 1977. 9. Binney, S. E. and Scherpelz, R. I., Technique for rapid analysis of uranium in porcelain dentures, Health Phys., 33, 341, 1977. 10. Binney, S. E. and Scherpelz, R. I., A review of the delayed fission neutron technique, Nucl. Instrum. Methods, 154, 413, 1978. 11. Mumba, N. K., Vas, L., and Buczko, Cs. M., Uranium and thorium analyses by delayed fission neutron counting technique using a small neutron generator, J . Radioanal. Nucl. Chem. Lett., 95, 311, 1985; Campbell, P., Gardy, E. M., and Boase, D. G., Assay of fissionable isotopes in aqueous solution by pulsed neutron interrogation, AECL (Rep), 5994, 1978. 12. Papadopoulos, N. N., Uranium and short-lived nuclide activation analysis by delayed neutron and gamma spectrum measurements with a new versatile sample transfer system, J . Radioanal. Chem., 72, 463, 1982. 13. Minor, M. M., Hensley, W. K., Denton, M. M., Garcia, S. R., An automated activation analysis system, J. Radioanal. Chem., 70, 459, 1982. 14. Papadopoulos, N. N., An inexpensive automatic uranium and short-lived radioisotope analyzer, J . Radioanal. Chem., 61, 81, 1981.

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15. Schlechte, P., Fast sensitive analysis of uranium and thorium in natural samples based on delayed neutron counting using an ultrafast conveyor tube system, J. Radioanal. Chem., 61, 55, 1981. 16. Millard, H. T., Jr. and Keaten, B. A., Precision of uranium and thorium determinations by delayed neutron counting, J . Radioanal. Chem., 72, 489, 1982. 17. Shenberg, C., Nir-El, Y., Alfassi, Z., and Shiloni, Y., Rapid and simultaneous determination of U, F, Al, Ca and V in phosphate racks by combination of delayed neutron and gamma-ray spectrometry techniques, J . Radioanl. Nucl. Chem., 114, 207, 1988. 18. Smith, A. Y., Armour-Brown, A., Olsen, H., Lyndberg, B., and Niesen, P. L., The role of geochemical prospecting in phased uranium exploration, Proc. Exploration for Uranium Ore Deposits, Internal Atomic Energy Agency, Vienna, 1976, 595. 19. Papadopoulos, N. N., An inexpensive automatic uranium and short-lived radioisotope analyzer, J . Radioanal. Chem., 61, 81, 1981. 20. Rosenberg, R. J., A simple method for the determination of uranium and thorium by delayed neutron counting, J. Radioanal. Chem., 62, 145, 1981. 21. Proctor, A. E., Harker, Y. D., Akers, D. W., and Mandler, J. W., Delayed neutron method for measurements of fissilelfertile content of samples ranging from environmental to irradiated fuel, Reports EGG-M-16084, EG & G Idaho Inc., Idaho Falls, 1984. 22. Schlechte, P. and Born, H. J., Rapid determination of uranium and plutonium components in mixtures by measuring the intensity of delayed neutrons after neutron irradiation, suitable for samples in the microgram region, Radiochem. Radioanal. Lett., 46, 103, 1981. 23. Hoffman, E. L. and Ernst, P. C., Analytical geochemistry advanced by neutron activation, J. Radioanal. Chem., 71, 447, 1982. 24. Rosenbeg, R. J., Pitkanen, V., and Sorsa, A. J., An automatic uranium analyzer based on delayed neutron counting, J . Radioanal. Chem., 37, 169, 1977. 25. Givens, W. W., Mills, W. R., and Caldwell, R. L., Cyclic activation analysis, Nucl. Instrum. Methods, 80, 95, 1970. 26. Guinn, V. P., Cyclic nuclear activation analysis, Radiochem. Radioanal. Lett., 44, 133, 1980. 27. Chatt, A., Desilva, K. N., Holzbecher, J., Stuart, D. C., Tout, R. E., and Ryan, D. E., Cyclic neutron activation analysis of biological and metallurgical samples, Can. J. Chem., 59, 1660, 1981. 28. Spyrou, N., Adessanmi, Ch., Kidd, M., Stephens-Newsham, L. G., Ortaoval, A. Z., and Ozek, F., Usefulness of thermal and epithermal cyclic activation analysis with a reactor system, J. Radioanal. Chem., 72, 155, 1982. 29. Hsu, H. H., The 870 keV gamma ray from PuO,, Nucl. Instrum. Methods, 193, 383, 1982. 30. Amiel, S. and Welwart, Y., Lithium and lithiumd analysis by counting delayed neutrons, Anal. Chem., 35, 566, 1963. 31. Gilat, J. and Gurfmkel, Y., Self-shielding in activation analysis, Nucleonics, 21, 143, 1963. 32. Amiel, S., Nondestructive Activation Analysis, Elsevier, Amsterdam, 1981, 49. 33. Amiel, S. and Peisach, M., Oxygen-18 determination by counting delayed neutrons of nitrogen-17, Anal. Chem., 35, 323, 1963.

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Chapter 7

USE OF X-RAY EMITTERS IN ACTIVATION ANALYSIS Mariana Mantel

TABLE OF CONTENTS General Considerations ......................................................... 112 Sensitivity and Limits of Detection .............................................112 A. Reactor Neutron Activation .............................................112 B. Fast (14 MeV) Neutron Activation ......................................116 Errors and Limitations ..........................................................116 A. Absorption .............................................................. 116 Interference .............................................................. 120 B. Discrete X-Rays .................................................120 1. Escape Peaks ....................................................121 2. 3. Secondary X-Rays ...............................................121 Beta Particles ....................................................122 4. Applications .................................................................... 126 A. Thermal Neutron Activation .............................................126 B. Epithermal Neutron Activation (ENAA) .................................127 C. 14-MeV Neutron Activation .............................................128 References .............................................................................. 128

112

Activation Analysis

I. GENERAL CONSIDERATIONS Since the pioneer work of Shenberg et al.' and Pillay2 who were the first to study the possibility of using X-ray spectrometry in activation analysis, this technique has been widely used to solve analytical problems where gamma-ray spectrometry fails to give satisfactory answers. The possibility of using X-ray spectrometry in activation analysis is based on the property of a number of nuclides to decay after neutron capture, by the emission of characteristic Xrays. The latter are due to different nuclear processes and as such may be divided into three groups: those due to internal conversion, X-rays of the parent element (Z); those due to electron capture, X-rays of the element (Z - 1); and those due to beta decay followed by internal conversion, X-rays of the element (Z + 1). The X-rays of all three groups decay according to the half-life of the parent element (Z). In some cases, the same nuclide may decay by two or even three of these nuclear processes increasing in this way the possibility of its detection and determination by X-ray spectrometry. As an example, neutron capture in 66Dy results in X-rays of Dy (Z) from '65mDy(1.26m), Tb (Z - 1) from 157Dy(8.1 h), and Ho (Z + 1) from '65Dy(2.3 h) with energies of 46.0,44.5, and 47.5 keV, respectively. This property may in itself present an advantage since the X-rays used for the determination of an element may be chosen according to their energy, their half-life, or the sensitivity of measurement, depending on the particular problem to be solved. The technique of X-ray spectrometry is attractive since it presents several advantages which are a result of the basic properties of X-rays. Thus, the precise measurement of the energy of the emitted X-rays permits the unambiguous indentification of the element due to the direct correlation between an element and its characteristic X-rays (Moseley's law). Furthermore, the limited number of peaks in the X-ray spectrum of each element permits the determination of many elements in a single spectrum. Finally, the reduced sensitivity of the Si(Li) and the Ge planar low-sensitivity photon detectors (LEPD) used for the measurement of X-rays, to high-energy photons, eliminates the interference of high-energy gamma rays and the Compton effect. As a result, the background is considerably lowered and the peak-to-background ratio much in~reased.~ In order to be able to evaluate correctly the merits of X-ray spectrometry, its limitations, such as absorption by the matrix and possible secondary enhancement effects, will have to be considered in every case together with its advantages.

11. SENSITIVITY AND LIMITS OF DETECTION A. REACTOR NEUTRON ACTIVATION High sensitivities may be obtained by measuring the characteristic X-rays produced following neutron activation. In addition to the parameters which influence the sensitivity of neutron activation in general, other factors, specific to X-rays, will have a great impact on the sensitivity of Xray spectrometry. These factors are the internal conversion coefficient or the percent of electron capture, the fluorescence yield, and finally, especially for heavy elements, the preference of emitting K or L X-rays and the ratio Loll LPI Ly X-rays. A few compilations of X-ray-emitting isotopes and the corresponding sensitivities of determination are reported in the l i t e r a t ~ r e . ~ , ~ - ~ Pillay2studied 21 elements by measuring with a proportional counter the X-rays emitted by their radioactive isotopes produced by the (n,y) reaction. He expressed the sensitivity as the amount of an element which after activation in a thermal neutron flux of -2 x 10" n cm-2 s-I for a period of 10 h, or until saturation, whichever is shorter, would produce Xrays with sufficient intensities to be measured by his experimental set-up (minimum 100 cpm).

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113

Mantel and Amie14 measured experimentally using a Si(Li) detector, the yield of X-rays obtained by the (n,y) reaction following reactor neutron activation. From the 59 elements which in principle could be determined by this method, only those were not studied whose X-ray-emitting radioisotopes have too short a half-life (less than 20 s) or the intensity of the emitted X-rays is too low to be detected by the experimental set-up used. Based on the results obtained, the sensitivities were calculated for irradiations of 1 half-life or 5 min, whichever is shorter, in a neutron flux of 1013 n cm-* S K I and expressed as d p s l ~ gat 0 time after irradiation. Highest sensitivities (> lo4 dpslpg) were obtained for Dy, Ir, Rh, Sc, Nb, Eu, Co, Ag, and Br. Habib and Minskis published a comprehensive compilation of X- and gamma rays emitted by 110 isotopes produced by 50 elements (2 = 27 to 92). The sensitivities expressed as counts s-' pg- were calculated theoretically using the computer program COMPAR for an irradiation time of 1 half-life (if t,,, < 135 h) or for a maximum irradiation time of 35 h and based on the use of a Ge LEPD. The highest sensitivities were obtained for Eu, In, Rh, and Dy (3lo4 c s- pg- I ) . The calculations are based on published values of thermal neutron cross-sections, resonance integrals, gamma-ray intensities, and on calculated X-ray intensities. The limits of detection obtainable by X- and gamma-ray spectrometry were also calculated5 and defined as the minimum concentration of an element which is detectable in a given matrix. IAEA-s-5-soil and Bowen's kale were chosen as reference standards for geological and biological materials, respectively. The minimum detectable signal was defined as 4.7(B = background) corresponding to a statistical precision of 30%. The experimental background continua for the two matrices were fitted with polynomial functions for the two detector systems (Ge and Ge[Li]) and for two standard irradiation and counting conditions. For the evaluation of intermediate values, the background counts of the continuum were assumed to be proportional to the irradiation and counting times and inversely proportional to the decay time. Table 1 shows the elements for which low X-ray limits of detection ( s lo-' ppm) have been calculated by Habib and Minski.' The same authors5 introduced the concept of "advantage factor" (AF), i.e., the ratio of X-ray and y-ray sensitivities. The use of X-ray spectrometry is recommended5 in those cases where AF is significantly greater than unity. Of the isotopes studied, half (66) were found to have an advantage factor greater than 2, and 9 an advantage factor greater than 100.5 Following the concept of Habib and Minski, we calculated the detection limit advantage factor (DLAF), the ratio of gamma-ray to X-ray detection limits for biological and geological matrices, obtained from the values calculated by Habib and M i n ~ k i . ~ Table 2 shows the radioisotopes with the highest detection limits advantage factors X-ray detection (DLAF > 100 for biological matrices), the corresponding X-ray sen~itivities,~ limits,' and A F . 3 s may be seen, a high DLAF does not always correspond to high sensitivities or high detection limits. However, these radioisotopes correspond to those for which high AF (AF > 10) have been calculated by Habib and M i n ~ k i . ~ Since the influence of the background was included in the calculation of the detection limit^,^ the DLAF will be the best expression of the performance of X-ray as compared to gamma-ray spectrometry. It is on this factor that the analytical chemist will have to rely for choosing between the two techniques for the analysis of an element in a specific matrix. As part of a study on the possible applications of neutron activation followed by X-ray spectrometry and magnetic deflection of beta rays, Mantel7 calculated the limits of detection for 18 trace elements in different biological matrices. The matrices studied were blood, urine, and five standard reference materials: milk powder, animal muscle, bovine liver, orchard leaves, and Bowen's kale.

114

Activation Analysis

TABLE 1 Elements5 with Low X-Ray Detection Limits ( ~ 1 0 ppm) - ~ X-rays measured Element

Radionuclide

Eu

152mEu

DY

lS2Eu 165Dy '="DY

Sm

9 m

Rh Hf Co Cs Ho Br Ha

IMmRn ll9mHf

Re Yb

W

I Au

In Ir Cu

u

Gd

Tn

Limits of detection ( ~ p r n ) ~

-0

134"'cs IMHo

-Br I9'Hg IE6Re IE8Re 169Yb 187W 1281 198Au 116rnIn 192mIr 64C~ 239Np 153Gd 233Pa

Note: B, biological matrices; G, geological matrices

Only those trace elements that, following neutron activation, produce radioisotopes that decay by the emission of K or L X-rays with energies up to 16 keV, were considered. For higher X-ray energies, it is not necessary to apply the method of magnetic deflection of beta rays since the same reduction in background may be obtained by plastic absorber^.^ The limit of detection was defined as the smallest amount of the element that permits the attainment of a signal-to-noise ratio of 3-(~ = background). The calculations were carried out for a thermal neutron flux of 10'' n cm-2 s-' and for irradiations and counting periods of one half-life after one half-life cooling time (if t,,, < 12 h) or for a maximum irradiation, counting, and cooling time of 12 h. The contribution of the background was taken into account for every matrix studied, over an energy range of 2 to 20 keV. The values were calculated based on the composition of the matrix (main elements: Na, K, C1, P, Mg, and Ca), the irradiation time, and the reduction in background due to the magnetic field. Table 3 shows the detection limits calculated for the matrices studied. The limits of detection are the averages obtained for all seven matrices. If the range between the results is higher than +- 40%, the lowest and highest limit of detection calculated for the isotope in question is given. Garmann9 used the technique of magnetic deflection of beta rays for the measurement of X-rays and low energy gamma rays (46 to 146 keV) emitted by geological materials after epithermal neutron activation. The "detection limits" (qualitative measurements) and "determination limits" (quantitative measurements) were calculated for an epithermal flux of 4.8 x 10" n cm-2 s-', an irradiation time of 24 h, and the use of a planar Ge detector. Only long-lived radioisotopes (t'/,> 30 h) were measured.

.

115

Volume 1

TABLE 2 Radioisotopes with High Detection Limit Advantage Factors5 (DLAF > 100 for Biological Matrices)

Radionuclide

Note:

X-ray measured

DLAF

y ray keV

B

G

AW5

X-ray sensitivityS c s-' p g - I

X-ray limits of detection5(ppm)

B

G

DLAF, detection limit advantage factor - gamma ray detection IimiVX-ray detection limit; B, biological matrix; G, geological matrix; AF, advantage factor - X-ray sensitivitylgamma-ray sensitivity.5

TABLE 3 Limits of Detection of Trace Elements in Biological Matrices Determined by INA using Ray Spectrometry and Magnetic Deflection of Beta Rays

Element Ir Co Br Se Nb Ge Sc Cu

u

Radionuclide LYZrnIr T o 82Br 7y"Se 94"Nb 75"Ge -sc 6"Cu 239U

Cr Th 0s Rb

51Cr 233Th 191"0s -Rb

Pt

198mpf

Hg

197Hg I-Y @Zn 8'mSr

Y Zn Sr a

X-ray measured

Average shown is of the results (range

IrL1 CoK, BrK, SeK, NbK, GeK, ScK, NiK, NPLI VK, Pal-,, 0&1

RbK, PtLd

YK, ZnK, SrK, %

X-ray energy (keV) 9.2 6.9 11.9 11.2 16.6 9.9 4.1 7.5 13.9 5.0 13.3 8.9 13.4 9.4 9.7 14.9 8.6 14.2

X-

Limit of detection (ppm) Average 6.0 7.0 1.4 1.5 2.0

Orchard leaves

Animal muscle

x lo-' x lo-)

x x x 10-2

2.8 x 5.0 x 7.0 x

1.8

1.1 x l o - '

3.4 x 10-1

3.0 x 10-I

7.0 x 10-I

X

lo-'

1.0 x lo-' 3.0 x lo-' 1.o 2.0 2.8

4.2

5.0

12.6

8.1 1.3

X

102

40%) obtained from all seven matrices.

Modified from Mantel, M., Analyst, 108, 1190, 1983. With permission.

116

Activation Analysis

The limits of determination obtained for the X-ray-emitting radioisotopes studied (Tb, Yb, Ta, U, and Th) are in the range of 10-lo to lo-" g.

B. FAST (14 MeV) NEUTRON ACTIVATION The most frequently occurring nuclear reactions by activation with 14 MeV neutrons are the (n, n') (n, 2 4 , (n, p) and (n, a)reactions. A number of radioisotopes produced by these reactions decay by internal conversion or electron capture emitting characteristic Xrays during their decay. The possibility of using these X-rays for analytical purposes has been investigated.I 0 ~ ' l Janczyszyn and Kwiecinski10 studied 68 elements, from Z = 24 (Cr) to Z = 92(U), and found that 50 of them emit X-rays after 14-MeV neutron activation. For these elements, the factor k defined as:

where a = abundance of the target isotope, 6 = reaction cross-section, X = number of characteristic X-rays per decay, X = decay constant, and f, t,, t, = irradiation, counting, and decay times was calculated. This factor directly influences the sensitivity of a nuclear reaction if the other parameters (neutron flux, counting efficiency, sample weight, etc.) remain constant. Of the elements for which a high k factor was found, 24 were studied experimentally. The "best" nuclear reactions, i.e., those reactions which could theoretically lead to high sensitivities, were chosen. Samples of thicknesses close to the "infinite layer" were irradiated in a 14-MeV neutron s-I. Table 4 shows the results generator with an average neutron flux of 2 X lo8 n obtained for the elements considered as most promising by the authors.I0 A determination limit of 0.1% may be obtained for the elements listed in Table 4. The same may be expected from selenium, rubidium, rhodium, hafnium, and iridium, judging from the values of their k factors. However, these elements have not been studied experimentally. It may be concluded that the technique of X-ray spectrometry following 14-MeV neutron activation may be applied to the determination of main and minor constituents but is not suitable for trace element analysis."

111. ERRORS AND LIMITATIONS A. ABSORPTION One of the most important sources of error in X-ray spectrometry is the absorption of X-rays by the matrix. This phenomenon may represent a serious limitation due to the low energy of X-rays. The mass absorption coefficient of an element, p,, expressed as cm2 g-I, defines the absorption process quantitatively. The mass absorption coefficient is dependent on energy; it decreases continuously with increasing photon energy, and for the same energy, it increases with the atomic number of the absorbing element. At a given energy, the mass absorption coefficient of a compound is the weighted sum of the absorption coefficients of the constituent elements.

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TABLE 4 Elements for which High Sensitivities were Obtained by 14-MeV Neutron Activation followed by X-Ray Spectrometry'' Element

Nuclear reaction'

X-ray measured

k

t,,,

mb.h

Target material

Thickness (mglcm~

Countsb

Si(Li) detector - 30 mm2 Co Ni In Sb Sn Ho Dy

10.5 min 9.7 min 10.9 min 4.2 min 15.9 min 37.0min 29.0 min

1.6 x 10-I 9.7 x 3.7 x 10-I 6.6 4.7

Metal

7.1

Ho@,

In,O, Sb20,

1.2

Ge(Li) detector - 80 cm3

Hg a

2wHg(n.2n)Iw"Hg

Hg

42.6 min

9.3

X

10-I

Hg(C,H,O,),

620

It is presumed that the measurements are based on the nuclear reaction with the highest k factor. Integrated K X-ray peaks, expressed as counts per 1000 s, after 600-s irradiation and 60-s delay time

where C, and p,(E) are, respectively, the concentrations (%) and the mass absorption coefficients (cm2 g-') at the energy (E) of the individual elements. Since tables of mass absorption coefficients for the elements are available,12those for compounds can easily be calculated. An example for the calculation of the mass absorption coefficient of a compound from the absorption coefficients of the constituent elements is given in Table 5 for an igneous rock at 16.0 keV. It follows that in a given matrix, the absorption of X-rays depends on their energy and on the composition of the matrix, which means that, for the same element, the absorption in a matrix composed of light elements (for instance organic materials) will be much lower than in a heavy-element matrix. For monochromatic X-rays, the degree of absorption (attenuation factor, A) in a given matrix may be estimated, provided the sample-to-detector distance is greater than the detector diameter,14 according to:

where, I and I, are, respectively, the attenuated and unattenuated X-ray intensity expressed as counts per unit time, p, = mass absorption coefficient of the matrix (cm2 g-I), p = density (g ~ m - ~and ) , d = sample thickness (cm). Thus, if p and d are known, the attenuation factor (A) may be evaluated for any matrix since the absorption coefficient may be calculated. Holzbecher and Ryan15 evaluated the attenuation of low energy photons in different materials by comparing their attenuation in the material studied (Am) to that in water (Aw). The ratio AmIAw was measured experimentally and calculated according to Equation 3. Very good agreement was obtained, in most cases, between the calculated and measured values. The nearly linear decrease of the absorption coefficient with increasing energy is interrupted by the absorption edges, i.e., abrupt increases of the absorption coefficient at energies

.

118

Activation Analysis

TABLE 5 Calculation of the Mass Absorption Coefficient of an Igneous Rock at 16.0 keV (Equation 2) Composition (oxides)

"

Weight (9%)" Ci

~ ~ ( 1 6 keV)b .0

p,Ci

According to data on the composition of rocks.13 Absorption coefficients of the oxides calculated according to their elemental composition and to the absorption coefficient of the elements.I2

equal to the bond energy of an electron in the K- or L-shells of the elements. The absorption edges occur at strictly defined energies for each element and their energy increases with increasing atomic number. The elements, with absorption edges in the vicinity of the energy of the X-rays to be analyzed, have to be considered in the evaluation of the possible absorption during the measurement. The quantity of these elements which in a specific material will produce an preventing increase of maximum 1% in the overall error of the method, may be ~alculated,~ in this way unexpected high errors. At the absorption edges, the increase in absorption with energy is sharp and the decrease is mild. It follows that the highest absorption will be obtained from elements whose absorption edges are at energies just below those of the X-rays to be measured. For example, in the determination of Cu in geological materialsI6 by measuring the NiK,-X-rays (7.47 keV), the presence of iron (K, = 7. l l keV) presents a serious problem and reduces the sensitivity of the method due to the absorption of the Ni-K, photons by iron. Different techniques are used to overcome the errors due to absorption of X-rays by the matrix. The preparation of very thin samples reduces the absorption to a negligible value. Weaver1' prepared such samples by introducing a very thin layer of VaselineB into the bottom of the flat plastic containers used for the counting of irradiated coal samples. A similar approach was proposed by Mantel and ArnieP who used very small plastic containers (I.D. = 12 mrn) as irradiation and counting vessels and 10- to 30-mg samples evenly distributed on the bottom of the container by a few drops of hot paraffin added to the sample. Very thin liquid samples were prepared by Rapaport et al.I9 by dropping a few microliters of the liquid between two ultrathin mylar foils. Another approach, used by most worker^^^^^^ is the preparation of standards with similar composition to that of the samples.

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TABLE 6 Absorption Coefficients of Different Organic Matrices at 4.95 keV Matrix

Absorption coefficient [p, (cm2 g-I)]

Kidney

Liver

Heart

Cellulose

36.8

34.6

36.4

31.2

TABLE 7 Relative Quantities of CaCO, and MgCO, Necessary to Obtain Standards with Different Absorption Coefficients at Different Energies Standard Pm

cm2. g-'

energy (keV)

CaCO,

MgCO,

(%)

(%)

The absorption coefficient of the matrix as a whole is only slightly influenced by the small variations in the relative abundance of its major constituents. Thus, it will be possible to use, as standard, matrices with a composition similar to that of the sample without it being imperative that standard and sample match perfectly; the same type of rock for geological samples or the same type of tissue, drug, or food for organic materials (see Table 6). The presence of trace elements, even of those with high atomic numbers will influence only slightly the absorption coefficient of the matrix. However, as mentioned before, great attention must be given to the presence of elements, sometimes trace elements, with absorption edges near the energy of the X-rays to be determined. If standards similar to the sample are not available and the composition of the matrix is known, it is possible to prepare standards with absorption coefficients similar to that of the sample. The mass absorption coefficient of the sample, at the energy of the X-rays to be measured, is calculated according to Equation 2 and another matrix with a similar absorption coefficient at the desired wavelength, chosen as standard. As an example, such a standard may be obtained by vacuum drying a mixture of CaCO, and MgC0,22 in the necessary proportion, in a solution of the element to be determined. Table 7 shows the relative quantities of CaCO, and MgCO, necessary to obtain various absorption coefficients at different energies. ~ the In order to be sure to prevent all errors due to abdrption, some ~ o r k e r p !prefer k particular use of "infinite thick samples", i.e., samples which ptre infinitely thick for t energy of X-rays being measured. The maximum thickness of sample through which a

120

Activation Analysis

particular X-ray could pass may be calculated with the help of the absorption coefficients of the major elements in the sample. If standards and samples are similar in composition, then the X-rays counted come from equal volumes, so that the actual weight can be neglected in the calculation of the concentration. However, it has to be taken into account that only X-rays which originate from a portion of the sample nearest to the detector will be measured. Finally Bode et a1.I4describe a method for the correction of absorption losses in a matrix, using only information from the sample spectra. The attenuation coefficients can be determined as a function of photon energy from sets of intensity ratios between related peaks in the spectrum. The unattenuated intensities are calculated based on these absorption coefficients which were found sufficiently reliable to obtain correct results.

B. INTERFERENCE The possible interference with the measurement of X-rays obtained following neutron activation may be due to discrete X-rays and low-energy gamma rays, secondary X-rays, and beta rays. 1. Discrete X-Rays Due to the direct relationship between the atomic number of an element and the energy of its characteristic X-rays, the interference with the measurement of these X-rays will result chiefly from X-rays emitted by neighboring elements in the periodic table. Another possible source of interference are L X-rays emitted by elements with high atomic numbers in the same energy range as the K X-rays of an element with a low atomic number, or vice versa. Finally low-energy gamma rays, which may be emitted from the irradiated sample, must also be taken into account. Obviously, the possibility of determining in the same spectrum two elements with close X-ray energies will depend on the resolution of the detector. In other words, for two neighboring elements in the periodic table, the difference between the energy of the K,, Xrays of one element and that of K,, X-rays of its immediate predecessor should be greater than the resolution of the detector. This difference increases with the atomic number but at the same time the intensity of the K,, X-rays also increases and may interfere with the measurements. The resolution of the Si(Li) detectors generally used for energies up to 50 keVZ3decreases sharply with increasing energy (from 160 eV at 6.4 keV to about 450 to 500 eV at 50 keV) and the resolution of the LEPD detectors used for higher X-ray energies lies between 450 to 500 eV (for 60 and 122 keV, re~pectively).~ As an example, for elements with atomic numbers Z = 60 to 70, a problem arises in the simultaneous determination of element Z and (Z + 4); the Kp, X-rays of element Z 4) since their energies are too cannot be separated from the Ka, X-rays of element (Z close to be separated by the detector. However, since we are dealing with X-rays obtained after neutron activation, several other factors may facilitate the simultaneous determination of two elements whose characteristic X-rays have very close energies. These factors are the type of decay process responsible for the emission of X-rays (electron capture, internal conversion, p-), the halflife of the X-rays involved, and the relative sensitivities of measurement based on the respective fluorescent yield of the emitted X-rays. In the case of fast (14 MeV) neutron activation, another important factor is the nuclear reaction which is responsible for the production of X-rays, since different isotopes, or the same isotope, of an element may emit various X-rays according to the nuclear reaction (n, 2n), (n, n'), (n, a) or (n, p) which took place. A very good example of X-rays obtained after the (n, y) reaction are the elements silver (Z = 47), cadmium (Z = 48), and palladium (Z = 46) which all produce radioactive isotopes which decay by X-ray emission. All three elements may be determined in the same sample considering differences in decay processes, half-lives, and sensitivities.'*

+

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Another example is the determination of niobium in steels by instrumental neutron activation (INAA) based on the measurement of the 16.6-keV niobium K, X-rays.'' The elements in the periodic table which could interfere are yttrium (Z = 39), zirconium (Z = 40), molybdenum (Z = 42), and technetium (Z = 43), by the emission of K X-rays and uranium and thorium by the emission of L X-rays. From all these elements, only zirconium and molybdenum could be present in steels. Since zirconium does not emit X-rays following neutron activation, Mo remains the only possible interference. However, due to the difference in sensitivities5 and half-lives, the amount of molybdenum in the steel sample would have to be lo4 times that of niobium to produce any interference. An interesting approach to overcome interferences was used by Zaitsev." He applied differential filters to select a part of the X-ray spectrum specific to the element to be determined. For the separation of a radiation with a defined energy, a pair of filters is used. One of the filters consists of an element with a K-absorption edge just below the energy of the line to be separated, while the other consists of an element with a K-absorption edge above that energy. The width of the transmission band is generally 0.3 to 0.4 keV for light elements and 2 to 2.5 keV for heavy elements. Recent compilations of X-ray-emitting radioisotopes include the possible interferences with the measurement of X-rays emitted after neutron activation. Habib and M i n ~ k i in ,~ their comprehensive compilation of X-ray sensitivities from isotopes produced by the (n,y) reaction, identify for each isotope the interfering X-rays obtained from neighboring elements. The percentage of interference for the optimum irradiation, decay, and counting conditions of the isotope of interest, are calculated assuming an equal mass of 1 g for both the interfering and the measured element. Baedecker et al. ," during a study on the utility of low-energy measurements in epithermal neutron activation analysis of geological materials, indicate the most probable interfering radiation for each line, estimated on the basis of common elemental abundances in silicate rocks and resonance activation integrals. Finally, Hertogen and Gijbels3 studied the spectral interferences in the gamma spectra obtained from two U.S. Geological Survey standard rocks and a series of volcanic rock samples from the Canary Islands, measured with a LEPD detector. Though the emphasis of the study was on gamma rays, a few X-ray-emitting radioisotopes (especially rare-earth elements) are also included. 2. Escape Peaks The contribution of escape peaks was studied by Keith and LoomisZ3and Reed and Warez5for Si(Li) detectors. They found that the silicon escape peak is geometrically similar to the main peak. The ratio E = height of escape peak/height of main peak (both heights having been corrected for background) is of the order of at 3.5 keV and decreases with increasing photon energy to a negligible value at 25 keV. For higher energies, in spectra recorded with a LEPD detector, intense lines are accompanied by small peaks about 10 keV lower, due to the Ge-K X-ray escape efect (K, = 9.9 keV, Kp = 11.0 keV). The escape peak ratio has been measured by Hertogen and Gijbels3 (from 24 to 121 keV) and Ungrin and Johnsz6 (from 40 to 41 1 heV), for different LEPD detectors. The results showed that the ratio escape peak areatphotopeak area is of the order for 121.9 keV.3 for 24 keV and decreases to 1.5 X of 5.27 x

3. Secondary X-Rays Strong beta or gamma rays from the radioactive samples may interact with major elements present in the samples or materials in the vicinity of the detector and induce secondary fluorescent X-rays. Thus Au-K X-rays were reported by Hertogen and Gijbels3 in all spectra obtained with a small LEPD detector as a result of the excitation of the gold-electrode atoms by intense

122

Activation Analysis

beta emitters or by gamma rays with energies near the K-absorption edge. The same authors3 report intense Ba-K X-ray fluorescent radiation when glass counting vials are used. Generally the intensities of the X-rays and gamma rays obtained by neutron activation are too low to produce fluorescent X-rays in sufficient yield to interfere with the measurements. The possibility of secondary fluorescent X-rays must be taken into account only when very long irradiations are carried out or if one of the main components of the matrix is known to produce especially strong gamma or beta radiation following neutron activation.

4. Beta Particles The beta particles emitted from an irradiated sample are another serious source of interference in X-ray spectrometry. These beta particles produce a high background which completely obscures the X-ray peaks obtained from trace amounts of low and medium Z elements making their quantitative determination practically impossible. In addition, beta particles may increase the dead time of the Si(Li) detector which results in inaccurate integration of the X-ray peaks. The elimination of this interference becomes of critical importance for the determination of trace elements in matrices, such as biological materials or seawater whose major components, sodium, potassium, chlorine, and phosphorous, are themselves strong beta emitters. The interference of beta particles may be overcome by the use of plastic absorbers or by the deflection of the beta particles by magnetic fields. The latter technique has been extensively studied in our laboratory at the Soreq Nuclear Research Centre8p27.28 and its possible applicatim to neutron activation analysis, investigated. Theoretical calculations were carried out to determine the fraction of beta particles in a beta-particle beam moving perpendicular to the detector, which will be deflected by a given magnetic field. The results showeds that the intensity of the magnetic field necessary to deflect beta particles, depends on their energy and the source detector distance. For example, at a distance of 27 mm (the source-detector distance used in our experiments), a 0.2-T magnet is necessary to remove 91% of the beta particles emitted by "P (1.7 MeV) an3 a 0.5-T magnet is needed for a similar reduction of those emitted by 35C1(4.92 MeV). The influence of the magnetic fields on the performance of Si(Li) detectors and on the background obtained from irradiated samples was studied experimentally. A 100 mm2 Si(Li) detector was used throughout the experiments. Figure 1 is a picture of the experimental set-up used. The magnet shown in the picture is a 0.35 T magnet (manufactured by AEI for a Minimass mass spectrometer). This magnet was replaced in part of the experiments by a 1.3 T electromagnet (18-mm gap between poles). The influence of magnetic fields on the disturbance produced by beta particles in the performance of the Si(Li) detector was studied by measuring the 6.4-keV Fe K, X-rays and the 14-keV gamma rays emitted from a 1 0 - ( ~ C i ~ ~source C o in the presence of 50-(LC~ 32P (E max = 1.7 Mev) with and without a magnetic field. The results showed that a 0.4 T magnet corrects the peak to background ratio to 10.7 as compared to 16 obtained for the 57 Co source alone and the resolution of the detector approaches its experimental value of 500 eVZ3 The background obtained from 24Na, 38Cl, and 32P, the principal beta-emitting radioisotopes which may be present in an irradiated sample, was measured over an energy range up to 20 keV, both with and without a magnet. The results showed that the decrease in background is not the same for all the nuclides and varies with energy for the samc nuclide. Table 8 shows the reduction in background obtained for each of the four nuclides with a 1.3 T electromagnet. The greatest reduction in background is seen to be obtained for 32P.

the magnet. For th'e other nuclides which are beta and gamma emitters, the Compton peaks of the gamma rays will also contribute to the background.

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FIGURE 1 . Experimental set-up used for X-ray spectrometry and magnetic deflection of beta rays (0.35 T magnet and 100 mm2 Si(Li) detector).

Recently Garmann9 published a study on the influence of magnetic fields on the measurement of X-rays and low-energy gamma rays obtained from geological materials after epithermal neutron activation. A 200-mm2planar hyperpure germanium detector and an electromagnet, as deflector, were used throughout the experiments. The sample to detector distance was adjusted to 50 to 80 rnm; X-rays and low-energy gamma rays with energies higher than 45 keV were measured. The results showed that the reduction in background is smaller than that obtained for Si(Li) detectors,' however, a substantial improvement in the peak to background ratio, better resolution of the detector, and improvement in the shape of the peaks, were observed. Table 9 shows the improvement in the peak to background ratio obtained by Garmann9 by measuring different photopeaks in the presence of magnetic fields. Furthermore, better limits of detection (see "sensitivity and limits of detection") and higher precision and accuracy of the results, were obtained by the use of magnetic fields as deflectors. Table 10 shows the improvement in the detection and determination limits for the X-ray emitting-radioisotopes ~ t u d i e d . ~

124

Activation Analysis

TABLE 8 Reduction in Background Obtained with a 1.3 T Electromagnet

P-

Radionuclide

"

Remaining background (I)'

(Mev)

Average of values obtained for different energies (up to 20 keV) expressed as percentage of the background obtained without the magnet.

Modified from Mantel, M., Analyst, 108, 1190, 1983. With permission.

TABLE 9 Improvement in Peak to Background Ratios Energy Radionuclide

a

t,,

Gamma (keV)

p-max (MeV)

Peak background ratioa Without magnetic field

Measured with a 200-mm2 planar germanium detector-approximate

With magnetic field

Improvement

(%I

values.

From Garmann, L. B., J . Radioanal. Nucl. Chem., 99, 75, 1986. With permission.

As mentioned before, the interference of beta particles may also be overcome by the use of plastic absorbers. However, the latter absorb not only beta particles but also X-rays k of magnetic fields at a rate which decreases with increasing X-ray energy. ~ h advantage is the fact that they have no influence on X-rays and thus do not produce any reduction in the X-ray activity. However, due to the physical size of the magnet, the source-to-detector distance is increased which causes a reduction in sensitivity. For choosing between a magnet and a perspex absorber, all these factors have to be taken into account. For low-energy Xrays, a magnet is generally preferable especially for high activity samples where the emitted X-rays may be easily detected in spite of the reduction in sensitivity due to the geometric factor. For samples of low activity, the following equation may be used as a guideline to decide whether to use a magnet or a perspex ab~orber.~

where p, and p,@ are the linear absorption coefficients in perspex for X- and beta rays, respectively, a is the decrease in X-ray activity due to the geometric factor (size of magnet), and b the reduction in the background obtained from a given sample, by the same magnet. For X-ray energies where F, > pB(In alln ab), a magnet is preferable; in the opposite case, a perspex absorber is a better choice.

Energy (keV)

Calculated values. Without magnetic deflection. With magnetic deflection.

Type

Valueb

Valuec

Detection limit' (%)

Improvement

From Garmann, L. B., J. Radioanal. Nucl. Chem., 99, 75, 1987. With permission.

a

Element

X-rays Valueb

Valuec

(%)

Improvement

Determination limit'

TABLE 10 Improvement in the Detection and Determination Limits for Epithermal Neutron Activation Analysis (EINAA); Measured With and Without Magnetic Deflection on a Planar Detector

126

Activation Analysis

For a 1.3 T electromagnet and a sample to detector distance of 27 mm (the experimental set-up used at the Soreq Nuclear Research Centre), the use of a magnet was found to be preferable, up to 16 keV, for the analysis of trace elements in biological matrices. It is obvious that for different matrices and for every magnet and absorber pair, another borderline energy will be found.

IV. APPLICATIONS The present chapter is not intended to be an exhaustive bibliography of all the works published on the use of X-rays in activation analysis. Only a 'few examples are given to illustrate the possible analytical applications of X-ray spectrometry.

A. THERMAL NEUTRON ACTIVATION Most workers studied the analytical applications of X-rays obtained by the (n,y) reaction following thermal neutron activation. Single elements were determined in complex matrices and multielement nondestructive analyses were carried out. Allen and BrookhartI6 determined copper in biological materials and in several United States Geological Survey (U.S.G.S.) standard rocks by measuring the Ni K, X-rays which result from the 43% electron capture decay of TU (tlIz = 12.8 h). Standards similar to the sample and infinitely thick samples (see Section 1II.A.) were used to overcome errors due to absorption. Sensitivities of tenths of ppm were obtained for plant material (National Bureau of Standards [NBS] orchard leaves) and in the ppm level for more complex biological materials (NBS bovine liver). The analysis of geological materials was found to be problematic due to the absorption by iron of the Ni K, photons. Weaver17 used a LEPD detector for the determination of mercury and selenium in coal. The method is based on the measurement of the Au X-rays emitted by '97Hg (tlI2 = 65 h) and two low-energy gamma rays emitted from 75Se(t,,, = 120 d). The sensitivities obtained with a 10 to 14 h irradiation at a lOI3 ncm-2 s-' neutron flux are 50 ng of mercury and 150 ng of selenium per gram of coal. The possibility of determining thallium and bismuth by X-ray spectrometry following neutron activation was studied by Holzbecher and Ryan.29Detection limits of 50 pg/g and 250pg/g, respectively, in biological materials, were obtained by measuring the Pb X-rays obtained from 206Tl (tlI2 = 4 min) and the Bi and Po X-rays obtained from 'I0Bi (tlI2= 5.0 d). The interference of lead and bismuth with the measurement of thallium, and of mercury with that of bismuth, was studied and the maximum quantity of these elements which could be present in the sample without being a serious interference, calculated. Allen and Steines30 determined niobium in geological samples by solvent extraction in the presence of carrier-free 95Nbtracer prior to neutron irradiation. After reextraction into 1.5% H202, the aqueous samples were irradiated for 5 min and counted twice for 1 min, with and without a plexiglass absorber. The Nb K, + K, X-rays (16.6 and 18.7 KeV) were measured with a planar intrinsic germanium detector. A total of 15 U.S. geological survey standard rocks with considerably different compositions were analyzed. A limit of detection g was obtained. Due to the preirradiation separation, the contamination limit of 5 x was generally inferior to the detection limit of the method. Magnetic deflection of beta rays was applied by the group at the Nahal Soreq Nuclear Research Centre to the determination by INAA and X-ray spectrometry of bromine in blood serum,lg niobium in steels,,' and trace elements in active charcoal3' and in the Dead Sea.32 All the irradiations were carried out at la neutron flux of loL3ncm-' S - ' and a Si(Li) spectrometer was used for the measurement of X-rays. The determination of bromine in synthetic blood serum samples has been carried out for the first time by INAA and X-ray spectrometry by Peisach et However, the reduction

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in background due to the deflection of beta rays by magnetic fields made possible the determination of bromine in real blood serum sample^.'^ An average value of 7.38 r 0.44 mg Br per liter was obtained. This result is in the range of values considered as normal for bromine in blood serum. The 16.6-keV Nb-K X-rays were used for the determination of Nb by INAA in steel samples.20An error of + 20% was obtained by using Nb,O, in cellulose as standard. This error may be reduced to f 10% by the use of a National Bureau of Standards SRM steel standard of similar composition to the steel to be analyzed. A different approach was applied to the determination of trace elements in the Dead Sea.32Due to the very high salinity of the Dead Sea (340 gll), a concentration step was necessary. The trace elements of interest, cobalt, copper, and mercury, were first co-precipitated with lead-APDC, separated from the bulk of the dissolved salts, and then determined by X-ray spectrometry in the neutron-activated APDC precipitate. Results of 1.3 ppb for Co, 2.0 ppb for Cu, and 1.2 ppb for Hg were obtained in Dead Sea surface water, sampled after the complete overturn of the sea a few years ago which resulted in the homogenization of the seawater in the Dead Sea. A number of papers3.2',34-36 deal with the determination of rare earth in geological samples by X-ray spectrometry, since the lanthanide elements (except for La and Pr) produce radioactive isotopes on neutron capture which decay with emission of X-rays. The technique is especially attractive since the simultaneous determination of these elements by other analytical methods, including gamma spectrometry, presents a very difficult problem. V ~ l d e determined t~~ ten rare-earth elements (REE) in basic rocks (45 to 52% SiO,) with very low REE concentrations. A preirradiation chemical separation was used, consisting in a separation of the REE by ion-exchange chromatography followed by purification and concentration as REE oxalates. The latter are submitted to reactor neutron activation. Two irradiation and counting sequences were used: 1-h irradiation at a thermal-neutron flux of lo9 ncmp2s-' and 600-s counting time (after 7-min cooling time) for short-lived isotopes (Eu, Dy) and l-h irradiation at 1.6 X 1013 n cm-'sp' and 600-s counting time, after 2 to 5-d cooling time, for the other REE. The proposed technique presents several advantages: the chemical separation of the REE group before irradiation considerably reduces the cooling time required before measurement of the activities; a convenient choice of the cooling time eliminates the interferences between the REE themselves. A precision of 5 to 10% was obtained for REE concentrations in the range 0.1 to 50 ppm by using standards similar in composition to the rocks analyzed. Hertogen and Gijbels3 investigated whether X-rays could be useful in the INAA of silicate rocks. Special attention was given to 170Tm.'66Ho, 175Hf,and 169Ybwhose characteristic X-rays were well resolved by the LEPD detector, permitting their quantitative measurement. B. EPITHERMAL NEUTRON ACTIVATION (ENAA) Baedecker et al." applied ENAA to the instrumental neutron activation analysis of silicate rocks using planar intrinsic Ge detectors. A total of 8 U.S. Geological Survey standard rocks (7 rock samples and a marine sediment) ranging from granitic to basaltic composition, were analyzed for 15 trace elements. The irradiations were carried out for 2 d at a neutron ' a cadmium ratio of about 3.0 for '97Au. The samples flux of 1.5 x lOI3n ~ m - ~ s -and were counted for l-h, 1 week, and 3 weeks after irradiation, and the spectra processed on an IBM 370 computer using the program "SPECTRA". The results were compared with those previously obtained by the usual INAA procedure using the whole reactor neutron energy spectrum and a coaxial Ge(Li) detector. The measurement of X-rays gave very satisfactory results for Ba, Sm, Tb, and Ta (KX-rays) and for Th and U (L X-rays). Holzbecher and Ryan15 attempted to determine niobium in rocks, iodine in table salt, and uranium and thorium in Canadian certified reference material, uranium-thorium ore, by

128

Activation Analysis

irradiating the samples in a cadmium-shielded site, taking advantage of the low Cd ratios of the elements of interest. They arrived at the conclusion that for low energies (Nb-X-rays, 16.6 keV), the determination is not reproducible due to problems associated with absorption. With increasing energies, the method becomes more accurate. A standard addition method was necessary to obtain correct results for iodine (Te X-rays 27.5 keV) whereas for U and Th (74.6 and 86.5, low-energy gamma rays) the use of standards of similar matrix composition to the sample could adequately compensate all the errors. Garmann9 determined trace elements in geological materials by ENAA. Magnetic deflection of beta rays was used to reduce the bremstrahlung interference due to the high Na and Fe content of the samples (see Section III.B.4). A total of 18 elements were analyzed in alkalisyenite and meteoritic material (Grefsheim meteorite from Norway and Allende meteorite from New Mexico). Tb, Yb, Ta, U, and Th were determined by X-ray spectrometry using a planar hyperpure Ge detector and the U.S. Geological Survey basalt BSR-1 as standard.

C. 14-MeV NEUTRON ACTIVATION The possible analytical applications of X-ray spectrometry following 14-MeV neutron activation, have been in~estigated.'~,",~' AS mentioned before, this technique is not suitable for the determination of trace elements. However, it may be applied to the analysis of main and minor constituents of different matrices. Janczynszyn et al.1° successfully used this technique for the determination of tantalum in niobium and other industrial materials, with a detection limit of 0.1%. The same authorslo experimentally studied 24 elements and found that Sb, In, Cu, Ho, Ta, and Hg could be determined by this method with the same detection limit of O.1%, after optimizing the sample thickness to obtain optimum counting conditions. El Barouni et al." also studied the possible analytical applications of characteristic Xrays induced by 14 MeV neutrons and arrived at the conclusion that Se, Br, Ag, Cd, Sb, and Pr could be quantitatively determined by this technique.

REFERENCES 1. Shenberg, C., Gilat, J., and Finston, H. L., Use of X-ray spectrometry in activation analysis, Anal. Chem., 39, 780, 1967. 2. Pillay, K. K. S., Characteristic X-rays from (n, y) products and their utilization in activation analysis, J . Radioanal. Chem., 2 , 97, 1969. 3. Hertogen, J. and Gibels, R., Instrumental neutron activation analysis of rocks with a low-energy photon detector, Anal. Chim. Acta, 56, 61, 1971. 4. Mantel, M. and Amiel, S., X-ray spectrometry, in Nondestructive Activation Analysis, Amiel, S., Ed., Elsevier, Amsterdam, 1980, chap. 3. section 2. 5. Habib, S. and Minski, M. J., A compilation of X- and gamma-ray sensitivities from isotopes produced by the (n,y) reaction for utilization in instrumental neutron activation analysis, J . Radioanal. Chem., 62, 307, 1981. 6. Negi, B. S. and Sadasivan, S., X-ray emission intensities of radioisotopes produced by neutron activation, X-Ray Spectrom., 9, 159, 1980. 7. Mantel, M., Limits of detection of trace elements in biological materials analysed by instrumental neutron activation analysis using x-ray spectrometry and magnetic deflection of beta-rays, Analyst, 108, 1190, 1983. 8. Alfassi, Z. B., Biran-Izak, T., and Mantel, M., The removal of beta particles in the measurement of Xrays with a Si(Li) detector, Nucl. Instrum. Methods. 151, 227, 1978. 9. Garmann, L. B., Reduction of beta-interference in gamma-spectrometic measurements of neutron irradiated geological material, J . Radioanal. Nucl. Chem., 99, 75, 1986. 10. Janczynszyn, J. and Kwiecinskl, S., Utilization of characteristic X-rays in 14 MeV neutron activation analysis, J . Radiqanal. Chem., 56, 153, 1980.

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11. El Barouni, A. M., Bokos, L., and Zemplen-Dapp, E., Preliminary study on the analytical application of characteristic X-rays induced by 14 MeV neutrons, J . Radioanal. Nucl. Chem. Lett., 126, 407, 1988. 12. Strom, E. and Israel, H. Y., Photon cross sections from 1 keV to 100 MeV for elements 1 through 100, Nucl. Data Tables, 7, 565, 1970. 13. Weast, R. C., Ed., Chemical composition of rocks, F-151, Handbook of Chemistry and Physics, 64th ed., CRC Press, Boca Raton, FL. 1983-44. 14. Bode, P., De Bruin, M., and Korthoven, J. M., A method for the correction of self-absorption of low energy photons for use in routine INAA, J. Radioanal. Chem., 64, 153, 1981. 15. Holzbecher, J. and Ryan, D. E., Evaluation of same X-rays and low energy gamma-rays in instrumental neutron activation analysis, I . Radioanal. Nucl. Chem., 102, 507, 1986. 16. Allen, D. R. and Brookhart, W., Determination of copper in complex matrices by neutron activation analysis using X-ray detection, Anal. Chem., 46, 1297, 1974. 17. Weaver, N. Y., Determination of mercury and selenium in coal by neutron activation analysis, Anal. Chem., 45, 1950, 1973. 18. Mantel, M. and Amiel, S., Application of high resolution X-ray spectrometry to activation analysis, Anal. Chem., 44, 548, 1972. 19. Rapaport, M. S., Mantel, M., and Nuthman, R., Determination of bromine in blood serum by neutron activation analysis and X-ray spectrometry, Anal Chem.., 51, 1356, 1979. 20. Mantel, M., Shenberg, C., and Rapaport, M. S., Non-destructive determination of niobium in steel by neutron activation followed by X-ray spectrometry, J. Radioanal. Chem., 75, 145, 1982. 21. Baedecker, P. A., Rowe, J. J., and Steinnes, E., Application of epithermal neutron activation in multielement analysis of silicate rocks employing both coaxial Ge(Li) and low energy photon detector systems, J. Radioanal. Chem., 40, 115, 1977. 22. Mantel, M., Sung-Tung, P., and Amiel, S., Neutron activation analysis of thorium in rocks and ores by multiple y-ray peak ratio determination, Anal. Chem., 42, 267, 1970. 23. Keith, H. D. and Loomis, T. C., Calibration and use of a lithium-drifted silicon detector for accurate analysis of X-ray spectra, X-Ray Spectrom., 5, 93, 1976. 24. Zaitsev, I. E., Activation analysis based on characteristic and soft gamma radiation using differential filters, J . Radioanal. Chem., 11, 241, 1972. 25. Reed, S. J. B., and Ware, N. G., Escape peaks and internal fluorescence in X-ray spectra recorded with lithium drifted silicon detectors, J. Phys. E., 5 , 582, 1972. 26. Ungrin, A. and Johns, M. W., Germanium X-Ray escape peaks in the 40 to 41 1 keV range produced by small "windoless" Ge(Li) detectors, Nucl. Instrum. Methods, 70, 112, 1969. 27. Mantel, M., Alfassi, Z. B., and Amiel, S., Magnetic fields to eliminate beta-ray interference in measurement of X-rays following neutron activation, Anal. Chem., 50, 441, 1978. 28. Amiel, S., Mantel, M., and Alfassi, 2. B., Development of a new approach to trace element analysis using neutron activation followed by high resolution X-ray spectrometry, J. Radioanal. Chem., 37, 189, 1977. 29. Holzbecher, J. and Ryan, D. E., Study of thallium and bismuth characteristic X-rays, after neutron activation, for their determination, J . Radioanal Nucl. Chem., 81, 153, 1984. 30. Allen, 0. R. and Steinnes, E., Determination of niobium in geological materials by activation analysis with pre-irradiation separation, Anal. Chem., 50, 903, 1978. 31. Mantel, M. and Stiller, M., The determination of trace elements in active charcoal by INAA, J . Radioanal. Nucl. Chem. Lett., 105, 185, 1986. 32. Stiller, M., Mantel, M., and Rapaport, M. S., The determination of trace elements (Co, Cu, and Hg) in the Dead Sea by neutron activation followed by X-ray spectrometry and magnetic deflection of beta ray interference, J. Radioanal. Nucl. Chem.. 83, 345, 1984. 33. Peisach, M., Maziere, B., Loc'h, C., Comar, D., and Kellersohn, C., Rapid neutron activation of bromine using 6.1 minute bromine-82m: Application to the determination of bromine in blood plasma, J . Radioanal. Chem., 19, 269, 1974. 34. Voldet, P., Determination of the rare earths elements in basic rocks by neutron activation and high-resolution X-ray or gamma-ray spectrometry, Trends Anal. Chem., I , 262, 1982. 35. Muminov, V. A., Kheiderov, R. A., and Khamrakulov, T., Determination of rare earths using an Xray spectrometer, Zh. Anal. Khim., 34, 703, 1979. 36. Mantel, M. and Amiel, S., Determination of rare earths by high-resolution X-ray spectrometry following neutron activation, J. Radioanal. Chem.. 16, 127, 1973. 37. Navilikhin, L. V., in Metody Kontroloya Poluprovodn. Mater. Met., Khabibullaev, P . K., Ed., Tashkent, U.S.S.R., 1984, 37; Chem. Abstr., 102, 55229, 1985.

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Chapter 8

STABLE ISOTOPE DILUTION ACTIVATION ANALYSIS Masuo Yagi

TABLE OF CONTENTS I.

Introduction .....................................................................132

I1.

Description of the Method ......................................................132 A. Method for General Cases ...............................................132 B. Method for Special Cases ...............................................133

I11.

Discussion of the Method ....................................................... 134 A. Classification of the Method ............................................. 134 B. The Range of Elements which can be Determined ......................134 C. Obtaining the Enriched Stable Isotope ...................................135 D. Some Comments on the Chemical Processing ...........................136 E. Sensitivity of the Method ................................................ 137 F. Accuracy of the Method ................................................. 138 1. Amount of y ..................................................... 138 2. Interference Effects and Instrumental Errors .....................138

IV .

Applications ....................................................................139

References .............................................................................. 143

132

Activation Analysis

I. INTRODUCTION Many problems of activation analysis relate to accurate and precise determination of a particular element, often in very low concentrations. To solve the above problems, activation analysis itself requires a new method which has excellent precision and accuracy as well as sensitivity. Stable isotope dilution activation analysis' is the most suitable method and it is the purpose of this chapter to review the method, its advantages and limitations, and to indicate some research in which it has proved to be valuable.

11. DESCRIPTION OF THE METHOD A. METHOD FOR GENERAL CASES1 If it is assumed that an element of atomic mass M to be determined consists of at least two stable isotopes, MI and M, (abundance: 0, and 0,), which are converted easily to the radionuclides through particular nuclear reactions, the radioactivity ratio between two radionuclides produced in the sample containing x g of the element, R = a,/+, is expressed as

+

where F = flux of bombarding particles given by JOE- 4 (E) dE, where (E) is the flux of particles in the energy interval between E and E dE; a , , a, = cross-sections for the particular reactions of MI and M,, defined as So& ui (E) 4 (E) dE/F, where a (E) is the cross-section as a function of the particle energy; A,, A, = decay constants of radionuclides produced from MI and M,; t = time of irradiation, and A, = Avogadro's number. On the other hand, the reaction due to M, can be emphasized by adding y g of the isotopically enriched M, to the above duplicated sample. When the comparator as an isotopic mixture is prepared by using an isotopically enriched M, (atomic mass: M*), having the isotopic compositions of 0; for MI and 0; for M, (0; > 0,), and irradiated for the same time as above by particles with flux F*, and with the same energy distribution as that used for the sample, the radioactivity ratio between two radionuclides produced in this comparator, R* = a;/&, can also be written as

+

In this case, R* is given in a smaller value than that of R. Dividing Equation 2 by Equation 1, the R*/R ratio is given in a simple form as

Finally, the following equation is obtained for x

In Equation 3, it should be noted that several important terms for activation, such as flux, cross-section, Avogadro's number, and saturation factor are eliminated completely. The conditions for Equation 4 are x > 0,y > 0, and 1 > R*/R > 0~0210,0;. In the case of x

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= 0, the present method is unnecessary. In Equation 4, only the value of R*/R is unknown, but all the others are known. In order to evaluate x, therefore, the R*/R ratio should be measured cautiously. The radioactivity ratios of R and R* due to two radionuclides produced in the sample and comparator can be determined by measuring their detectable nuclear radiations. Gammaor X-ray spectrometry using germanium or silicon semiconductor detectors is the standard method of measurement and has become almost synonymous with instrumental activation analysis. If the above two radionuclides are measured by such a detector under the same conditions, R can be obtained as the ratio of the count rate in the net photopeak area under the selected gamma or X-line of a, at the end of irradiation to that of a, in the sample. Similarly R* can be obtained as the ratio of a; to a; in the comparator. Thus, Equation 4 makes it possible to determine the quantity of element A initially present in the sample even through the sample and comparator are irradiated separately by particles with different flux. On the other hand, when a highly enriched stable isotope is used as a spike to prepare the comparator, the value of 0;/0, ratio in Equation 4 is negligibly small as compared with the value of (R*/R) (0;/0,). In such a case, Equation 4 can be simplified to

B. METHOD FOR SPECIAL CASESZ The special method in which the self-shielding effect is negligible can also be introduced in a similar manner as above. Supposing that an element of atomic mass M to be determined also consists of at least two stable isotopes, MI and M, (abundance: 0, and O,), if an isotopically enriched M, (atomic mass: M*), having the isotopic compositions of 0; for M, and 01 for M, (0; > O , ) , is spiked to a sample containing x g of the above element, the nuclear reactions due to M, will be more emphasized than that in the natural element. When the sample is spiked with y g of the above enriched M,, and irradiated by particles with flux F*, the radioactivity ratio between two radionuclides produced in the sample, R* = a;/a;, is given by Equation 2. On the other hand, when an arbitrary amount of the element to be determined, z g, is irradiated as a comparator for the same time by particles with flux F f , but with the same energy distribution as that used for the above spiked samples, the radioactivity ratio between two radionuclides produced in this comparator, R' = a;/&, is also given by

In this case, R' is always given as a larger value than that of R*. Dividing Equation 2 by Equation 6, the following equation is finally obtained for x

It should be noted that the self-shielding effect of the comparator clearly differs from that of the sample spiked with the enriched isotope. Accordingly, Equation 7 should be applied only when the self-shielding effect of the sample is negligible, as in the case of photon activation. The conditions for Equation 7 are also x > 0, y > 0, and 1 > R*IRr > O;O,/ 0, 0;. In this case, the quantity x of element to be determined in the sample can also be evaluated by measuring only the value of R*/Rf ratio.

134

Activation Analysis

On the other hand, when the isotopically enriched M, of low quality is used as a spike, it is permitted to irradiate an arbitrary amount of it as a comparator instead of the natural element under investigation. If an arbitrary amount of the isotopically enriched M, of low quality, w g, is irradiated as a comparator for the same time as that of the spiked sample by particles with flux F', but with the same energy distribution, the radioactivity ratio between two radionuclides produced in this comparator, R" = a';/$, is also given by

In this case, R" always has a smaller value than that of R*. Dividing Equation 2 by Equation 8 also yields the following equation

The conditions for Equation 9 are x > 0, y

> 0, and 0, 0;/0;0, > R*/RV> 1.

111. DISCUSSION OF THE METHOD Discussion of the method may be divided into the following sections: 1.

2. 3. 4.

5. 6.

Classification of the method The range of elements which can be determined Obtaining of the enriched stable isotope Some comments on the chemical processing Sensitivity of the method Accuracy of the method

A. CLASSIFICATION OF THE METHOD This stable isotope dilution activation analysis is placed under the category of the internal standard m e t h ~ d Although .~ the method imposes a necessary condition that an element to be determined in the sample consists of at least two stable isotopes, one of the stable isotopes, MI, assumes an important role as an internal standard. Accordingly, this method makes it possible to evaluate accurately and precisely the amount of trace element to be determined in the sample, even though the sample and comparator are irradiated separately by particles with different flux.

B. THE RANGE OF ELEMENTS WHICH CAN BE DETERMINED As a result of mass-spectrographic investigations, it is well known that the elements with atomic numbers between 1 and 83 have on the average more than three stable isotopes each. All of 61 elements possess naturally occurring plural isotopes, and a further 20 elements are monoisotopic. Among the elements below bismuth, only two, technetium and promethium, do not have any stable species. The relative abundances of these isotopes in most cases are independent of the source of the material, and the stable isotopes of a given element occur together in constant proportions. Accordingly, the atomic weight determinations of a given element in widely different sources agree within experimental errors, although there are a few notable exceptions, such as abundances of lead isotopes, especially in ores containing uranium and thorium, and the abundance of 87Srin rocks containing rubidium.

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d B

* **

C N 0 FNe

9 Ir * * LaCePrNdhSmEuGdTbDyHoErTm)lbLu h

4

1

R

Ac Th Pa U N p P u AmCmBk C f Es FmMdNoLr

FIGURE 1. Elements which are likely to be determined by means of neutron activation.

Based on the above rule of the constant isotopic composition, the present method is established as well as the isotope dilution method in mass spectrometry. The method is characterized by that it can be applied only to elements which have more than two stable isotopes, and that it can also be applied to multielement determination in a wide variety of materials by various activation methods without any standard reference material. As a matter of course, however, the above applications would be limited to the elements having the following characteristics: two nuclides produced by particular nuclear reactions are radioactive, and formed in reasonable amounts. Both also have suitable half-lives, and decay by emission of detectable nuclear radiations. Such characteristic elements which are likely to be determined by activations using reactor neutrons and high energy photons' are shown in Figures 1 and 2, respectively. Though clear advantages of charged-particle activation are a variety of particles and a wide range of bombarding energies, as an example, elements applicable by proton activation are also shown in Figure 3. On the other hand, it has been proved that most of the elements containing the other inapplicable elements and monoisotopic can be determined effectively by applying another method,3-" i.e., the internal standard method coupled with the standard addition method developed recently by the present author.

C. OBTAINING THE ENRICHED STABLE ISOTOPE When one isotope is enriched, the others are depleted. This fact permits production of a radioactive nuclide in higher yield than with the natural element. Many enriched stable isotopes andlor isotopic mixtures sufficiently enriched in some isotopes are now available, and widely used to study nuclear chemistry, to produce specially designed radioisotopes, and to prepare useful labeled compounds. Such an enriched stable isotope is used as an effective spike to prepare the comparator containing the isotopic mixture which is different from the natural isotopic abundance. The enriched stable isotopes of most elements are available in milligram to gram quantities from the Isotopes Division of the United States Atomic Energy Commission at Oak Ridge, TN, and from the Atomic Energy Research Establishment in Harwell, England. Usually, the certification of isotopic analysis by mass spectrometry is served to such an enriched stable isotope together with the data of chemical impurities. The above isotopic

136

Activation Analysis

* **

t

r

r

L a c e PrNdRnSrnEu A c T h P a U NpPuAmCmBk C f E s F m M d N o L r

FIGURE 2. Elements which are likely to be determined by means of photon activation. (From Masumoto, K. and Yagi, M., J. Radioanal. Chem., 79(1), 68, 1983. With permission.)

* LaCeP r N d R n S m E u G d T b D y H o E r T m Y b Lu ** AclThPa U N p P u AmCmBkCf E s F m M d N o L r I

7

I\

f

r

r

\

-

-

F'IGURE 3. Elements which are likely to be determined by means of proton activation.

analysis can easily be checked by a suitable activation method, comparing with the natural element.

D. SOME COMMENTS ON THE CHEMICAL PROCESSING In preparation of the comparator, the chemical processing must ensure that the sample and the enriched stable isotope are mixed thoroughly in physical meaning. The comparator spiked with the enriched stable isotope in a known concentration may be prepared by either of the following two ways.

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In the first way, an exactly known amount of the enriched stable isotope or a material containing the same amount of that is mixed mechanically with a known quantity of the sample. In the second way, an aliquot of solution containing the enriched stable isotope is added to a known amount of the sample which was dissolved in a suitable diluent. The former is often very difficult to achieve adequate mixing of the sample and the enriched stable isotope in order to ensure homogeneity, and this difficulty is highly increased when more than two enriched stable isotopes are spiked. In the latter way, the chemical processing involves decomposition or dissolving of the sample and also necessarily requires a step where the final mixture is converted to a suitable physical form for irradiation. By this way, even the comparator spiked with several enriched stable isotopes can be easily prepared in such a manner that the solution containing a known amount of the sample is mixed with an aliquot of solution containing all of the enriched stable isotopes related to the elements to be determined. In the case of charged-particle activation, the self-shielding effects, which are often observed in the case of reactor neutron activation, can essentially be considered to be negligible. It is noteworthy, however, that the charged particles lose their energy within the target by excitation and ionization of the target atoms, and consequently, the nuclear reaction cross-section is varied markedly in the target. The rate of such energy loss per unit length inside the target, dE/dx, is well known as the stopping power of material, and it is closely related to the chemical compositions of the sample and comparator. Because of this stopping power, the sample and comparator should be prepared in the same chemical and physical form, and irradiated under the same conditions. The sample in this case, therefore, should also be processed chemically in the same manner as that of the above comparator. The above processings of the sample and comparator prior to irradiation produce the possibility of contamination. This is particularly dangerous in the cases of trace analysis for common elements. In the cases of minor constituent analysis or trace analysis for rare elements, this aspect becomes less important. In either case, however, it is preferred that the chemical processing is as simple as possible in order to minimize such contaminations, and that blank determinations would be included. On the other hand, when the relatively few predominant radionuclides in the gammaray spectrum of the sample obscure the contributions from less abundant indicator radionuclides, or when the half-lives and the radiation characteristics are very similar to each other, it may not be possible to resolve them simply by a purely instrumental method. This means that some elements at low concentrations are not detected, even though their actual sensitivities are sufficiently high. In such cases, it becomes necessary to resort to chemical separation. The decided advantage of activation analysis in comparison with the other analytical methods is the absence of blank values from reagents added after the end of irradiation. This opens the possibility of intentionally adding known amounts of the elements to be determined, which may serve as a chemical carrier. In the present method, moreover, it makes it possible to determine the interesting element without quantitative separation as well as that of the isotope dilution method in tracer chemistry.

E. SENSITIVITY OF THE METHOD In the present method, two radionuclides produced by particular nuclear reactions are proportional to amounts of two stable isotopes selected. These proportionality constants can be determined by parameters, such as activation cross-sections, flux density of activating particles, duration of irradiation, and half-lives of radionuclides produced. When these parameters are known, the sensitivity of the method may be calculated. Since the quantity of interesting elements in the present method is determined by measuring the ratios between two radioactivities produced in the sample and comparator, the above sensitivity is influenced by one of the selected stable isotopes, having a small value in the proportionality constant.

138

Activation Analysis

1

FIGURE 4. The relationships between x/y and R*/R or R*/R1 ratios in the case of Zn. (From Yagi, M. and Masumoto, K . , J . Radioanal. Nucl. Chem., 99(13), 293, 1986. With permission.)

On the other hand, the above sensitivity may also be affected by the following relevant facts. (1) The real detection limit of the element to be determined may well be dictated by the contamination introduced during the chemical processing of the sample or comparator prior to irradiation. This problem seriously affects the sensitivity of the method, so that special care should be taken to minimize blank corrections. (2) The activation by particles is not very specific, because the radioactivity is produced not only in the element to be determined, but also in many other elements contained in the sample. For this reason, measurement of the radioactivities must be made with the highest selectivity, most often in combination with radiochemical separation.

F. ACCURACY OF THE METHOD The main sources of errors in activation analysis are generally due to the facts that sample and comparator are not exposed to the same flux of bombarding particles, and that self-shielding effects are not always possible to eliminate, particularly when the matrix has a high absorption cross-section. In the present method, however, the nonuniformity of flux and differences of the self-shielding effects between the sample and comparator are compensated completely. Therefore, other sources of errors must be taken into account. A discussion of errors in the present method may be divided into the following two sections: (1) amount of y to be added, and (2) interference effects caused by other elements and instrumental errors.

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TABLE 1 Some Investigations by Stable Isotope Dilution Activation Analysis Investigation

Pair of isotopes

Ref.

Determination of Ca, Zn, and Ce by photon activation

Determination of Sr in biological materials by photon activation Determination of Sr in biological materials by proton activation Simultaneous determination of Ca, Rb, Sr, and Ce in environmental materials by photon activation

Determination of Sb in environmental materials by neutron activation

1. Amount of y When the present method is applied in practice, the quantity of y to be added to the sample is of importance. For example consider zinc. This element has five stable isotopes, 64Zn, 66Zn, 67Zn, 68Zn, and 70Zn, and in the natural element @Zn predominates - its abundance is 48.6%. An enriched 68Zn may be obtained with a @Zn/68Znratio of 0.00384. The relationship between x/y and R*/R in Equation 4 is shown in Figure 4 together with that between the x/y and R*/Rf ratio in Equation 7, because both relationships are identical. If the x/y ratio is small, the value of R*/R or R*/Rf ratio results in a small one which presents in the sensitive region to variation of y , so that the value of x to be determined will be subjected to large errors. If, on the other hand, the x/y ratio is large, the value of R*/R or R*/Rr ratio is obtained in a large one which also exists in the too sensitive region to variation of y. Consequently, determination of x is forced to serious errors. In this case, the optimum x/y ratio would be given when the value of R*/R or R*/R1 ratio is in the range from 0.3 to 0.7. Furthermore, much attention must also be paid to the following terms when the optimum x/y ratio is determined, that is, the count rate of gamma events in the net photopeak area of the selected gamma ray, the branching ratio of the selected gamma ray, the solid angle of the detector, and the detection efficiency of the detector for the selected gamma ray. Under normal conditions, the optimum x/y ratio may be given at R*/R or R*/ R' = 0.5, and this is found to be 4.5 for zinc. From this, the optimum quantity of the enriched stable isotope to be spiked to the sample can be determined easily.

2. Interference Effects and Instrumental Errors The interference effects caused by the other isotopes in the same element or the other elements may be subdivided into production of the same radionuclide by different nuclear reactions and overlapping of photopeaks in the same gamma-ray spectrum. Similarly, the instrumental errors may also be classified into random and systematic errors. However, these interference effects and instrumental errors would be given similar treatments to those in ordinary activation analysis.

IV. APPLICATIONS Some examples of the present method are listed in Table 1. The method using Equation 4 was carried out by proton activationI2 to determine strontium in tomato leaves [NBS] (National Bureau of Standards SRM-1572) and citrus leaves (NBS SRM-1573) using isotopically enriched 86Sr as a spike, and by neutron activationI4 to determine antimony in

140

Activation Analysis

TABLE 2 Isotopic Compositions of Enriched "Ca, 87Rb, %r, and '"Ce, and the Natural Abundances of the Respective Elements

Isotope

Enriched isotope composition (%)'

Element abundance (%)

"Ca 42Ca 43Ca "Ca T a "Ca Atomic mass 8SRb 87Rb Atomic mass "Sr 87Sr 88Sr Atomic mass 13Te W e '"Ce 14Te Atomic mass

"

As reported by Oak Ridge National Laboratories, Oak Ridge,

TN . From Yagi, M. and Masumoto, K., . I . Radioanal. Nucl. Chem., 99(13), 289, 1986. With permission.

environmental materials using isotopically enriched lZ1Sb.The method applying Equation 7 was also performed by photon activationZto determine strontium in the same biological materials as above using isotopically enriched 86Sr,and by photon activationI3to determine calcium, rubidium, strontium, and cerium in coal fly ash (NBS SRM-1633a), estualine sediment (NBS SRM-1646), and lake sediment (IAEA SL-1) using a mixture of isotopically enriched 48Ca,87Rb, and 142Ce. Although determinations of strontium in the above biological materials were carried out by proton and photon activations using a different pair of isotopes, the strontium fractions in the sample and comparator were isolated chemically together with most of the calcium before irradiation to avoid thermal decomposition of the sample and comparator during irradiation, and to eliminate interfering nuclear reactions due to matrix elements. As a matter of course, the above strontium fractions were isolated without quantitative separation. In order to prepare the most suitable physical and chemical form for irradiation, the above isolated fractions and/or the sample and comparator in the other cases were all processed finally as silica gel pellets, according to the procedure reported by Mitchell et al.I5 As a representative example of the method, the following describes the case of the simultaneous determination of four elements in the environmental materials.13 In Table 2, the isotopic compositions of the enriched 48Ca,87Rb,86Sr,and I4Te, and the natural isotopic abundances16 of the respective elements are listed together with their atomic masses. On the other hand, the principal photonuclear reactions leading to radionuclides on the four elements

1

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141

TABLE 3 Principal Photonuclear Reactions on Ca, Rb, Sr, and Ce Target nuclide (abundance, Half-life

%)

12.36 h 22.3 h 4.536 d 20.5 min 32.9 d 18.8 d 32.4 h 68 min 64.8 d 2.80 h 17.8 h 34.4 h 9.0 h 137.2 d 32.5 d

43Ca(0.135) MCa (2.09) " T a (0.187) 85Rb(72.17) 85Rb(72.17) 87Rb(27.83) @Sr(0.56) (9.8) 86Sr(9.8) (82.6) I3Te (0.190) (0.254) (0.254) I4"Ce (88.5) 14ZCe( 1 1.1)

Decay mode

Principal y-ray energy (MeV)

PPPIT

WP+ PECP'

IT ,EC EC IT EC,P+

IT,EC

WP+ EC

P

From Yagi, M. and Masumoto, K., J . Radioanal. Nucl. Chem., 99(13), 291, 1986. With permission

TABLE 4 Determination of Ca, Rb, Sr, and Ce in Environmental Materials Concentration of element (ppm) Sample

Ca

Rb

Sr

Ce

Coal fly ash

Average Certified value Estuarine sediment

Average Certified value Lake sediment

Average Certified value From Yagi, M. and Masumoto, K., J . Radioanal. Nucl. Chem., 99(13), 295, 1986. With permission.

to be determined are summarized in Table 3 together with their nuclear characteristics. Among them, the produced radionuclides and their gamma rays used for determination are underlined. The relationships between the x/y and R*/R or R*/R1 ratios for the interesting elements obtained by Equation 4 or Equation 7 are shown in Figure 5. From this, if the optimum x/y ratio would be given at R*/R = 0.5, it is found to be about 350 for calcium, about 3 for rubidium, 10 for strontium, and 7 for cerium, respectively. On the basis of the above ratios, the optimum quantities of four enriched isotopes to be spiked to each sample can easily be determined. As results, the concentrations of four elements on triplicate runs in three environmental materials are obtained as shown in Table 4. Although the sample

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Activation Analysis

FIGURE 5. The relationships between x/y and R*/R or R*IR1 ratios in the cases of Ca, Rb, Sr, and Ce.

and comparator in this case were irradiated separately under different beam intensities, all the results obtained are to be reproducible within the statistical limits. Moreover, the mean values are also in good agreement with either the certified or noncertified value within the usual limit of elemental analysis. The homogeneities of the sample and comparator processed chemically before irradiation can be checked by evaluating the reproducibility of data obtained on triplicate runs. As indicated by the values given in Table 4, the standard deviations from the means based on four elements are within 6% even in the case of maximum. From the above facts, it is concluded that the present stable isotope dilution activation analysis was demonstrated to be sensitive, highly specific, and reasonably accurate.

REFERENCES 1. Masumoto, K. and Yagi, M., Stable-isotope dilution activation analysis, and determination of Ca, Zn and Ce by means of photon activation, J . Radioanal. Chem., 79, 57, 1983.

2. Yagi, M. and Masumoto, K., Stable-isotope dilution activation analysis for special samples in which the self-shielding effect is negligible, J . Radioanal. Nucl. Chem., 90, 91, 1985.

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3. Yagi, M. and Masumoto, K., A new internal standard method for activation analysis and its application. Determination of Co, Ni, Rb and Sr in pepperbush by means of photon activation, J . Radioanal. Nucl. Chem., 83, 319, 1984. 4. Yagi, M. and Masumoto, K., A new internal reference method for activation analysis and its application. Determination of Ti, Cr, Ni and Zr in aluminium alloys by means of photon activation, J . Radioanal. Nucl. Chem., 84, 369, 1984. 5. Yagi, M. and Masumoto, K., Simultaneous determination of Ti, Cr, Fe, Cu, Ga and Zr in aluminium alloys by charged-particle activation analysis using the internal standard method, J. Radioanal. Nucl. Chem., 91, 379, 1985. 6. Masumoto, K. and Yagi, M., Highly accurate and precise multielement determination of environmental samples by means of photon activation using the internal standard method, J . Radioanal. Nucl. Chem., 100, 287, 1986. 7. Yagi, M. and Masumoto, K., Instrumental photon activation analysis of environmental materials using the internal standard method, J. Radioaml. Nucl. Chem., 109, 237, 1987. 8. Masumoto, K. and Yagi, M., Simultaneous determination of P, C1, K and Ca in several control serums by alpha-particle activation analysis using the internal standard method, J. Radioanal. Nucl. Chem., 109, 449, 1987. 9. Yagi, M. and Masumoto, K., Instrumental charged-particle activation analysis of several selected elements in biological materials using the internal standard method, J. Radioanal. Nucl. Chem., 111, 359, 1987. 10. Masumoto, K. and Yagi, M., Instrumental photon activation analysis of soil samples using the internal standard method coupled with the standard addition method, J . Radioanal. Nucl. Chem., 116, 375, 1987. 11. Masumoto, K. and Yagi, M., Revaluation of the internal standard method coupled with the standard addition method applied to soil samples by means of photon activation, J. Radioanal. Nucl. Chem., 121, 131, 1988. 12. Masumoto, K. and Yagi, M., Determination of strontium in biological materials by charged-particle activation analysis using the stable isotope dilution method, J . Radioanal. Nucl. Chem.. 91, 369, 1985. 13. Yagi, M. and Masumoto, K., Simultaneous determination of Ca, Rb, Sr and Ce in environmental materials by photon activation analysis using the stable isotope dilution method, J . Radioanal. Nucl. Chem., 99, 287, 1986. 14. Masumoto, K. and Yagi, M., Unpublished data, 1988. 15. Mitchell, J. W., Blitzer, L. D., Kometani, T. Y., Gills, J., and Clark, L., Jr., Homogeneously doped silica materials for trace element standards in neutron activation analysis, J. Radioanal. Chem., 39, 335, 1977. 16. Lederer, C. M. and Shirley, V. S., Eds., Table of Isotopes, 7th ed., John Wiley & Sons, New York, 1978.

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Chapter 9

SUBSTOICHIOMETRIC RADIOACTIVATION ANALYSIS Nobuo Suzuki

TABLE OF CONTENTS I.

Introduction ..................................................................... 146

I1.

Principle ........................................................................146

I11.

Substoichiometric Separation ...................................................147 A. Adsorption .............................................................. 147 B. Precipitation .............................................................148 C. Solvent Extraction .......................................................149

IV .

Application ..................................................................... 155 Determination of Oxygen using Substoichiometric Adsorption A. and Precipitation ........................................................155 Determination of Silicon using Substoichiometric Precipitation..........156 B. C. Determination of Chromium using Chelate Extraction ................... 156 Determination of Lanthanum using EDTA and 8D. Hydroxyquinoline .......................................................157 Determination of Uranium using Substoichiometric Separation E. of Barium or Lanthanum ................................................158 F. Determination of Antimony using Redox Substoichiometry ............. 160

References..............................................................................163

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Activation Analysis

I. INTRODUCTION Radioactivation analysis is one of the powerful techniques for the determination of many elements in a variety of matrices with different compositions such as geological, biological, and environmental samples. Especially neutron activation analysis (NAA) is particularly suited for this purpose. In particular, with the advent of sophisticated and computerized high resolution y-ray spectroscopy, NAA is adequate for instrumental determination of many elements. However, for example, the presence of predominant radionuclides, such as 24Na and 32Pfor biological samples masks the y-ray spectrum of the less abundant radionuclides. In this case, selective radiochemical separations must be introduced before the measurement of the neutron-activated samples. Various radiochemical separation procedures for NAA have been proposed, but usually the chemical separation is tedious to yield the complete recovery of the element of interest. Substoichiometric radioactivation analysis involves a chemical separation, but this does not require the complete recovery of the element of interest, hence the separation technique can be simplified. The substoichiometric radioactivation analysis has several advantages which can not be expected in instrumental radioactivation analysis and also in ordinary radiochemical radioactivation analysis.

11. PRINCIPLE The principle and the practical examples of the substoichiometric analysis are described in several review articles and monographs.'-6 The substoichiometric analysis is a definitive analytical method and has many excellent features compared with ordinary analytical methods including the radioactivation analysis. One of the characteristics of this method is summarized below: the element of interest can be determined by only the radioactivity measurement without any comparison with a standard sample. The present concept can easily be combined with the radioactivation analysis. The substoichiometric radioactivation analysis can be classified into the following three methods: 1. Direct method - The specific activity of a radioisotope induced by irradiation is NM,. If a known amount of the element m is separated, where m might be less than the total amount of the element M, (hence the term "substoichiometry"), the amount of the element of interest M, is easily determined by the measurement of radioactivity a

2. Method of carrier amount variation - The irradiated sample is divided into two equal parts; each contains the element of interest by the amount, M,. The specific activity, therefore, is AIM, = S. When a known amount of carrier (M) is added to one part, the specific activity becomes

From each part, the same amount of the element of interest is separated substoichiometrically and the radioactivities, a and a', are measured. The amount of the element is determined by Equation 3. This is based on the reverse isotope dilution principle, but by applying the stubstoichiometry, an unknown amount of the element of interest can be simply determined without any comparison with a standard sample. M, = Mal/(a - a')

(3)

3. Comparison method -After the samples to be analyzed have been irradiated together

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with the standard containing M,, a greater but equal amount of carrier M(M >> M,, M,) is added to both of them. The same quantity m(m > M) is separated from each and the radioactivities a, and a, are measured.

where A denotes produced radioactivity, accordingly the amount of the element of interest M, is determined by the following equation

In this method also, it is not necessary to pay any attention to the absolute amount of the separated portion m. The chemical procedure for the separation is much simpler than that of any ordinary destructive radioactivation analysis. Among these three, the first and the second methods do not require comparison with any standard, which means the errors to be involved in the radioactivation, such as fluctuation of neutron flux and self shielding can be avoided. Any interfering nuclear reaction produced the same nuclide to be measured does not introduce any error in contrast with conventional radioactivation analysis. It follows that for the successful determination of the element of interest by these methods, it is necessary to fulfill the following conditions: (1) the isotopic equilibrium between radionuclide and carrier must be achieved perfectly, (2) the exactly equal portion of the element must be constantly isolated from solutions. As clearly seen from Equations 3 and 4, the second and the third methods do not require any attention be paid to the chemical yield in the preliminary separation of the element of interest from an unknown sample with complex matrix composition. The second method is best among these three methods, however, in practice, the third method is widely used mainly due to simple substoichiometric separation in the presence of greater amount of carrier.

111. SUBSTOICHIOMETRIC SEPARATION Substoichiometric separation is performed by ordinary chemical separation methods, such as solvent extraction, ion exchange, precipitation, and electrochemical methods. In recent years, however, ion-exchange and electrochemical methods have not been used so much in substoichiometric separation. The precipitation technique is often used due to its simplicity, but the solvent extraction is most widely employed. This is because the procedure of solvent extraction is very simple and an appropriate extraction system can be selected from the great numbers of research papers dealing with solvent extraction of many different elements. A. ADSORPTION Adsorption of a fixed portion of the element of interest onto a constant amount of an adsorbent can be used for a simple substoichiometric separation. In general, adsorption mechanism is not so simple and can not be explained quantitatively, but the experimental procedure is simple and rapid, hence, if the substoichiometric separation is achieved, apparently this technique can be also used for the present purpose. Adsorption of a reproducible portion of fluorine onto various inorganic ion exchangers was examined.' A high adsorption

148

Activation Analysis

yield was observed in hydrated tin oxide, activated alumina, and commercial tin oxide. Similar substoichiometric adsorption of fluorosilicate onto hydrated tin dioxide was again clear.

B. PRECIPITATION The precipitation reaction of fluoride with a substoichiometric amount of lanthanum was used for the substoichiometric separation of fluoride, and this separation was also applied for the determination of oxygen in silicon crystals. Condition of the substoichiometric precipitation is theoretically di~cussed.~ If 50% of fluoride added (C,,) reacts with lanthanum in the substoichiometric precipitation, the following relation is obtained [HF]

1 2

= - CHF

where C, is the concentration of lanthanum added initially. If 99.9% added lanthanum reacts with fluoride, that is (La3+) = 0.001 C,, and by using the solubility product of lanthanum fluoride and the dissociation constant of hydrogen fluoride, the threshold pH can be given by Equation 6.

On the other hand, it can not be expected at pH higher than 8 that 99.9% of added lanthanum reacts with fluoride because lanthanum hydroxide is also precipitated at that pH. By substituting the solubility product of lanthanum hydroxide, the following expression is obtained for the value of the threshold pH:

When the concentration of fluoride added initially is 0.12 M, the pH range at which fluoride can be precipitated substoichiometrically with lanthanum is given by Equation 8:

Experimental results showed that fluoride can be precipitated substoichiometrically in the range between pH 2 and 8. The experimental results are little different from the theoretical one in lower threshold pH, but is in good agreement with that in higher threshold pH. It seems that this difference of lower threshold pH is caused by the kinetic effect in the precipitation of fluoride. A high reproducibility of the substoichiometric precipitation of fluoride with La was observed. The effect of coexistent elements for the precipitation of substoichiometric precipitation of fluoride, but sodium, copper, and manganese do not. A high reproducibility of the substoichiometric precipitation of fluorosilicate (SiF,Z-) with lanthanum was again clear, and in this case, the precipitation reaction was assumed as NhSiF, 2 La(NO,), + 3 H,0+2 LaF, + 2 NaNO, H,SiO, + 4 HNO,. The substoichiometric precipitation of fluorosilicate anion with barium as a counter cation was reproducible and used for the substoichiometric determination of ~ i l i c o nFigure .~

+

+

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t

FIGURE 1. Reproducibility of substoichiometric precipitation of fluorosilicic acid with barium (0.5 mmol barium added). (From Shikano et al., J. Radioanal. Nucl. Chem., 92, 309, 1985. With permission.)

1 shows the reproducibility of the substoichiometric precipitation of silicon as barium fluorosilicate using 3'Si tracer. The high reproducibility of the substoichiometric separation is only the essential point and the reaction ratio of the precipitate is out of concern in the substoichiometry, but the reaction ratio between fluorosilicate and barium was calculated simply from the inflection point in Figure 1 as 1:1 . The substoichiometric precipitation of the cation of interest with a substoichiometric amount of anion was examined. The example is a system of barium and sulfate ions.I0 The optimum conditions for the substoichiometric precipitation with an inorganic reagent can be derived from the solubility product, when 50% of barium initially added is used to form BaSO, in the substoichiometric separation without breaking down more than 0.1% of BaSO,, the threshold pH can be calculated from Equation 9.

where K,, is the solubility product of BaSO,, K, is the dissociation constant of HSO;, Cua2+and C,,: are the initial concentrations of Baz+ and SO:-, respectively. Assuming M, the threshold pH for the substoCBa2+= 4.5 X lo-' M, and C,,: = 2.25 x ichiometric precipitation is more than - 1.71. Figure 2 shows the dependence of the precipitated fraction of barium on the concentration of SO:-. The slopes of these curves indicate that the substoichiometric precipitation of barium can be performed with good accuracy and precision. From the equivalent points, the reaction ratio between barium and SO:- is found to be 1:l. The flat plateaus of these curves mean that SO:- is in excess of the barium content, and the unreacted barium is absent.

C. SOLVENT EXTRACTION Two extraction systems are commonly used: the chelate extraction system of metal ions with chelating agents, and the ion-association extraction system of metal ions with simple negative or positive ions. The ion-association extraction of ammonium phosphomolybdate (APM) has been frequently used for phosphorus separation. The ternary compounds of phosphomolybdate with organic agents, such as a-picoline, quinoline, oxine, and tetraphenylarsonium (TPA), have also been reported." All these extraction systems were used for the substoichiometric separation of phosphorus. Solvent extraction of 32P-labeledphosphorus as an ion associate with a substoichiometric amount of organic reagents, TPA and tri-n-octylamine (TNOA) is shown in Figure 3 where the dependence of phosphorus extraction

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Activation Analysis

A

"

0

0,2

0.4

0.6 SO:-,

0,8

1,0 1,2 1,4

mmo 1

FIGURE 2. Effect of the amount of SO:- on the substoichiometric separation of barium. Ba2+ (mmol): (1) 0.223; (2) 0.447; (3) 0.893. (From Katoh, M. and Kudo, K., J. Radioanal. Nucl. Chem., 84, 283, 1984. With permission.)

x10-~ phosphorus concentration, M FIGURE 3. Extraction of phosphorus with a substoichiometric amount of organic reagent. C,, = 9.29 x lo-) M, C,,, = 1.13 x 10-'M, and q , = 3.76 x M. (1) TPA and (2) TNOA. (From Shigematsu, T. and Kudo, K., J. Radioanal. Chem., 67, 33, 1981. With permission.)

on the quantity of phosphorus carrier was examined in the presence of excess molybdenum. It is clear that phosphorus can be separated substoichiometrically with a substoichiometric amount of TPACl or TNOA. The phosphorus concentrations at the equivalence points, for M and 1.30 x M, respectively, then the added TPA and TNOA are 3.60 x ratio of phosphorus to organic reagent is 1:3 for both TPA and TNOA. The substoichiometric extraction of phosphomolybdate with TPA in dichloroethane was applied to niobium sample. For a 100-mg sample, and irradiation time of 12 h with a thermal neutron flux of 8 x 10"

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FIGURE 4. Effect of pH on the substoichiometric extraction of chromium: (a) TPACI, (b) TNOA, ( c ) DDDC, (d) APDC, (e) NaDDC. (From Shigematsu, T. and Kudo, K . , J. Radioanal. Chem., 59, 66, 1980. With permission.)

n cmp3s-I, the detection limit of the substoichiometric activation analysis is estimated as low as 2 ng g-I. The substoichiometric extraction of chromium with a chelating reagent, such as sodium diethyldithiocarbamate (NaDDC), or with an ion-association reagent, such as TPA, was s t ~ d i e d . ' ~The - ' ~ substoichiometric extraction of chromium with TPA, TNOA, NaDDC, diethylammonium diethyldithiocarbarnate (DDDC), and ammonium pyrrolidinedithiocarbamate (APDC) was examined in detail.15 The effect of pH on the substoichiometric extraction of Cr(V1) with these reagents is shown in Figure 4, where the Cr(V1) concentration M for is 1.96 x lo-' M for extraction with ion-association reagents and 3.84 X chelating reagents, and substoichiometric amounts arbitrarily chosen are used in each extraction system. The optimum pH ranges for ion-association systems differ from those for chelate-extraction systems. The good reproducibility of substoichiometric extraction with these reagents was observed by extraction using a constant but substoichiometric amount of reagent from a series of Wr(V1) solutions containing different amounts of Cr(V1) carrier. Substoichiometry by using two complexing agents, a complexan and an extracting agent, is the method based on the formation of water-soluble complex with a substoichiometric amount of complexan followed by the separation of the unmasked element of interest by chelate-solvent extraction. This method is based on the differences of the reactivity of the competitive complexing agents used. The use of complexans as masking reagents for the substoichiometric separation is expected to extend the number of systems which can be applied for the substoichiometric determination of trace elements, and to increase the selectivity in the substoichiometric determination by nobel combination of two complexing agents with different formation constants for the element of interest and the coexisting elements. Possibility and applicability of this type of substoichiometry have been studied. 16-16 Equations have been derived to calculate the optimum conditions for the substoichiometric separation of elements and these theoretical results were examined experimentally. The substoichiometric separation of lanthanum by using EDTA and 8-hydroxyquinoline was examined and applied to the determination of lanthanum by activation analysis.19 The optimum conditions for the substoichiometric separation of the element by using two complexing agents were calculated from the following equations.

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Activation Analysis

where D and D' are the distribution ratios of the element of interest, M, between the aqueous and the organic phases in the absence and in the presence of a complexan in aqueous phase, respectively. HiY is complexan and i is its basicity. MY is the water-soluble complex of M with HiY. HA is the extracting reagent. K,, is the thermodynamic stability constant of MY. K,, is the extraction constant of M, ai is a factor which determined the quantitative distribution of any species of the complexan H,Y as a function of the pH of medium, and a, is a factor expressed as 1 + UP,, + KJ(PHA[H])where P,, and K, are the partition coefficient and the dissociation constant, respectively. In the extraction system of M with HA in the presence of H,Y, the extracting agent for the substoichiometric separation of the element of interest can be chosen from the relationship between the distribution ratios D' and D by using Equation 10. The condition for the substoichiometric separation is given as the constant region of D, and this constant region is dependent on the substoichiometric fraction shown as C,/CHlY. Equation 11 represents the concentration region of the extracting agent which can extract the unmasked element from a solution at the condition of C,/CHiy = 2. This condition is 50% stoichiometry without breaking down more than 0.1% of the complex with the complexan. EDTA was chosen as a complexan for the substoichiometric separation of lanthanum. Log Dr-log D plots of the lanthanum-EDTA system are shown in Figure 5. Three regions in Figure 5 can be observed on the log D' curves. In the region of log D' = 0, lanthanum can be extracted substoichiometrically since equilibrium exists between the competitive complexan and the extracting agent. The log D' < 0 region corresponds to the region where lanthanum unreacted with EDTA can not be extracted completely, while the log D' > 0 region corresponds to the region where the substitution of EDTA from the lanthanum-EDTA complex by the extracting agent takes place. 8-Hydroxyquinolinewas chosen as an extraction agent for uncomplexed lanthanum. From Figure 5 8-hydroxyquinoline was expected to be suitable for the extraction of unreacted lanthanum. Figure 6 shows the effect of pH on the substoichiometric separation of lanthanum under the condition of 50% stoichiometry; in other words, half of the lanthanum is complexed with EDTA. The flat plateau indicates that the substoichiometric separation of lanthanum can be performed in this region. The pH region corresponding to this plateau is 7.3 to 8.5, and this is in good agreement with the optimum pH region calculated theoretically. It is well known that a synergic extraction system involving an acidic chelating agent and a neutral basic ligand provide better extraction efficiency of the element of interest. In contrast to the number of research papers on the synergic extraction with various combinations of different types of reagents, very few papers on substoichiometry using synergic extraction have appeared. Applications of synergic extraction to substoichiometry are very interesting and sugggest further possibilities of substoichiometric separation. The application of the synergic extraction system to substoichiometric determinaton of calcium, uranium, manganese, and vanadium has been In the substoichiometric extraction of synergic extraction system using two reagents, two substoichiometric combinations are possible: the system with a substoichiometric amount of chelating agent and an excess of neutral ligand, and vice versa. Both systems for the substoichiometric extraction of uranium were theoretically disc~ssed.~' When 100x% of the chelating agent HA initially added is used to form the extractable adduct complex MA,L, in the presence of an excess of the neutral ligand L, pH of the aqueous phase can be readily calculated from the synergic extraction constant K,,.,

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b

6

10

8

log

D

FIGURE 5. Log Dl-log D plot of lanthanum in the presence of a constant substoichiometric amount (C,,,/C, = 0.5) of EDTA in the aqueous phase and \arious pH values. C,,, = M; pH: 7.0 for Curve 1 , 8.0 for Curve 2 , and 9 . 0 for Curve 3. (From Katoh, M. and Kudo, K., J. Radioanal. Chem., 79, 28, 1983. With permission.)

where C, V, P, and K,,,, are initial concentration, phase volume, partition coefficient, and synergic extraction constant, respectively, and subscript o denotes organic phase. For the substoichiometric determination of uranium, the extraction system involving a substoichiometric amount of 2-thenoyltrifluoroacetone (TTA) and an excess of tributylphosphate (TBP) was compared with a system involving a substoichiometric amount of trioctylphosphine oxide (TOPO) and an excess of TTA." In both systems, a high reproducibility of the substoichiometric extraction under 1% relative standard deviation (RSD) is possible. Another modified synergic extraction system was examined for the substoichiometric extraction of lower amounts of uranium in complicated matrix samples as phosphate rock. Hexafluoroacetylacetone (HFA) is one of P-diketones but stronger acid than TTA and can scarcely extract uranium into nonpolar solvent, furthermore, it is expected to have a larger synergic effect for U(V1) in the presence of a neutral ligand than the synergic system containing TTA. Effect of pH on the substoichiometric extraction of U(V1) with a substoichiometric amount of HFA in the presence of a large excess of TOPO was examined as shown in Figure 7.24A constant amount of U(V1) is extracted at pH over 4, and the extracted

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Activation Analysis A

I

FIGURE 6. Effect of pH on the substoichiometric separation of lanthanum. (From Katoh, M. and Kudo, K., J . Radioanal. Chem., 79, 33, 1983. With permission.)

FIGURE 7. Effect of pH on substoichiometric extraction. [U(VI)] = 3.0 x lo-' M; @[HFA] = 6.4 X 10-'M, [TOPO] = 1.8 X M, O[HFA] = 1.3 X l o - * M,[TOPO] = 1.0 X M. (From Suzuki et al., J . Radioanal. Nucl. Chem.. 97, 83, 1986. With permission.)

amount of U(V1) is in good agreement with the amount expected from the substoichiometric reaction of TOPO by considering the extracted species of UO,(HFA),TOPO. The extraction of U(V1) with an excess of HFA alone was negligible. A substoichiometric extraction was made to aqueous solutions containing various amounts of U(V1). Table 1 indicates the high reproducibility of the present substoichiometric extraction.

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TABLE 1 Reproducibility of

Substoichiometric Extraction Amount of U ( ~ g m

Activity of organic phase ( C P ~

Average: 7338 + 69 R.S.D.: 0.94% Note: [HFA] = 1.3 X lo-' M ,[TOPO] = 1.1 x lo-' M, pH 4.1 to 4.6.

From Suzuki et al., J . Radioanal. Nucl. Chem., 97, 85, 1986. With permission.

IV. APPLICATION A. DETERMINATION OF OXYGEN USING SUBSTOICHIOMETRIC

ADSORPTION AND PRECIPITATION The substoichiometric adsorption of fluoride onto hydrated tin dioxide was applied for the determination of oxygen in silicon crystals by neutron activation analysis via the nuclear reaction of 6Li ( n p ) t+160(t,n) 18F.' After irradiation of test and standard samples in a nuclear reactor, the test sample was dissolved in concentrated sodium hydroxide solution containing 12 mmol fluorine carrier by gentle heating and pH of the solution was adjusted to 8 to 9 with nitric acid. The hydrated silicon dioxide precipitated was filtered. The filtrate was adjusted with 1 M nitric acid and passed through the column packed with hydrated tin dioxide (2 g). The radioactivity of the column was measured by a coincidence counting system connected with two 3 in. X 3 in. NaI(T1) detectors. After appropriate dissolution of the standard sample, the solution obtained was treated exactly in the same way as the test sample. The oxygen content was calculated simply from both activities (Equation 4). The oxygen content of 28 ppm determined by the present method agreed with that determined by a nondestructive method 26 ppm. In the activation analysis of oxygen, "F produced by charged-particle activation as 160('He,p) 18F has frequently been employed for the determination of trace amounts of oxygen. The radiochemical separation of "F is required for the accurate determination of oxygen at ppb level in various materials inducing high radioactivity by irradiation. In the charged-particle activation analysis of oxygen in gallium arsenide, 76Br produced by the nuclear reaction of 75As('He, 2n) 76Brinterferes with the measurement of 'F activity. It is tedious to separate I8Fwith high purity and to correct the chemical yield in its radiochemical separation. Substoichiometric precipitation of fluoride with lanthanum was effectively introduced. *' Gallium arsenide sample wafer with the size of 20 mm x 20 mm X 0.5 to 1.0 rnm was set on a target holder and irradiated in a cyclotron with 3He at 15 MeV for 20 rnin. As a standard sample, fused quartz with the size of 20 rnm x 20 mm x 0.5 m m was irradiated. The irradiated sample wafer was etched to remove the surface contamination and dissolved

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Activation Analysis

TABLE 2 Analytical Result for Oxygen in Gallium Arsenide Sample no.

Location in crystal

Oxygen (ppb)

1 2 3 4 5 6 7

Front Front Middle Middle Tail Tail Tail

13 10 19 18 24 26 36

From Shikano, et al., J . Radioanal. Nucl. Chem., 91, 89, 1985. With permission.

in a solution of nitric and hydrochloric acids containing 12 mmol sodium fluoride. After two fractional distillations, a substoichiometric amount of lanthanum solution (1 mmol) was added to the filtrate and fluoride was precipitated as lanthanum fluoride at pH 5 to 6. The standard sample was washed and fused with sodium carbonate containing sodium fluoride. The melt was dissolved in hot water and the pH of the solution was adjusted to 8 to 9 with nitric acid. Hydrated silicon dioxide was centrifuged and the filtrate was treated in the same way as the test sample. Analytical results of oxygen are summarized in Table 2. It is found from the table that the concentration of oxygen is 10 to 36 ppb and increases a little along the position such as fron,t, middle, and tail in gallium arsenide crystal. The time required for the separation was 2 h, the detection limit was 10 ppb, and the decontamination factor of 76Brwas approximately lo6.

B. DETERMINATION OF SILICON USING SUBSTOICHIOMETRIC PRECIPITATION Silicon-doped gallium arsenide was used as test sample and a small piece of silicon wafer was used as standard sample. The test and standard samples were irradiated in a nuclear reactor for 5 min. After irradiation, the test sample was etched in the mixture of sulfuric acid, hydrogen peroxide, and redistilled water. Test sample was dissolved in a mixture of nitric and hydrochloric acids containing a silicate solution of 2 mmol as a camer. After addition of sulfuric acid and hydrofluoric acid, silicon was distilled as silicon tetrafluoride. The distillate was collected in a mixture of boric acid and hydrochloric acid. A small portion of hydrofluoric acid and a substoichiometric amount of barium chloride (0.5 mmol) were added to the distillate and silicon was precipitated as barium fluorosilicate. Standard silicon wafer was dissolved in a mixture of nitric and hydrofluoric acids. The solution was added to a mixture of boric acid and hydrochloric acid and treated in the same way as the test sample. The concentration of silicon in the test sample was determined by comparing the radioactivities of 31Sifrom the test and standard samples. The results are summarized in Table 3.9

C. DETERMINATION OF CHROMIUM USING CHELATE EXTRACTION The substoichiometric extraction of chromium with DDDC and APDC was applied for the determination of chromium in high purity calcium carbonate.'* Sample and standard were irradiated in a nuclear reactor at a thermal flux of 3 X lOI3 n s-' for 260 h. After an addition of 2 mg Cr(V1) carrier to the irradiated samples, these samples were dissolved by appropriate treatment. Cr(V1) was extracted from acidic media with methyl isobutyl ketone and was back-extracted with distilled water into aqueous solution. Chromium

.

.

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TABLE 3 Analytical Result for Silicon in Gallium Arsenide

Sample no.

Sample weight (mg)

Content in test sample (MA (14

Concentration (PP~)

0.303 0.274 0.364 0.280 0.320 0.321 0.253 0.254 Mean value Sequential substoichiometric precipitation from the same final solution of sample no. 1-1 or 4-1. From Shikano et al., J. Radioanal. Nucl. Chem., 92, 313, 1985. With permission.

TABLE 4 Analytical Results for Chromium in Calcium Carbonate Extracting agent

DDDC APDC

NaDDC

Weight of test sample (9)

Content (I%)

Concentration ( P P ~

0.2035 0.2035 0.2038 0.2085

0.045 0.044 0.043 0.045

0.22 0.22 0.21 0.22 0.22

From Shigematsu, T. and Kudo, K., J . Radioanal. Chem., 59, 70, 1980. With permission.

was extracted with a substoichiometricamount of DDDC or APDC from the aqueous solution of pH 5.6. The radioactivity of the organic extract was measured by a Ge(Li) detector. Analytical results of chromium in high purity calcium carbonate are summarized in Table 4.

D. DETERMINATION OF LANTHANUM USING EDTA AND 8HYDROXYQUINOLINE The substoichiometric separation of lanthanum by the solvent extraction with oxine in the presence of a substoichiometric amount of EDTA was applied to the lanthanum determination in some material^.'^ Orchard leaves and spinach of National Bureau of Standards (NBS) standard reference materials were dried in an air oven at 85°C for more than 4 h. About 0.5-g sample of NBS standard reference materials was irradiated together with a lanthanum standard in a nuclear reactor for 10 min at a thermal neutron flux of 8 X 10-13 n cm-2 s ~ ' After . irradiation, lanthanum carrier was added to the sample and standard. NBS standard reference materials were dissolved in a perchloric acid and nitric acid mixture by adding hydrogen peroxide, and evaporated nearly to dryness. The evaporated residue

158

Activation Analysis

was dissolved in water and hydrofluoric acid and was added to the solution. The precipitated lanthanum fluoride was centrifuged and dissolved in a boric acid and nitric acid mixture. To the solution, aqueous ammonia was added and lanthanum hydroxide was separated by centrifugation. After dissolution of lanthanum hydroxide, pH of the solution was adjusted above 5.5. Lanthanum was extracted with 0.5 M ?TA in methyl isobutyl ketone. To the organic phase, a diluted perchloric acid was added and lanthanum was back-extracted. Rock crystal sample was dissolved in a hydrofluoric acid and perchloric acid mixture. The solution was evaporated nearly to dryness. The residue was dissolved in hydrochloric acid and lanthanum was extracted with di(2-ethylhexy1)phosphoric acid (HDEHP) in toluene. Lanthanum was back-extracted with diluted hydrochloric acid. The pH of the lanthanum solutions obtained here was adjusted to greater than 9.5. A 10-ml portion of 0.25 M 8-hydroxyquinoline in chloroform was added and lanthanum was extracted. The organic phase was shaken with 10 rnl of 3.0 x M EDTA solution and lanthanum was separated substoichiometrically into the aqueous phase followed by the radioactivity measurement. The analytical results of lanthanum in orchard leaves, spinach of NBS standard reference materials, and rock crystal were 1.4 ppm, 315 ppb, and 52 ppb, respectively. The substoichiometric method using EDTA and 8-hydroxyquinoline is suitable for the determination of trace amount of lanthanum.

E. DETERMINATION OF URANIUM USING SUBSTOICHIOMETRIC SEPARATION OF BARIUM OR LANTHANUM Substoichiometric isotope dilution analysis for U in a synergic extraction system has been r e p ~ r t e d . ~Substoichiometry '.~~ was applied to the determination of trace amounts of uranium via nuclear fission of 235Uas 235U(n,f)'40Ba+140La. This is based on the substoichiometric separation of a fission product lmBa or its daughter product lWLaafter neutron irradiation of samples.'O Three methods were examined for the determination of uranium using the substoichiometric separation of 140Baor ""'La. In Method 1, I4OBa is separated substoichiometricallyas barium sulfate, and left to reach radioactive equilibrium between 140Baand ''"'La. Uranium is determined by measuring the activity of I4"La. In Method 2, a fission product of 14Ba is separated, and left to reach radioactive equilibrium. I4OLa is separated substoichiometrically, and uranium is determined by measuring the activity of lNLa. The substoichiometric separation of lanthanum was made by extraction technique using two complexing agents, DTPA (diethylenetriarninepenta-acetic acid) and TTA. In this method, it is necessary to correct the chemical yield of lmBa, because barium is separated by an ordinary method prior to the final substoichiometric separation. Therefore, a known quantity of 13'Ba tracer is added to the test sample for the chemical yield correction. Method 3 is based on the substoichiometric separation of lanthanum. When the radioactive equilibrium between IWBa-lWLais established, the substoichiometric separation of lanthanum is carried out using two complexing agents described above and uranium is determined by measuring the activity of '@La. In this method, lWLaproduced independently by (n,y) reaction of lanthanum in sample interferes in the determination of uranium. Thus, it is necessary to apply the substoichiometric separation for '40La produced by fission of "W after decay out of laLa produced by (n,y) reaction of lanthanum, or to correct the contribution of '@La produced by (n,y) reaction. Uranium is determined by Equation 4 in Methods 1 and 3, and Equation 13 in Method 2.

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I

Ba ,La carries HNOj -HCI mixture NH40H,Na2C03 solution

dil.HCI ~ e carrier ~ '

P Ba solution

Fe(OHh ,La(OHI3

[+, Method-1

Method - 2

Method-3

FIGURE 8. Analytical procedure for the determination of uranium using substoichiometric separation of barium or lanthanum. (From Katoh, M. and Kudo, K . , J. Radioanal. Nucl. Chem.. 84, 281, 1984. With permission.)

where M and a are the amount of uranium and the activity of I4OLa, y is the chemical yield of barium (100 y %). Analytical procedure for uranium in molybdenum sample as an example is shown in Figure 8. After irradiation of standard and molybdenum samples, barium and lanthanum carriers were added. In Method 2, barium carrier labeled with '33Ba was added for the chemical yield correction of 140Ba. Molybdenum was dissolved in nitric and hydrochloric acid mixture, other samples of rock crystal and glass were dissolved in perchloric and hydrofluoric acid mixture, and silicon wafer sample was dissolved in nitric and hydrofluoric acid mixture. Barium and lanthanum were finally precipitated as barium carbonate and lanthanum hydroxide, respectively. In Method 1, barium carbonate was dissolved in dilute hydrochloric acid. By adding a diethyl ether-hydrochloric acid mixture to the solution, barium was separated as barium chloride precipitate. The precipitate was dissolved in diluted hydrochloric acid and barium was precipitated substoichiometricallyas barium sulfate by adding a substoichiometric amount of sodium sulfate solution. In Method 2, after reaching radioactive equilibrium between I4OBa and I4'La, lanthanum and ferric carriers were added and barium carbonate was dissolved in dilute hydrochloric acid. Lanthanum and ferric hydroxydes were precipitated by aqueous ammonia. These hydroxides were dissolved in hydrochloric acid, and the ferric carrier was extracted with isopropyl ether. Lanthanum was extracted with HDEHP in toluene solution, and then back-extracted with hydrochloric acid. Lanthanum was separated substoichiometrically by extraction with TTA in the presence of a substoichiometric amount of DTPA. In Method 3, lanthanum was separated by the same treatment as in Method 2 and the substoichiometric separation of lanthanum was carried out successively.

160

Activation Analysis

TABLE 5 Analytical results for uranium (ppb) Samples Mo-1 Mo-2

Rock crystal Glass Silicon wafer

Method 1

Method 2

Method 3

80 k 5

84 2 5 60?5

81 +. 5

194

?

<2

24

196

?

16

194 2 29 <0.08 <0.05

From Katoh, M. and Kudo, K., J. Radioanal. Nucl. Chem., 84, 288, 1984. With permission.

Analytical results for uranium by these three methods are summarized in Table 5, and uranium contents in molybdenum samples determined by three methods are in good agreement with each other. Uranium contents in rock crystals determined by the three methods are also in good agreement each other. Uranium contents in glass and silicon wafers were determined by Method 3. The analytical results were less than 0.08 ppb and less than 0.05 ppb, respectively. From these values, the detection limit was considered to be about 0.05 ng g-'. It is found that the substoichiometric methods are superior for the determination of trace amounts of uranium.

F. DETERMINATION OF ANTIMONY USING REDOX SUBSTOICHIOMETRY Redox substoichiometry is based on the partial oxidation or reduction of the element of interest with a substoichiometric amount of oxidizing or reducing agent. Redox substoichiometry can be combined with the neutron activation analysis.26In the radioactivation analysis of antimony, after the substoichiometric oxidation of antimony(II1) with permanganate or potassium bromate, unoxidized antimony (111) was extracted with an excess of Nbenzoyl-N-phenylhydroxylamine (BPHA). Irradiated tin sample (about 100 mg) was dissolved in hydrochloric acid containing hydrogen peroxide. After complete dissolution, sulfur dioxide gas was bubbled through the solution for 20 min to completely reduce all antimony(V) to antimony(III), with gently heating. To a portion of the solution, 6.02 mg of antimony(II1) carrier was added. The substoichiometric amount of antimony(II1) was then oxidized by the addition of a small portion of potassium bromate, and unoxidized antimony(II1) was extracted with an excess of BPHA. An aliquot of the organic phase (or the aqueous phase) was taken out for the measurement of activity. The antimony standard was treated in the same way. The reproducibility of substoichiometric oxidation of antimony(II1) was studied by oxidizing various amounts of antimony(II1) with a constant but substoichiometric amount of potassium bromate. Figure 9 shows that the amounts of antimony(V) produced increase linearly until an equivalence point with the substoichiometric amount of the oxidizing agent and after which the amounts of antimony(V) produced remain constant. The inflection point in Figure 9 is in good agreement with the antimony quantity equivalent to the added quantity of potassium bromate. The same experiment was performed in the presence of target element of tin(IV), but tin(1V) had no disturbing effect on the quantitative substoichiometricoxidation. In the application of this method to the determination of antimony in metallic tin, it is not necessary to separate antimony(II1) from the matrix and any other elements, such as arsenic(II1) before substoichiometric oxidation. In the substoichiometric oxidation after an addition of large amounts of antimony(II1) carrier, the amount of the oxidizing agent consumed by trace amounts of other elements is negligibly small. By the proposed method, the synthetic tin sample (10 mg) containing antimony(II1) of the known quantity (3.92 ~ g was ) analyzed as 3.96 ? 0.06 kg with good agreement and reproducibility. The analytical results of antimony in 10 mg metallic tin (chemical pure reagent grade) was 1.22 + 0.14 ~g .

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FIGURE 9. Substoichiometric oxidation of antimony(II1) with potassium bromate. (From Kambara et al., Radioisotopes, 29, 592, 1980. With permission.)

Finally, some recent papers on the practical application of the substoichiometric to the It is clear that the substoichiometry radioactivation analysis are summarized in Table 6.3,27 is a reliable analytical method and has many excellent features compared with ordinary analytical methods. The characteristics of the substoichiometric radioactivation analysis are summarized as follows. In principle, the element of interest can be determined by only the radioactivity measurement without any comparison with calibration curve; this means the present method has a possibility as a definitive analytical method. High accuracy and precision of the analytical results can be expected. The method involves a chemical separation procedure, but never complicated compared with ordinally analytical methods involving chemical separations. It is not necessary to pay any attention to the complete recovery or the chemical yield correction in the separation of the element of interest. The sensitivity is nearly equal or much better than that of the instrumental radioactivation analysis because the chemical separation of the element of interest is favorable to the reliable radioactivity measurement of the radioactive nuclide of interest free from interference due to matrix background radioactivity. Considering these characteristics of the substoichiometric radioactivation analysis, more popular application of this method can be expected in the near future.

162

Activation Analysis

TABLE 6 Substoichiometric Radioactivation Analysis Element Ag As Au

Cd Co Cr Cu

Ga Hg I In La La, Eu, Tb

Mo Na Ni 0

P Pd

n Sb

Separation system Ex. Dithizone-CC1, Ex. KBrO,Ce(SO,),, Thionalide1,2-dichloroethane Ex. TNOPS-cyclohexane Ex. TNOPO-cyclohexane Ex. Rhodamine B-CHCI, EX. INAP-CHCI, Ex. PMBP-isoamylalcohol Ex. I-nitroso-2-naphthol-CHCI, Ex. DDDC-Bz, APDC-BZ EX. NaDDC-MIBK EX. ETAA-CHCI, Ex. Neocuproine CHCI, Ex. Dithizone CCI, Ex. TNOPO-cyclohexane Ex. Bindscheller's Green-1.2dichloroethane EX. TPA-CHCI, Ex. NaDDC-CCl, Ex. Oxime-CHC1, Ex. EDTA-oxime-CHCl, Ex. DTPA-TTA-Bz EX. DTPA-TTA-BZ

Ex. TNOA-Toluene Ex. Crown-TPB-CHC1, Ex. NaDDC-toluene Ex. INAP-CHC13 Ad, SnOjnH,O Ppt, LaF, Ppt, L a h Ex. AM-MIBK Ex. AM-TPA-I ,2-dichloroethane EX. HEINA-CHC1, Ex. Dithizone-CC1, Ex. HEINA, CETAB-nitromethane EX. KBIO,, BPHA-CHCl, Ex. TPA-CHCI,, 1,2-dichloroethane EX. BPHA-CHCI, Ex. KMnO,, BPHA-CHCI, Ex. Cupferron-CHC1,

Sample

Content determine

ZnSe Biological materials

42, 32 ppb 9.7, 60.4 ppm

Water sample SiO, ZnSe Environmental samples Biological materials ZnSe Brewers yeast Glass, ZnSe Muscle Biological materials Biological materials, glass, ZnSe Al alloy Biological materials

0.42 pg/ml 26.6 2 0.3 ng 0.36, 0.076 ppb 0.012-5.4 pprn 0.03-3.06 ppm 160, 7.9 ppb 2.0 2 0.02 pprn 0.035-1.8 ppm 9.9 & 0.51 pg/g 4.88, 66.3 ppm 0.06-1 1.6 pprn

Biological materials Sn, Cd, glass Si Spinach Spinach Orchard leaves

3.2 ~ 8 4 . 3 2 % 0.09 ppb-28.2 pprn <2 ng 320 ppb 0.35 % 0.01 pprn 1.24 ? 0.01 ppm (La), 25 1.8 ppb (Eu), 17 ? 1.8 ppb (Tb) 3-40 pprn 3.82 pglg 0.4-6.5 pprn 0.05, 4.2 pprn 28 PPm 6-27 pprn 10-36 ppb 0.22 '-- 0.02% 8.2 2 2.6 nglg 1 Pg 0.022-20.8 pprn 0 . 1 - 4 kg, 3.49%

Al, Ni(NO,), SiO, Biological materials, glass Steel, oyster homogenate Si crystal Si crystal GaAs Orchard leaves Nb A mixture of noble metals Glass Synthetic samples, Ag alloy Sn metal Rock Biological materials Zn ZnSe

0.021% 0.158,0.36 pprn

1.22 2 0.14 kg 87-100 pprn 77, 96 ppb 18.2 ? 5.3 pprn 8.5, 5.9 pph

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TABLE 6 (continued) Substoichiometric Radioactivation Analysis Element

Separation system

Si

Ppt, BaSiF,

Ta

Ex. DAPM- 1,2-dichloroethane Ppt, BaSO, Ex. DTPA-TTA-Bz Ex. INAP-CHC13

u Zn

Sample GaAs Sediment Rock crystal Rock crystal Wheat flour

Content determine 13.1 ? 0.8 ppm 0.52 % 0.05 pglg 194 & 25 ppb 194 29 ppb 26.4 2 2.7 pglg

Note: Ad, adsorption; Ex., extraction; Ppt, precipitation; AM, ammonium molybdate; APDC, ammonium pyrrolidinedithiocarbamate; BPHA, N-benzoyl-N-phenylhydroxylamine; Bz, benzene; CETAB, N-cetyl-N,N,N,-trimethylammonium; bromide; Crown, dicyclohexyl-18-crown-6; DAPM, diantipyrylmethane; DDDC, diammonium diethyldithiocarbamate; DTPA, diethylenetriaminepentaacetic acid; ETAA, ethylthioacetoacetate; HEINA, ethyl-a-isonitrosoacetoacetate;INAP, isonitrosoacetophenone; MIBK, methyl isobutyl ketone; NaDDC, sodium diethyldithiocarbamate; PMBP, 1-phenyl-3-methyl-4-benzoylpyrazol-5-one; TNOPO, tri-n-octylphosphine oxide; TNOPS, tri-noctylphosphine sulfide; TPA, tetraphenylarsonium; TPB, tetraphenylborate; l T A , thenoyltrifluoroacetone. From Kudo, K. and Suzuki, N., J. Radioanal. Chem., 59, 621, 1980. With permission.

1. Ruzicka, J. and Strary, J., Substoichiometry in Radiochemical Analysis, Pergamon Press, Oxford, 1968 2. Kudo, K. and Suzuki, N., Recent development of substoichiometry in trace analysis, J. Radioanal. Chem., 26, 327, 1975. 3. Kudo, K. and Suzuki, N., Recent aspect of substoichiometry in trace analysis, J. Radioanal. Chem., 59, 605, 1980. 4. Alimarin, I. P. and Bilimovich, G. N., Substoichiometry in radioanalytical methods, Bunseki Kagaku, 31, E213, 1982. 5. Kudo, K. and Suzuki, N., Substoichiometry in trace analysis, Trends Anal. Chem., 3, 20, 1984. 6. Suzuki, N., Radiometric determination of trace elements, in Chemical Applications of Nuclear Probes, Stumpe, R., Ed., Springer-Verlag, Heidelberg, 1988, chap. 2. 7. Shikano, K., Kudo, K., and Kobayashi, K., Radiochemical separation of fluorine using hydrated tin dioxide as an inorganic ion exchanger, J. Radioanal. Chem., 74, 73, 1982. 8. Shikano, K. and Kudo, K., Substoichiometric precipitation of fluoride with lanthanum, J. Radioanal. Chem., 78, 71, 1983. 9. Shiiano, K., Kudo, K., and Kobayashi, K., Substoichiometric precipitation of silicon as barium fluorosilicate, J. Radioanal. Nucl. Chem., 92, 307, 1985. 10. Katoh, M. and Kudo, K., Determination of uranium by neutron activation analysis using substoichiometric separation, J. Radioanal. Nucl. Chem., 84, 277, 1984. 11. Shigematsu, T. and Kudo, K., Substoichiometric extraction of phosphorus, J. Radioanal. Chem., 67, 25, 1981. 12. Baishya, N. K. and Heslop, R. B., The substoichiometric determination of chromium in aluminium alloys, Anal. Chim. Acta, 51, 69, 1970.

164

Activation Analysis

13. Zmijewska, W., Application of tri-n-octylamine for the substoichiometric separation of chromium(V1) in activation analysis, J . Radioanal. Chem., 10, 187, 1972. 14. Kudo, K., Shigematsu, T., and Kobayashi, K., Substoichiometric determination of chromium by neutron activation analysis, J. Radioanal. Chem., 36, 65, 1977. 15. Shigematsu, T. and Kudo, K., Substoichiometric extraction of chromium, J . Radioanal. Chem.. 59, 63, 1980. 16. Bilimovich, G. N., Atrashkevich, V. V., and Alimarin, I. P., Calculation of the optimal conditions for the substoichiometric isolation of trace amounts of the elements in the presence of two chelating agents, J. Anal. Chem. USSR., 29, 555, 1974. 17. Akol'zina, L. D., Bilimovich, G. N., Alimarin, I. P., and Churkina, N. N., Substoichiometric isolation of zirconium in the presence of two chelating agents and its determination by isotope dilution, J. Anal. Chem. USSR., 30, 1467, 1975. 18. Katoh, M. and Kudo, K., Study on the optimum conditions for the substoichiometric separation of lanthanum by using EDTA and 8-hydroxyquinoline, Bunseki Kagaku, 32, 1, 1983. 19. Katoh, M. and Kudo, K., Substoichiometric determination of lanthanum by using edta and 8-hydroxyquinoline, J. Radioanal. Chem., 79, 23, 1983. 20. Yuzawa, M. and Suzuki, N., Substoichiometric isotope dilution analysis of calcium in biological materials, J . Radioanal. Chem., 62, 115, 1981. 21. Suzuki, N., Yoshida, K., and Imura, H., Substoichiometric isotope dilution analysis of uranium by synergic extraction, Anal. Chim. Acta, 129, 221, 1981. 22. Suzuki, N., Nakamura, S., and Imura, H., Substoichiometric determination of manganese in a synergistic emtraction system and its application to the analysis of biological materials, J. Radioanal. Nucl. Chem., 81, 37, 1984. 23. Suzuki, N., Takahashi, M., and Imura, H., Substoichiometric liquid-liquid extraction and determination of vanadium based on the synergic effect of thenoyltrifluoroacetone and trioctylphosphine oxide, Anal. Chim. Acta, 160, 79, 1984. 24. Suzuki, N., Hanzawa, K., and Imura, H., Substoichiometric determination of uranium in phosphate rock, J. Radioanal. Nucl. Chem., 97, 81, 1986. 25. Shikano, K., Kudo, K., and Kobayashi, K., Determination of oxygen in gallium arsenide by 'He activation analysis using substoichiometric precipitation, J. Radioanal. Nucl. Chem., 91, 81, 1985. 26. Kambara, T., Suzuki, J., Yoshioka, H., and Nakamura, T., Redox substoichiometry in neutron activation analysis - Determination of antimony in metallic tin, Radioisotopes, 29, 590, 1980. 27. Suzuki, N. and Imura, H., Substoichiometric analysis, Bunseki. 106, 1987.

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Chapter 10

UTILIZATION OF CHEMICAL DERIVATIVES IN ACTIVATION ANALYSIS

.

William D Ehmann

TABLE OF CONTENTS I.

Background ..................................................................... 166

I1 .

Principles of Derivative Activation Analysis ....................................166

111.

Applications of DAA ........................................................... 167 A. Determination of Mg ....................................................167 B. Determination of Si .....................................................167 C. Determination of P ...................................................... 168 D. Determination of Ni .....................................................169 E. Determination of T1 .....................................................169 F. Speciation of Organic Compounds ......................................169 G. Speciation by Oxidation State ...........................................171

IV . Summary .......................................................................171 References .............................................................................. 172

166

Activation Analysis

I. BACKGROUND Conventional instrumental neutron activation analysis (INAA) with a nuclear reactor as a neutron source is capable of simultaneous, high sensitivity, multielement analyses in a wide variety of matrices. Sometimes, however, the naturally occurring stable isotope that would normally be the target nuclide for the analytical nuclear reaction may have a low isotopic abundance or a low reaction cross-section. The potential indicator radionuclide may also have nuclear properties that are unsuitable for detection by gamma-ray spectrometry, e.g., decay by pure alpha- or beta-particle emission, a low branching ratio for gamma emission, a high internal conversion coefficient, a low specific activity due to a very long half-life, or a half-life that is too short for convenient counting. A few examples of elements not ordinarily determined, or determined with poor sensitivity, by conventional reactor INAA include Li, Be, Ni, P, Nb, Rh, Si, Sn, T1, Pb, and Bi. Determination of several of these elements in biological and environmental samples is of interest, since they may be either toxic or essential. Prompt gamma neutron activation analysis (PGNAA), fast (14 MeV) neutron activation analysis (FNAA), or charged-particle activation analysis (CPAA) can sometimes be used, where the special facilities required are available. However, in a given matrix, the purely instrumental determination of any element can be restricted by reaction or spectral interferences, even if the fundamental nuclear properties of both the stable target nuclide and the product indicator radionuclide are favorable.

11. PRINCIPLES OF DERIVATIVE ACTIVATION ANALYSIS Derivative activation analysis (DAA) is a method to enhance the sensitivity of nuclear activation analysis for the more elusive elements. It may also allow a degree of chemical speciation for the element of interest. DAA uses a preirradiation chemical reaction on the sample to initiate the formation of, or an exchange with, a chemical complex which contains a surrogate element, M. As a result, the amount of the element or the chemical species to be determined, X, is now represented by measurement of the amount of the surrogate element, M, that is made part of, or released by the complex species. The surrogate element is selected for its superior properties for nuclear activation analysis and the absence of interference reactions in its final determination by INAA after some preconcentration or separation chemistry. Published DAA studies have been limited to neutron activation analysis. Young, in his dissertation research at the University of Kentucky, examined the requirements for successful DAA. ' His work and other DAA studies in the same laboratory have been recently summarized by Ehmann et a1.'

radionuclide with a suitable half-life for convenient handling and counting. The indicator radionuclide produced from irradiation of M should undergo decay with the emission of one or more gamma rays which yield photopeaks in interference-free regions of the gamma-ray spectrum from the irradiated processed matrix. When element M is to be complexed with element X, additional characteristics are desirable. The molar stoichiometric ratio, M:X, should be 1.0 or greater, any excess M from the original complexing medium which has not reacted stoichiometrically with element X should be easy to remove, and the final chemical form containing the surrogate element should be stable and easily isolated. Young1 considered three approaches by which the surrogate element, M, may be substituted for the analyte of interest, X. In the first, the element of interest in an aqueous phase, X,, is combined with the surrogate indicator element, M,. The complex formed,

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167

(XM), is extracted into an organic solvent to separate it from excess complexing reagent, Ma,, and also effect a concentration of the analyte. The reactions are X,

+ Maq-, (XM), + excess M,

organic solvent

+ (XM), + M,

+

(XM),,

(1)

+ M,

(2)

The separated complex (XM),, is then irradiated and the induced activity produced from M is used as an indirect measure of X in the original sample. The separation procedure in all DAA approaches must be quantitative, or have a reproducible, measured chemical yield. In the second approach, the element or chemical species of interest, X,, is exchanged for the surrogate element, M, which has been introduced to the aqueous solution of the analyte as a complex, (ML),,, in an organic solvent. L is usually an organic ligand.

The surrogate element released from the complex and transferred to the aqueous solution, M,, is collected, quantified by INAA, and used as a measure of X in the original sample solution. This method has the advantage that, since the complex, (ML),, is introduced in an organic solvent which is immiscible with the analyte solution, the excess complexing agent can be separated from the released M simultaneously with the exchange reaction. A further purification step may not be needed. The third DAA approach is, in part, a reverse of the second approach. The element or species of interest, X,,, is first quantitatively converted to the complex (XL), and then isolated by solvent extraction into an organic solvent of volume V,,. (XL), is then exchanged with the surrogate element, M,, to form (ML),,, which is separated, irradiated and counted for the induced activity from M.

(XL),

+ organic solvent + (XL),,,

(5)

In this method, there is a potential for preconcentration of X (Equation 5), if Vowis smaller than V,. An additional advantage is that both the exchange reaction and the separation of the complex from excess reagent can be effected in a single step (Equation 6).

I n . APPLICATIONS OF DAA A. DETERMINATION OF Mg Smathers et a]., increased the sensitivity of NAA Mg determinations by forming a 1:2 chelate of Mg with 5,7-dibromo-8-hydroxyquinolinein CHCl, (in the presence of 2,4,6trimethylpyridine), separating the chelate from excess chelating agent by two-dimensional paper chromatography, and measuring the activity of 82Brfollowing reactor neutron irrareaction diation. A fivefold increase in sensitivity over use of the conventional 26Mg(n, was reported. The much longer half-life of 82Br is an advantage in sample handling and counting, compared with the use of 27Mg. An absolute sensitivity of 0.1 pg Mg using a reactor flux density of loL2n cm-2 s-' was reported. B. DETERMINATION OF Si Oltmann and Ryan4 used a DAA approach to determine reactive Si in natural waters.

168

Activation Analysis

In their procedure, Si is complexed as molybdosilicic acid and extracted with methyl isobutyl ketone. The 10'Mo-'OITcparent-daughter decay pair induced by reactor neutron irradiation of the extracted complex is used for detection and indirect determination of Si. Specifically, 50-ml water samples were treated with 10% ammonium molybdate and dilute H2S04to yield a pH of 1.2 to 1.4. Any isopoly acids, molybdosphosphoric acid, or molybdoarsenic acid present were destroyed by treatment with 1:l H2S0, and the molybdosilicic acid complex was extracted into 2 ml of methyl isobutyl ketone and washed with dilute H2S0,. The organic phases were irradiated at a neutron flux density of 5 x 10" n c n r 2 s-I for 30 min in a SLOWPOKE I1 reactor and the 307-keV photopeak of lolTc counted to quantify the Si. Interferences caused by tannic acid in the waters were removed through treatment with dichromate. Oltmann and Ryan4 report a detection limit of 0.2 ng Si ml-'. They note that the detection limit could be improved, if better methods to purify molybdate reagents could be developed.

C. DETERMINATION OF P DAA determinations of P in natural waters have been reported by Allen and Hahn.$ Direct measurement of the beta decay of 32Pfrom the 31P(n,y)32Preaction using a liquid scintillation counter is hampered by interferences from the 32S(n,p)32Pand 35Cl(n,a)32P reactions. Allen and Hahn formed tungstomolybdophosphoric acid by adding molybdate and tungstate ions to aqueous solutions containing phosphate. The complex was then extracted into 2,6-dimethyl-4-heptanonewith a measured 38% efficiency. The complex was irradiated with reactor neutrons and the resulting lS7Wactivity used as a measure of P. Detection limits of 4.0 ng P per gram of water were achieved, using a reactor neutron flux density of 1013 n per square centimeter per second. The DAA determination of P also avoids problems associated with the 28Si(n,p)28A1interference reaction in the FNAA determination of P by the 31P(n,~)28A1 reaction. Kleppinger et a1. ,6 Ehmann et a1. ,2 and Oltmann and Ryan7 have also reported P determinations by DAA. In these latter studies, phosphovanadomolybdate complexes (i.e., ternary heteropoly acids containing P, V, and Mo) are formed from aqueous analyte solutions containing phosphate anion. The complex may be selectively extracted from dilute acid solutions into an organic solvent (usually methyl isobutyl ketone), irradiated, and the 1.434MeV gamma rays from the induced activity of 52V(t1,2 = 3.75 m) counted as an indirect measure of the P present. The sensitivity obtained by measuring 52V is superior to that obtained by Allen and Hahn5 by measuring IE7W. Extraction yields for the complex were reported by Ehmann et a1.2 to be nearly quantitative (>97%) from dilute HNO, solutions with pH < 1.0. The yield was also found to be insensitive to the presence of many common anions and cations up to interference concentrations of 1000 pg mlpl. However, fluoride, arsenate, and Fe3+ ions were found to significantly reduce extraction yields, when present at the 1000 pg ml-' level. The relationship between 52Vactivity and sample P content was found to be linear in the ranges from 1.0 to 20 ~g P per milliliter (r = 0.997) for 1.25 mg 252Cfsource irradiations at a flux density of lo7 n per square centimeter per second and from 1.0 ng P per milliliter 1.0 pg P per milliliter (r = 0.995) for reactor irradiations at a flux density of 1013 n per square centimeter per second. Ehmann et a1.2 used their DAA procedure to analyze for P in natural waters, coal, human brain tissue, meteorites, geological samples, and various standard reference materials at levels ranging from ng g-' to several percent P. They found that a 1.25 mg 252Cfneutron source was adequate for many of the determinations. Kleppinger et a1.6 reported DAA analyses for P in commercial plant food as part of an instructional radiochemistry laboratory experiment. Kleppinger and Yates8later reported that student data for the P content of plant food using DAA compare favorably with data obtained by FNAA.

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Oltmann and Ryan7 reported DAA detection limits of 0.2 ng P per milliliter in natural water samples, using a SLOWPOKE I1 reactor with a neutron flux density of 1012 n per square centimeter per second. They note that this sensitivity was four times greater than they could obtain using the 31P(n,a)28A1reactor fast neutron reaction. It was also superior to that obtained by use of 32Pbeta counting. They investigated possible interferences from arsenate, germanate, and silicate anions which might form heteropoly acids and fluoride ion and tannic acid which might compete as complexing agents. Under their experimental conditions, these potential interferences were minimal. D. DETERMINATION OF Ni Ehmann et al.2 have reported the DAA determination of Ni by formation of a Nidithiozone complex which is first isolated by solvent extraction into chloroform (CHC1,) and then treated with Au3+ to substitute AU for Ni in the complex. The gold complex is then irradiated and the induced 19'Au (tIl2 = 64.8 h) gamma rays at 411 keV are detected as an indirect measure of the original Ni content. A preconcentration step was recommended prior to the DAA procedure. In this step, Ni is extracted into chloroform from an aqueous solution (adjusted to pH 8.0 to 8.5 with NH,OH) to which alcoholic dimethylglyoxime has been added. The solvent extraction of the Ni-dimethylglyoxime complex serves as both a preconcentration step and, in the presence of masking agents, a decontamination step to remove potential interferences. In the DAA portion of the procedure, some problems were encountered with the incomplete removal of uncomplexed Au3+ from the CHC1, layer during the exchange pro~ e d u r eThis . ~ resulted in a detection limit of only 200 ng Ni, using reactor irradiations at a flux density of lOI3 n ~ m s - l. ~A linear response curve (r = 0.977) resulted from the analysis of Ni standard solutions over a range of 0.1 to 10 pg total Ni in 25 ml solution aliquots. Other than tests with standard solutions, the only "real" sample analyzed was the Allende stony meteorite. A 2s2Cfneutron source with a flux density of lo7 n cm-2 s-I was used to obtain a value of 1.41 ? 0.02% Ni, in excellent agreement with literature values. E. DETERMINATION OF TI Ehmann et a1.2 have reported a DAA procedure for the determination of TI which is based on an ion association between thallium(II1) chloride and iodonitrotetrazolium chloride. The T1-complex formed was extracted into benzene, irradiated with reactor thermal neutrons, and the 443-keV gamma rays from lZ8I(tIl2 = 25 m) were counted with scintillation detectors. Since T1 concentrations in environmental samples are very low, a preconcentration step which used a Chelex 100 column, or a relatively specific preseparation step based on solvent extraction of TlBr, into ethyl ether was recommended. In the DAA portion of the procedure, a 1.0 M HCI solution containing T1 was treated with 1.0 ml of a 0.2% (vlv) solution of iodonitrotetrazolium chloride. This solution was extracted twice with benzene. The benzene extracts were combined and evaporated under an air flow. The residues were then irradiated for 5 min at a flux density of 1013 n cmP2 s-l. Following a 5- to 15-min delay, the Iz8Igamma rays at 443 keV were counted for 5 min with a NaI(T1) scintillation detector and the activity used as an indirect measure of the T1 content of the sample. Ehmann et a1.2 only reported analyses for a series of T1 standard solutions. They found that the analytical response was linear (r = 0.997) for sample solutions containing 0.2 to 20 pg T1 and estimated their detection limit to be 150 ng TI. As in DAA for Ni, the detection limit appeared to be governed by incomplete removal of umeacted complexing agent from the benzene layer. F. SPECIATION OF ORGANIC COMPOUNDS Steim and Benson9 tagged various organic compounds with derivatives containing Br.

170

Activation Analysis

After separations from excess reagent were effected by paper chromatography, the papers were irradiated with reactor thermal neutrons, the locations of the desired species determined by autoradiography, and the amount of the compound of interest calculated from the 8ZBr activity produced. Both simple Geiger-Muller and liquid scintillation counters were used. Carboxylic acids were determined by formation of p-bromophenacyl esters, amino acids by formation of p-bromophenylsulfonamides, and keto acids and sugars by formation of pbromophenylhydrazones. Steim and Benson also treated unsaturated fatty acids with mercuric acetate to form Hg complexes that were irradiated to produce '03Hg. Fatty acid contents were estimated following paper chromatographic separations and autoradiography. Ehmann et al.' used DAA to speciate the organically bound oxygen in coal. Hydroxyl functional groups in powdered coal samples were treated with hexarnethyldisilazane and subsequently determined by measurement of stoichiometricallyreacted silicon, as silyl ethers. The method was originally proposed by Friedman et a1.I0 Ehmann et a1.2 modified it to nuclear reaction, rather than measure the Si uptake by 14-MeV INAA via the 31Si(n,a)28A1 by conventional wet chemical methods. In their procedure, dried coal samples ranging in weight from 1 to 1.5 g, were ground to -200 mesh, refluxed with 10 ml pyridine, 20 ml hexarnethyldisilazane, and 0.2 ml trimethylchlorosilane as a ~atalyst.'~."Silyl ethers were formed according to the following reaction: 2ROH

+ [(CH,),Si],NH

+ 2ROSi(CH3),

+ NH,

(7)

All sample manipulations including grinding and drying were conducted under nitrogen. Excess reagents were removed by vacuum distillation. Final drying was done under vacuum at 110°C to constant weight. Both dried raw coal and dried demineralized samples were analyzed. Hydroxyl contents were calculated as the difference in equivalents of Si in the original underivatized sample and the washed derivatized material. Although the analytical results for hydroxyl content they obtained were well within the range of literature values for similar coals using other techniques, it was noted that further work would be required to evaluate potentially compensating errors resulting from incomplete derivatization of the solid coal matrix coupled with partial retention of umeacted silylation agent in the treated coal. Ehmann et aL2 also used DAA to determine the carbonyl functionality in coals. Their procedure takes advantage of the reaction of carbonyl groups with hydroxylarnine to form the corresponding oxime.l2,I3The reaction is represented as follows: R2-C=0

+ OHH2N . HCl

-*

R,-C=NOH

+ H20 + HCI

(8)

The derivatized coal was recovered and an aliquant was used directly for 14-MeV INAA determination of N. A second aliquant was reacted with acetone and sulfuric acid to decompose the oxime before analysis for N. The carbonyl oxygen (C=O) content was calculated from the difference in the N contents of the oxime derivative and the decomposed oxime coal samples. The 14-MeV INAA determination of N employed coincidence counting of the 0.51 1-MeV annihilation photons emitted by positron decay of I3N (tIl2 = 9.96 m) produced by the l4N(n,2n)I3Nreaction. Corrections for potential interfe~ncesfrom ''C(p,y)13N, "C(p,n)I3N, and '60(p,a)13Nreactions were applied. A 12.5-min delay before counting was used to minimize interference from formed by the 31P(n,2n)30Preaction. In their detailed procedure, 1 g of powdered coal was freeze-dried, weighed, and treated with 1 M HCl with stirring for 24 h to ensure that the oxygen functional goups were in the free state, washed free of acid with distilled deionized water, and again freeze-dried and weighed. The dried coal was treated with 1 g of hydroxylamine hydrochloride and 10 rnl of pyridine, and then heated at 90°C with refluxing for 24 h to form the oxime. Following

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171

additional washing steps, the derivatized coal was freeze-dried, weighed, and the N content was determined by 14-MeV INAA. The derivatized coal sample was then refluxed with acetone and 1 M H2S0, at 90°C for 24 h to decompose the oxime derivatives. Finally, the oxime-decomposed coal was washed, filtered, freeze-dried, weighed, and again analyzed for N. The carbonyl content of the coal was calculated from the difference in the N contents of the derivatized coal and the oxime-decomposed coal, as described by Blom et a1.12 Ehmann et a1.2 reported carbonyl analyses for seven coal samples using their DAA procedure and found them to compare favorably with literature values for similar rank coals. G . SPECIATION BY OXIDATION STATE Cheng et al.I4 used a DAA approach to speciate Fez+ and Fe3+. Chelates of p-bromobenzoyltrifluoroacetone with Fe2+ and Fe3 were first formed and then isolated by paper chromatography. Quantification of the two iron species was accomplished by reactor neutron irradiation of the separated sections of the chromatography papers followed by counting the induced activity of "Br. A detection limit of 0.1 kg Fe was reported using a reactor neutron flux density of 1012 n cm-2 s- '. +

IV. SUMMARY In the use of DAA, the analyst gives up one of the most important advantages of NAA, that of minimal opportunity for sample contamination. However, if the element or species to be determined is in moderately high abundance (e.g., P in brain, Ni in meteorites), preirradiation chemical processing is quite feasible without concern for low levels of laboratory and reagent contamination. Here, DAA can achieve considerable savings in time, when compared with use of conventional analytical methods. If trace level determinations are desired, extreme care must be taken to evaluate sources of reagent and laboratory contamination. Chemical yields for the derivatization and separation processes must be quantitative, or at least reproducible. As in conventional RNAA, the method is destructive and opportunities to obtain additional information about the sample after DAA are minimal. DAA can offer the analyst some important advantages. It can determine elements, functional groups, or chemical species which cannot be determined directly by INAA, FNAA, PGNAA, or CPAA procedures. When compared with conventional RNAA, there are fewer precautions with respect to handling of intensely radioactive samples, since the chemistry is done before the irradiation. The preirradiation chemistry may also eliminate many interferences that might occur in INAA and, through use of an appropriate surrogate element, can place the analytical gamma-ray line in an interference-free region of the gamma-ray spectrum. The greatest potential use of DAA may be in the area of chemical speciation. In this application, DAA may be regarded as just an extension of tracer methodology using activatable tracers. When applied to the determination of molecular species, it is a useful extension of molecular neutron activation analysis." Since chemical separations in DAA result in "clean" gamma-ray spectra from the samples that are finally counted, scintillation detectors with high counting efficiencies may often be used. The high resolution associated with germanium detectors may not be required. For separations that are very selective, the use of Cerenkov counting may be possible.' Guzzi et a1. l6 reported that the ratio of Cerenkov to NaI(T1) scintillation counting rates for IzXIis 3 1.4:1. Since the energy resolution inherent in Cerenkov counting is poor, care must be taken to avoid potential spectral interferences. However, some degree of energy selection may be achieved by varying the liquid medium in which the radionuclide is counted. Applications of DAA are largely unexplored. However, a review of the extensive literature of classical methods of analysis based on the formation of derivatives suggests that

172

Activation Analysis

the innovative radiochemist may wish to return to the wet chemistry laboratory, at least part of the time.

REFERENCES 1. Young, R. C., 111, Derivative Activation Analysis Applied to the Determination of P, Ni and TI, Ph.D. dissertation, University of Kentucky, Lexington, 1979. 2. Ehmann, W. D., Young, R. C., 111, Koppenaal, D. W., Jones, W. C., and Prasad, M. N., Derivative techniques in activation analysis, J. Radioanal. Nucl. Chem., 112, 71, 1987. 3. Smathers, J. B., Duffey, D., and Lakshmanan, S., Chelate enhancement of the sensitivity of magnesium in neutron activation analysis, Anal. Chim. Acta, 46, 9, 1969. 4. Oltmann, P. and Ryan, D. E., The indirect determination of trace reactive silicon in waters by neutron activation analysis, Can. J. Chem., 63, 2585, 1985. 5. Allen, H. E. and Hahn, R. B., Determination of phosphate in natural waters by activation analysis of tungstophosphoric acid, Environ. Sci. Technol., 3, 844, 1969. 6. Kleppinger, E. W., Brubaker, E. H., Young, R. C., Ehmann,W. D., and Yates, S. W., Phosphorus determination by derivative activation analysis: a multifaceted radiochemical application, J. Chem. Educ., 61, 262, 1984. 7. Oltmann, P. and Ryan, D. E., Heteropoly acids in the determination of phosphoms by neutron activation analysis, J. Radioanal. Nucl. Chem., 110, 565, 1987. 8. Kleppinger, E. W. and Yates, S. W., Variations on the determination of phosphorus by neutron activation analysis, J. Chem. Educ., 63, 987, 1986. 9. Steim, J. M. and Benson, A. A., Derivative activation chromatography, Anal. Biochem., 9, 21, 1964. 10. Friedman, S., Kaufman, M. L., Steiner, W. A., and Wender, I., Determination of hydroxyl content of vitrains by formation of trimethylsilyl ethers, Fuel, 40, 33, 1961. 11. Roth, C. A., Silylation of organic chemicals, Ind. Eng. Chem. Prod. Res. Dev., 11, 134, 1972. 12. Blom, L., Edelhausen, L., and Van Krevelen, D. W., Chemical structure and properties of coal and related products, Fuel, 36, 135, 1957. 13. Ruberto, R. G. and Cronauer, D. C., Oxygen and oxygen functionalities in coal and coal liquids, in Organic Chemistry of Coal, Larsen, J . W . , Ed., Am. Chem. Soc. Symp. Ser. 71, American Chemical Society, Washington, D. C., 1978, 50. 14. Cheng, F. C., Lakshmanan, S., and Duffey, D., Derivative neutron activation analysis of ferrous and fenic iron, Trans. Am. Nucl. Soc., 10, 448, 1967. 15. Blotcky, A. J., Hansen, G. T., Opelanio-Buencamino, L. R., and Rack, E. P., Determination of trimethylselenonium ion in urine by ion-exchange chromatography and molecular neutron activation analysis, Anal. Chem., 57, 1937, 1985. 16. Guzzi, G., Pietra, P., Sabbioni, E., and Girardi, F., Cerenkov counting rates of radionuclides for activation analysis purposes, J. Radioanal. Chem., 20, 751, 1974.

Index

Volume I

INDEX

Absorption coefficient, 116, 1 17 Absorption errors in X-ray emitters, 1 1 6 1 2 0 Accuracy, 45, 138 Acids, 65, see also specific types Activation analysis, defined, 4 Activation product, 42 Adsorption, 63, 147-148 Advance prediction computer program (APCP), 5759 Advantage factor (AF), 113 AF, see Advantage factor Aluminum, 10l Amino acids, 170, see also specific types Analysis time, 45,46 Antimony, 37, 128, 16&161 APCP, see Advance prediction computer program Arsenic, 37 Asymmetrical distortion, 17 Automated radiochemical separations, 87 Automatic peak search, 19 Autoradiography, I70 Avogadro's numbec 132

Barium, 122, 158-160 Barycenter method, 37 Beryllium, 98 Beta particles, 6, 122-126 Betarays, 113, 114, 120, 126 Bethge apparatus, 66 Biological materials, 64, 6 5 , 7 6 8 7 , see also specific types group separation of, 82-85 ion-retention media for, 87 single-element separation of, 77-82 Bismuth, 126 Bisulphite fusion, 65 Boron, 102 Bremsstrahlung energy, 45,46 Bromine, 126, 128

Cadmium, 82, 120, 128 Calcium, 101, 140 Carbon, 87 Carboxylic acids, 170 Carius tube, 66 Carrier amount variation, 146 Carriers, 63, 65 Cesium, 140 Charged-particle activation analysis (CPAA), 135, 137, 166 Charged particles, 43 Chelate extraction, 1 5 6 1 5 7

Chemical derivatives, 165 applications of, 167-171 magnesium and, 167 nickel and, 169 organic compound and, 169-1 7 1 oxidation speciation and, 17 1 phosphorus and, 168-1 69 sensitivity of, 166 silicon and, 167-1 68 titanium and, 169 Chi-square, 34 Chromatography, see also specific types paper, 167, 170, 171 reversed-phase, 75 Chromium, 80, 1 5 6 1 5 7 Coal, 170 Cobalt, 127 Colloid formation, 63 Comparison methods, 146-147 Compton effect, 21, 49, 1 12 Computer graphics, 46,50 Contamination, 64 Convolution operation, 10 Copper, 82, 1 2 6 1 2 8 Coulombic barrier, 6 Coulombic repulsion energy, 6 Counting times, 42,43,45,46 CPAA, see Charged-particle activation analysis Cross-contamination, 64 Cryogenic homogenization techniques, 64 Cyclic activation, 103-107 Cyclic activation analysis, 43

DAA, see Derivative activation analysis Decay, 45,46 Decay time, 41--42,45 Decontamination factors, 66,67 Deformed Gaussian, 14, 17-18 Delayed neutron counting (DNC), 98 Delayed neutron precursors (DNP), 98,99 Delayed neutrons. 97 cyclic activation and, 103-107 experimental system for, 107-108 fissile elements and, 98-103 fission and, 100 nonfissile elements and, 98, 107 Derivative activation analysis (DAA), 1 6 6 1 6 7 Derivatized coal, 170 Detector background, 42 Differential filters, 121 Diffusion, 62 Dimethylglyoxime (DMG), 83 Direct computation, 48 Dissolution, 82 Distillation, 74, 83 Distortion, 17

175

176

Activation Analysis

DMG, see Dimethylglyoxime DNC, see Delayed neutron counting DNP, see Delayed neutron precursors Dry ashing, 64 Drying, 64 Dysprosium, 112

EDTA, 151, 152, 157-158 Effectivity parameter, 48 Electromagnets, 126 Electron capture, 112 Empirical methods for analysis of gamma-ray spectra, 31-37 ENAA, see Epithermal neutron activation analysis Energy, see also specific types absorption coefficient and, 116, 117 Bremsstrahlung, 45,46 Coulombic repulsion, 6 gamma-ray, 42 incident, 11 irradiation, 4 3 4 4 peak, 24 Enriched stable isotopes, 133-1 38 Epithermal neutron activation analysis (ENAA), 43, 121, 123, 127-128 Epithermal neutrons, 102 Equation for activation analysis, 40 Escape peaks, 121 Experimental parameters for instrumental activation analysis, 4 0 - 4 4 Expert systems, 51

Fano factor, 11 Fast neutron activation analysis (FNAA), 116, 128, 166 Fast neutrons, 102 Fatty acids, 170, see also specific types Fertile nuclides, 102 First derivation method, 10 Fissile elements, 98-103 Fissile nuclides, 102 Fission, 99, 100 Fitting intervals, 24 Fitting methods, 10, 14.34, see also specific types Fluorine, 101 Flux, 43 FNAA, see Fast (14 MeV) neutron activation analysis Fourier Transformation, 10 Freeze drying, 64 Fusion, 65

Gamma rays, 6, 122 energy of, 42 high-energy, 112 low-energy, 114, 120, 123

Gamma-ray spectra, 9,62, 138 analytic approximation of, 10-18 empirical method for analysis of, 31-37 nonlinear least-square fit for analysis of, 18-27 prediction of, 52 summation method for analysis of, 27-3 1 Gaussian function, 11-14, 18, 23 deformed, 14, 17-18 symmetrical, 17 Geological samples, 64, 100, 101 Geological samples in radiochemical neutron activation analysis, 68-76 Geological surveys, 102 Geometry, 4 2 4 3 Germanium, 16 Germanium detectors, 10, 62 Graphics, 46,50 Group separation of biological materials, 82-85

Hafnium, 127 High-energy gamma rays, 112 High-resolution germanium detectors, 62 Hold-back carriers, 63 Holmium, 127, 128 Hydride generation, 74 Hydrological samples, 101 Hydroxide precipitation, 83 Hydroxyl functional groups, 170 8-Hydroxyquinoline, 157-158

Incident energy, I I Indium, 128 Infinite thick samples, 119 Information profitability (IP), 47 Information'theory, 45,47 INNA, see Instrumental neutron activation analysis Inorganic ion retention media, 76 Instrumental activation analysis, 39-52, see also Instrumental neutron activation analysis (INAA) experimental parameters for, 4-4 future directions of, 5 1-52 multielement determination in, 4 5 4 8 optimization of, 49-5 1 performance prediction for, 4 8 4 9 response functions in, 4 4 4 8 . 5 1 single-element determination in, 4 4 - 4 5 Instrumental neutron activation analysis (INNA), 55-59.62. 121, 126, 127, 166 Interactive computer graphics, 46, 50 Interference, 67 in stable isotope dilution activation analysis, 138 in X-ray emitters, 1 2 G 1 2 6 Interference-free LOD, 56 Internal standard method, 134, 135 Iodine, 127 Ion-associated extraction, 149

Volume I Ion exchangers, 84 Ionizing neutron irradiation, 62 Ion-retention media, 84, 87 IP, see Information profitability Iron, 171 Irradiated sample transfer, 64--65 Irradiation. see also Radiation conditions of, 4 3 4 4 energy of, 4 3 4 4 ionizing neutron, 62 neutron, 62 time of, 4 1 , 44 Isotope dilution method, 135, 137 Isotopic carriers, 63 Isotopic composition, 133, 135

K-absorption edge, 121, 122 Keto acids, 170 K factor, 116 K X-rays, 112, 114, 118, 121, 126, 127

Lagrange interpolation, 33 Lanthanum, 82, 157-160 Lead, 126 Least-squares methods, 10, 14 in gamma-ray spectra analysis, 14 linear, see Linear least-squares method nonlinear, see Nonlinear least-squares method LEPD, see Low-sensitivity photon detectors Limits of detection (LOD). 56-57 Linear absorption coefficients, 124 Linear least-squares method, 10, 34 Line shape function, 18 Lithium, 98, I07 LLOD, see Lower limits of detection LOD, see Limits of detection Low-energy gamma rays, 114, 120, 123 Low-energy photons, 1 17 Lower limits of detection (LLOD), see Limits of detection (LOD) Low-sensitivity photon detectors (LEPD), 112, 121 Lutetium, 16.25 L X-rays, 112, 114, 118, 121, 127

Macroporous anion exchanger, 74 Magnesium, 167 Magnetic deflection, 113, 1 14, 126 Magnetic fields, 122, 123 Magnets, 124, 126 Mass absorption coefficient, 116, 1 17, 119 Mass spectrometry, 135 Matrix analysis, 40 Mercury, 77, 82, 1 2 6 1 2 8 Metals, see also specific types 14-MeV neutron activation analysis, see Fast neutron

177

activation analysis Mineralization, 6 5 6 6 Molybdenum, 121 Multielement activation analysis, 28-31.4548 Multielement determination in instrumental activation analysis, 4 5 4 8 Multielement radiochemical separations. 67 Multiple countings, 43 Multiple irradiations, 43

NAA, see Neutron activation analysis NADNC, see Neutron activation followed by delayed-neutron counting Neutron activation, 139 epithermal, 123, 127-128 reactor, 1 12-1 16 thermal, 1 2 6 127 Neutron activation analysis (NAA), 43, 146, see also specific types epithennal, 121 fast, 116, 128, 166 instrumental, see Instrumental neutron activation analysis (INAA) radiochemical, see Radiochemical neutron activation analysis Neutron activation followed by delayed-neutron counting (NADNC), 98, 100 Neutron generators, 100 Neutrons, 6,43 activation of, see Neutron activation activity of, 98 delayed, see Delayed neutrons epithemal, 102 fast, 102 irradiation of, 62 isotopic source of, 103-107 reactor, 102 self-shielding of, 44, 100, 133, 137, 138 thermal, 102, 1 2 6 1 2 7 Nickel, 126, 169 Nickel sulfide fire-assay technique, 70 Niobium, 121, 1 2 6 1 2 8 Nitrogen, 98, 107 Noble metals, 7 G 7 4 , see also specific types Nonfissile elements, 98, 107 Nonisotopic carriers, 63 Nonlinear least-squares method, 10, 26, 31, 37 Nonlinear least-squares method in gamma-ray spectra analysis, 18-27 NPAR, 16 Nuclear activation, 44 Nuclear chemistry, 6 7

Optimization of instrumental activation analysis, 49-5 1 Optimum decay time, 46 Organically bound oxygen, 170

178

Activation Analysis

Organic compound determination, 169-17 1 Organic ion exchangers, 68 Oven-drying, 64 Overall decontamination factor, 67 Oxygen, 98, 107 organically bound, 170 substoichiometric radioactivation analysis and, 155-156

PAA, see Polyantimonic acid Palladium, 120 Paper chromatography, 167, 170, 171 Peak area standard deviation, 47 Peak centroid, 3 1 Peak energy, 24 Peak integration, 28 Peak intensity, 3 1 Peak search, 19,21-23,27-28 Performance prediction for instrumental activation analysis, 4 8 4 9 Peroxide fusion, 65 Perspex absorber, 124 PGNAA, see Prompt gamma neutron activation analysis Phosphorus, 122, 168-169 Phosphovanadomolybdate complexes, 168 Photons, 43 activation of, 45, 133, 139 low-energy, 1 17 Photopeak parameters, 3 1 Platinum, 77 Plutonium, 102, 103 Polyantimonic acid (PAA), 85 Praseodymium, 128 Precipitation, 68, 148-149 Precision, 45 Preconcentration, 70, 167 Preirradiation chemical reaction, 56, 62, 166 Pressure build-up, 62,64 Prompt gamma neutron activation analysis (PGNAA), 166 Proton activation, 135, 139 Protons, 43 Pseudocyclic analysis, 43

Radiation, 6, see also Irradiation; specific types beta, 113, 114, 120, 126 damage from, 4 1.44 gamma, see Gamma rays measurement of, 4 0 , 4 2 4 3 X, see X-rays Radioactivation analysis, 146 Radioactivation analysis, substoichiometric, see Substoichiometric radioactivation analysis Radiochemical fire-assay procedures, 72 Radiochemical neutron activation analysis (RNAA), 61

automated radiochemical separations in, 87 biological materials for, see Biological materials carriers in, 63, 65 future prospects for, 90 geological samples in, 68-76 hazards of activation in, 6 2 4 3 irradiated sample transfer in, 6 4 - 4 5 low chemical concentrations in, 63 mineralization in, 6 5 - 6 6 preseparation of treatment sample in, 6 4 - 4 6 sample preparation in, 64 separation characteristics in, 6-8 Radiochemical operations, 62, see also specific types Radiochemical radioactivation analysis, 146 Radiochemical separation, 67, 87, 146 Radiochemical-separation NAA (RCS-NAA), 56 Radiolysis, 62, 64 Radionuclide migration, 62 Rare-earth elements (REE), 6 8 4 9 , 127, see also specific types RCS-NAA, see Radiochemical-separation NAA Reaction cross-section, 6 Reactor neutrons, 102, 112-1 16 Redox substoichiometry, 160--16 1 REE, see Rare-earth elements Reirradiation, 46 Replicate activation, 43 Resolution enhancement, 10 Response functions in instrumental activation analysis, 44-43, 5 1 Response surfaces, 44,47 Reversed-phase chromatography (RPC), 75 RNAA, see Radiochemical neutron activation analysis (RNAA) RPC, see Reversed-phase chromatography Rubidium, 140

Sample size, 44.45 SAMPO, 19,24--26 Search methods, 49-50, see also specific types peak, 19,21-23,27-28 simplex, 45,46,50 Secondary X-rays, 120--122 Second derivation operation, 10 Selectivity parameter, 48 Selenium, 82, 126, 128 Self-shielding, 44, 100, 133, 137, 138 Sensitivity, 41 of chemical derivatives, 166 of stable isotope dilution activation analysis, 137138 of X-ray emitters, 112-1 16 Separation methods, see also specific types multielement, 67 radiochemical, see Radiochemical separation single-element, 77-82 in substoichiometric radioactivation analvsis, 147154 Shoniger technique, 66

Volume I Signal-to-noise ratio, 43 Silicon, 156, 167-168 Silver, 120, 128 Simplex methods, 45,46,50 Simulation, 4849 Single-element analysis, 2S44-45 Single-element separation of biological materials, 77-82 Smoothing of experimental data, 19-21 Solvent extraction, 75, 149-154 SPAN, 19,25.26 SPECTRA, I27 Spectrometry, see also specific types gamma-ray, see Gamma-ray spectra mass, 135 Stable isotope dilution activation analysis, 131-140 accuracy of, 138 applications of, 139-140 chemical processes in, 136137 classification of, 134 description of, 132-l 34 instrumental errors in, 138 interference effects in, 138 method, 134-l 38 sensitivity of, 137-l 38 Standard addition method, 135 Standard deviation, 31 Standard peak translation. 33 Standard reference material, 135 Steepest descent method, 50 Stopping power of material, 137 Strontium, 139, 140 Substoichiometric radioactivation analysis, I45 adsorption and, 147-148 antimony and, 160-161 applications of, 155-161 carrier amount variation in, 146 chromium and, 156157 classification of, 146 comparison method in, 146-147 direct method of, 146 lanthanum and, 157-158 oxygen and, 155-156 precipitation and, 148-149 principles of, 146-147 separation methods in, 147-I 54 silicon and, 156 solvent extraction in, 149-154 uranium and, 158-160 sugars, 170 Sulfide precipitation, 83 Summation methods, 27-3 I, 37 Surface effects, 63 Surrogate elements, 166 Symmetrical Gaussian, 17

179

Synergic extraction system, 152 T Tantalum, 128 Tarry substances, 64 Tborium, 128 Technetium, 121 Terbium, 128 Thallium, I26 Thermal neutrons, 102, 126-127 Thorium. 102, 107, 121, 127 Threshold reactions, 43 Thulium, 127 Tin, 80 Titanium, 169 Total count time, 106 Trace elements, 74-76, see also specific types Tracer chemistry, 137 Transit time, 105 U Uranium, 101-103, 107, 121, 127, 128 Uranium, substoichiometric radioactivation analysis and, 158-160 V Vanadium, 10 I Volatilization, 62, 65

X X-my emitters, 11 I absorption errors in, 116-120 applications of, 126-128 interference in, 120-126 limits of, 112-l I6 sensitivity of, 112-l 16 X-ray intensity, 117 X-rays, 6, 112, see also specific types K, 112, 114, 118, 121, 126, 127 L. 112, 114, 118, 121, 127 secondary, 120-l 22

Y Ytterbium, 127, 128 Yttrium. 121

Z ZFC, see Zinc ferrocyanide Zinc, 82, 138 Zinc ferrocyanide (ZFCI, 87 Zirconium, 12 1

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