Multielement instrumental neutron activation analysis of biological tissue using a single comparator standard and data processing by computer

International Journal of AppliedRadiation and Isotopes, 1973,Vol.24, pp. 343-351. PergamonPres*. Printedin Northern Ireh

Multielement Instrumental Neutron Activation Analysis of Biological Tiss Using a Single Comparator Standard a Data Processing by Computer D. M. L I N E K I N * Physics Research Laboratory, Maxsachusetts General Hospital, Boston, Massachusetts, U.~

(Received22 May 1972; /n revisedform I 1 August 1972) A detailed description of a mulfielement instrumental neutron activation analysis approaet outine trace element measurements in biological samples is presented. The main new featt of this method are the use of a single element, cobalt, as the comparator standard, and the of a data smoothing computer technique employing a Fourier transform. A theoretical trq ment of the single comparator method is given, along with practicai experimental detMh: brief description of the computer program with special emphasis on the data smoothing asp~ follows. Some results of trace element measurements in biological tissue are then presenl and the paper is concluded by a discussion of the advantages and disadvantages of approach. L' ANA L Y S E I N S T R U M E N T A L E DE P L U S I E U R S E L E M E N T S PA R A C T I V A T I N E U T R O N I Q U E DE T I S S U B I O L O G I Q U E EN E M P L O Y A N T U N S E U L E T A L O N - C O M P A R A T E U R E T EN T R A I T A N T LES D O N N E E S PAR C O M P U T E U R On pr~sente une description ddtailMe d'une approche ~ l'analyse imtrumentale de plusi dldments par activation neutronique pour la mesure de routine des all,inChes-traces dan., ~chantillons biologiques. Les principaux nouveaux aspects de cette m~thode sont l'emploi c seul dldment, le cobalt, comme dtalon-comparateur et l'emploi d'une mdthode d'ajustcment donndes par computenr en utilisant une transforme Fourier. On donne un traiten thdorique de la mdthode ~ comparateur unic ainsi que des d~tails pratiques provenant d pdriences, suivis d'une br~ve description du programme ~ computeur avec appui parficuliex les aspects de l'ajusternent des donndes. Ensuite on pr&ente quelques r6sultats des rues d'dl~xnents-traces darts du tissu biologique, et on termine la communication avec une cution des avantages et des d~avantages de cette approche. M H O P O a J I E M E H T H B I H I I P H B O P H b I H H E H T P O H H b l H AKTHBAILHOHHbI] A H A J I H 3 BHOJIOPHqECHOIgI T H A H H HA OCHOBE C T A H ~ A P T A O ~ H H O q H HOMIIAPATOPA CO O BPABOTHOI;I ~ A H H b I X HOMIIbIOTEPOM rlpe~cTaB~eHo no~tpoSHoe o~caHHe no~xo~a it py~HH~M ~SMopeHH~M XSOTOm HH~[IIIHaTOpOBB 5HoJIorl~qec~t~txTHaHHXnp~l HOMOI~HMH0rO~JIeMeHTHoronpH6opHoro aHaJi~ IlplIHI~HIIaJII,H/aIeHOBtae0c06eHHOCTH: IIpHMeHeHHeO~tIH0qHOPOgJIeMeHTa,Ito6aJIBT,BKatle¢ cTaH~apTa ~oMnapaTopa, np~MeHe~me Me~o~a cr~amHBa~za ~a~HmX RO~n~Tepo~, ~CrI¢ ~y~ npeoSpasoBaHze ~ypse. ~aeTc~ ~eopeTz~ec~oe ~syqen~e MeTo~a O~IBO~HOrO I~O~ala~ Topa, a TaHoe npa~THqec~He s~cnepHMeHTaJi~H~e ~aHH~e. 8aTeM ~aeTcH Rop0T onHcaHHe nporpamMta HoMHB~orepa, rto~qepHHBaa, s t~aC~HOCT~t, HeRoTop~e BOnp CFJIa~RHBaHHH~IaHIi'I~tx. Tor~a npe~cTaB~IeH~ He~oTop~e peay~TaT~ HaMepeHllH HSOTOIII HHJIHHaTOpOB B 6Ho3orHqecHHx Tl~a~Lt~x. B 8aKJi~oqeitHll 06cy~eHI, I npeHMyn~ecTB~ n e ~ o c T a ~ ~TOrO no~xo~a. * Present address: Saskatoon.

Department of Nuclear Medicine, Saskatoon Cancer Clinic, Univcrsir

344

D. M. Linekin

INSTRUMENTELLE MEHRELEMENT-NEUTRONEN-AKTIVIERUNGSANALYSE VON BIOLOGISCHEM GEWEBE UNTER VERWENDUNG EINES EINZIGEN KOMPARATOREICHMASSES UND DATENVERARBEITUNG DURCH RECHENAUTOMATEN Eine amfuhrliche Beschreibung yon Schritten zur imtrumentellen Mehrelement-NeutronenAktivierungsanalyse ftir laufende Spurenelementenrnessungen in biologkschen Proben wird vorgelegt. Die haupts~chllchen neuen Ztige dieses Verfahrens sind die Verwendung eines einzigen Elements (Kobalt) als das Kornparatoreichrnass, und die Verwendung eines Rechenautomaten zur Datengl/ittung vermittels einer Fou.rierschen Bildfunktion, Eine theoretische Behandlung des Verfahrem mit einera Komparator folgt, zusamrnen mit praktischen Einzelheiten der Experimente. Eine kurze Beschreibung des Rechenautomatenprograrnms mit besonderer Betonung der Datengl~ttung schliesst sich dann an. Einige Ergebnisse der Spurenelementenrnessungen in biologischen Geweben werden dann vorgelegt, und der Beitrag endet mit einer Er6rterung der Vorteile und Nachteile dieses Vorgehens. 1. I N T R O D U C T I O N THE COMPARdkTORmethod of neutron activation analysis involves the determination of the mass of an element by simultaneous irradiation of a known mass of the same element and comparison of their respective activities. I f the irradiation and detection conditions are identical for both standards and unknown, the masses are directly proportional to the activities. T h e application of the lithium-drifted germ a n i u m detector to activation analysis made possible the simultaneous determination of several trace elements in biological materials without the necessity of any chemical separations, m T h e use of multielement rather than individual element comparator standards greatly improved the overall productivity of results in terms of technician, reactor, spectrometer and data processing time. However, the preparation and calibration of such standards required much careful effort. The author's experience with a multielement standard approach using two separate standards ~ was fairly successful. However, this method had certain disadvantages. First, in any given irradiation, one third to one quarter of the rabbit space consisted of the two standards. Second, each standard had to be carefully weighed and had to be counted twice, once for the short-lived isotopes and again for the longlived isotopes. Each standard also had to be computer processed twice, all of which led to less efficient use of time and money than would be desirable. Finally, this approach limited the number of elements which could be analyzed to those which were present in the two standards.

I f a different element was to be analyzed on a routine basis, a completely new multielement standard containing it had to be prepared. The resulting homogeneity and standardization checks etc., were costly and time consuming. I t soon became obvious that some sort of single comparator method was necessary.

2. T H E O R Y OF S I N G L E C O M P A R A T O R METHOD T h e following equation relates the photopeak counting rate at any time after irradiation to all the constants and variables of significance A - - rn6SD(rf~N°ef~

M

(1)

where A = photopeak counting rate, rn = mass of element, $ -----neutron flux, S = saturation factor, D = decay factor, (r -----thermal neutron cross s e c t i o n , f / = isotopic fractional abundance, .~ro = Avogadro's number, ~ = photopeak 7" efficiency, f / - = photopeak ?,-fractional abundance, M = atomic weight of element. Rewriting this expression so that all the nuclear constants are on one side of the equation

A

,,fdCof,

em~SD =

M

(2)

For a fixed geometry and detector, the photopeak efficiency is also a constant. Therefore, a constant aZ is defined as K =

c;fi N o f , e

M

(3)

K is a constant for each photopeak of each

MultMement instrumental neutron activation analysis of biological tissue

As before, Kr, may also be expressed as

isotope. It m a y also be expressed as K=

A,

(5)

is the saturation photopeak counting rate. From (4) and (5) it is seen that, physically, K is the counting rate per unit mass of element per unit neutron flux at the end of an irradiation long enough to produce saturation activation for a specific photopeak of a given isotope. In the case of activation analysis of a single element using a known mass of the same element as the comparator standard, the following relation is valid A, As*

me,

(6)

-

where the asterisks denote the corresponding quantities of the standard. The mass of the unknown element m a y then be expressed as m --- r~* .4~ 4"

A** ~ "

(7)

From (3) we saw that for a fixed geometry for each photopeak of each isotope, K was a characteristic constant. According to GIa-~au)~(a), this fact may be taken advantage of in calculating relative X-values; that is, the ratio of two different X-values. Girardi selected one e°Co photopeak and the lSSAu photopeak as his standards and calculated relative X-values for photopeaks of several other isotopes with respect to them. The expression for the K-value of the standard, corresponding to (3) is X * = ~ * f ' * ~Vof,* ~*

M*

(8)

From (3) and (8) the relative K-value, Kt. , is defined as aZ f ~eM * K r -- a . Z . f . e . M .

x, =

(4)

where A A, -- SD

345

(9)

This is a constant for each isotope photopeak, and m a y be calculated from efficiencies and nuclear constants.

A, / As*

(10)

Physically, the relative K-values, Kr, represent the ratio of the saturation photopcak counting rate per unit neutron flux per unit mass of any clement with respect to the corresponding quantity for the standard. The mass of the unknown clement m a y then bc expressed as 1 l 1 m --A,/m,K,f~A,

(11)

where

f , = 4/~* (12) is the ratio of the average thermal neutron flux activating the unknown sample to the corresponding flux activating the standard. The first term in (1 l) is a constant for each irradiation. It is identical for all photopeaks of all samples in a given irradiation. The second term is a constant for each photopeak of each isotope. It is independent of activation, decay and counting times or nature of the sample. The third term, the flux ratio, is the same for all photopeaks in a given sample but may be different for each sample of each irradiation. The final term must be determined for each photopeak of each sample in each irradiation. 3. P R A C T I C A L A P P L I C A T I O N OF SINGLE COMPARATOR METHOD In the routine practical application of the single comparator method to multielement neutron activation analysis, some of the quantities in (11) are determined indirectly. The saturation photopeak counting rate, As, is calculated from irradiation times, decay times, counting times, disintegration constants and photopeak areas. First, the photopeak counting rate at the moment the spectrometer is turned on, Ax, is determined. This quantity may be expressed by ~A~r A1 - - 1 - - e-aat (I 3) where 2 is the disintegration constant of the isotope whose photopeak is being analyzed, AN is the number of counts in the photopeak, and At is the counting time. The saturation photopeak counting rate is

D. M. Linekin

346 then computed from (5) As

--

1

A 1 eaArl -e-at

(14)

where T is the irradiation time and T 1 is the decay time between the end of the irradiation and the beginning of the count. A similar expression holds for the standard. The discussion of the practical application of the single comparator method is now divided into two sections depending upon the nature of the single comparator standard.

3.1 Pseudo-biological matrix standard In this case the standard is considered to be a single element uniformly distributed in a matrix similar to that of the unknown biological samples to be analyzed. An amount of the standard matrix approximately the same as that of the unknown biological samples is placed in a separate irradiation vial. T h e standard and several unknowns are then irradiated and counted under identical conditions. T h e neutron fluxes ~ and q~* are not measured directly. However, their ratios, f~, are determined by wrapping cobalt-aluminum alloy wires of uniform composition around the circumference of each irradiation vial, unknowns and standard, and calculating the ratios of their specific activities. T h e justification for this is seen upon examination of (4), (5) and (12) A[SDKm f* = A*/S*D*K*m* "

(15)

are proportional to their photopeak counting rates. Moreover, since the wires are highly homogeneous, the mass of each wire is proportional to the mass of cobalt in it. Equation (15) therefore reduces to

f , -- a/a*

where a and a* are the integral mode counting rates per unit mass of cobalt-aluminum wire for unknown and standard, respectively. These will be referred to as the specific activities of the cobalt wires. I f the mass of the element used as the single comparator standard in the pseudo-biological matrix standard, m*, and the relative K-value with respect to this element, Kr, are accurately known, (16) and (14) may be used with (11) to compute the mass of each unknown element. 3.2 Neutronflux monitor wire standard In this case, the method actually used by author, the single comparator standard is in the form of the highly homogeneous cobalt-aluminum alloy wires wrapped around the circumference of each unknown irradiation vial (there is no vial containing a standard). These wires are then irradiated under the same conditions as the unknown samples but are counted in a NaI well detector. As before, the specific activity of each cobalt wire is determined. T h e specific activity of the "standard", a*, is defined as the mean of each of the individual specific activities N

a * = Z aJN" Because all wires are irradiated for the same time, the saturation factors are identical. Normally the wires are counted sequentially in a NaI well counter operating in the integral mode about 2 weeks after irradiation. T h e counting rates are high enough so that 1- or 2-rain counts may be used. Therefore, the time interval between the first and the last wire counted is short enough that the decay factors may be considered equal. After 2 weeks any possible contaminating short-lived activity has decayed away and only n°Co is present. T h e K-values all refer to e°Co, and are therefore identical. Since all wires have approximately the same n0Co activity and are counted under identical conditions their integral mode counting rates

(16)

(17)

i~l

where a~ is the integral mode counting rate per unit mass of wire for the ith of N unknown vials. T h e flux factor for each vial is then determined from (16). Assuming that both receive the same thermal neutron flux, the ratio of (1) the photopeak counting rate, obtained with the Ge (Li) spectrometer, per unit mass of cobalt homogeneously distributed in a pseudo-biological matrix of the same size as the unknown biological samples to (2) the integral mode counting rate, obtained with the NaI well detector, per unit mass of cobalt-aluminum alloy wire wrapped around it is determined. AS before, the integral mode counting rate and the mass of cobalt wire

Multielement instrumentalneutron activation analysis of biological tissue are directly proportional to the S°Co photopeak counting rate and the actual mass of cobalt in the wire, respectively. Therefore a conversion factor C, which will be constant as long as the total efficiencies of the two detectors are kept constant, m a y be defined

C = A,*/m*

(18)

as*

where, for the sake of simplicity, the counting rates at saturation are employed. T h e saturation counting rate for each unknown photopeak detected with the Ge(Li) spectrometer m a y be calculated in the usual m a n n e r from (I 4). With (18), (17) and (16) in (10), the mass of an unknown element m a y be calculated from a given photopeak by 1 1 1 A, rn -- C a,* f ¢ K,"

(19)

T h e first term is a constant, and for a given irradiation the second t e r m need be calculated only once. T h e third term will be fixed for each unknown sample, and the last t e r m will be different for each unknown photopeak. 4. E X P E R I M E N T A L

METHODS

4.1 Determination of K, T h e relative K-values, K,, were determined experimentally from (10), (12) and (16). Accurately known amounts of each element of interest, including cobalt, were prepared in liquid solutions. These elements were irradiated in the same irradiation rabbit for the same a m o u n t of time. T h e y were counted with the Ge(Li) spectrometer in the same geometry used in the analysis of these elements in biological samples. Corrections for neutron flux variations from one vial to the next were m a d e using the cobalt flux monitor wires. In order to reduce the relative standard deviation and thus provide for more accurate values of K,, the 6°Co counting rate was calculated from a weighted average of two e°Co photopeak counting rates. T h e m e a n of several determinations of the experimental values o f K r for each isotope photopeak of interest are shown in T a b l e I.

347

TABLE 1. Mean experimental Kr values of principal photopeaks Isotope

Energy (MeV)

nVmSn VaSe =°aHg 61Cr 113Sn

0-158 0.265 0-279 0.320 0.393

K¢ 0.000394 0.0615 0.0591 0-0163 0-000411

198Au

0.412

0.0826

8ZBr ~4Sb ~a°mAg X~Cs 4SSc SSRb

0.554 0.603 0-658 0.796 0.889 1.078

59Fe

1.095

eSZn eOCo ~Na

1.115 1.173 1.368

0.00156 0-0890 0.0670 0-685 1.23 0-000921 0.0000760 0.00484 1.088 0-000746

4.2 Determination of C Several vials, each containing known amounts of cobalt in liquid solution, and each with a c o b a l t - a l u m i n u m wire wrapped around its outside mid-height circumference, were irradiated. After all activity due to interfering reactions and contamination had decayed, several counts were made, and m e a n counting rates were computed. T h e conversion factor C was determined from the ratio of (1) the Ge(Li) weighted average 6°Co photopeak counting rate per unit mass of cobalt in a liquid solution having the same geometry as a typical biological sample to (2) the N a I integral mode counting rate per unit mass of c o b a l t - a l u m i n u m wire in its usual counting geometry. T h e m e a n value of C was determined to be 0.324. 4.3 Sample preparation Extreme care was taken to minimize contamination and treatment of all biological samples before irradiation. Individual samples were placed in clean polyethylene irradiation vials and were lyophilized. After weighing, the vial caps were hermetically sealed by heat fusion. A 2-cm length of c o b a l t - a l u m i n u m wire, NBS No. SRM-953, was taped around the circumference of each vial at its mid-height.

348

D. M. Linekin

4.4 Activation Depending upon their size, six to eight of the vials containing biological samples were placed in an irradiation rabbit. Activation was carried out in the Massachusetts Institute of Technology 5 M W Research Reactor in a flux of 2 × 1013 thermal neutrons/crab/see for a period of 9 hr. 4.5 Counting Five or six days after activation the samples were quantitatively transferred from the irradiated polyethylene irradiation vials to polyethylene counting vials. Each sample was counted for 100 rain in a reproducible geometry 30 cm from the Ge(Li) detector. F r o m these results the concentration of Au, Br and Na were determined. Six weeks after irradiation each sample was counted again, this time for 1000 rain in a reproducible geometry 5 cm from the Ge(Li) detector. T h e analysis of the longer-lived species such as So, Cr, Fe, Zn, Co, Cs, Hg, Se, R b , Sb and Ag was m a d e from this count. T w o weeks after irradiation the cobalt wires were cleansed, weighed and counted in the integral mode in a N a I well counter which had been adjusted to obtain a reproducible total efficiency. 4.6 Detection equipment T h e N a I well counter was a Baird-Atomic automatic g a m m a counter and sample changer, Model No. 707. T h e Ge(Li) spectrometer system is shown in Fig. I and a block diagram of the spectrometer Ge ( L i ) Spectrometer

Shields

.-Liquid NilrQqen

"

.Vacuum Pump /To HV Bias Power Supply

I

Ge (Li) Deice,or

]

I

Chor I senslhve FE~FPre ornolifier O=.... ,SA ]

~ 1

i

ptinte¢.Ooto 307Nucleof

|

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t

pc~er Atomtc suppb¥ Boirci3t2~

..

1024 Channel ! analyzer Nucleor Oota t81

recorder Houston Omniq ropfliC 6~$0

oar forotor

Tatly 420

FIG. 2. Block diagram of spectrometer electronics. electronics is shown in Fig. 2. I n the energy range between 0,1 and 1.5 M e V the resolution was about 5 keV. T h e radioactive sample could not be placed closer than about 5 cm from the center of the useful volume of the Ge(Li) detector. As a result, in this position the absolute photopeak efficiency of the spectrometer varied from about 1 per cent at 100 keV to 0-1 per cent at 1.0 MeV. 5. C O M P U T E R D A T A P R O C E S S I N G T h e pulse-height analyzed raw data from the 1024 channel analyzer was transferred from the p a p e r tape to cards. These raw data cards were fed into the H a r v a r d University I B M 360/50 computer along with our T E G S (Trace Elements by G a m m a Spectrometry) computer p r o g r a m and were processed. T h e results were expressed in terms of micrograms of trace element per g r a m of freeze-dried tissue or per milliliter of blood.

~ron --.~

"~* ""'Charge Sensitive F.I=T. Preamplifier ""

-" Ga (Li} ""

Oefeafo¢

RQdioQcIivI Sample

FtQ. 1. Ge(Li) spectrometer system.

5.1 Smoothing of the raw data T h e main feature of our computer p r o g r a m is a data smoothing technique employing a Fourier transform, c4) T h e raw data was first Fourier-transformed into "energy-frequency" space. Here the low frequencies corresponded to the peaks and the higher frequencies corresponded to the background fluctuations. This Fourier transform was then multiplied by a filter function in which the low-frequency

Multielement instrumen¢alneutron a~tivationanalysis of biological tissue

349

tracted smoothed data were used in further peak analysis.

7000

6000

t !

5ooo!

• Row data *

Smoothed data

4 000 ! 2

I

3000 .

,

• ,

%~

~

z

"

"

i|

200C 360

370 380 Channel number" (1-5 k e / / C h )

Fro. 3. Effect of data smoothing using Fourier transform method. Note computer selected background curve. portion equalling unity was smoothly matched to the high-frequency portion equalling zero b y an appropriate Gaussian function. T h e choice of an o p t i m u m filter function is very i m p o r t a n t and was m a d e only after several trials and errors. T h e resulting smoothed data was now obtained by an inverse Fourier transform back to energy space. As shown in Fig. 3, for a properly chosen filter function, the smoothed data contained essentially the same spectral information as the original data but the r a n d o m background fluctuations have been eliminated. This smoothing technique enabled a better c o m p u t e r selection of the background u n d e r the peaks and therefore a more accurate deterruination of peak areas than would be possible from the raw data alone. T h r o u g h the use of the C o o l e y - T u k e y Fourier transform, computation time was greatly reduced.

5.2 Background subtraction A one-to-five-point averaging criterion was used to determine the magnitude of the background level at each chosen m i n i m u m . By linearly joining these averaged background points, a background curve was obtained, as shown in Fig. 3, which was a very close approximation to the true background curve. Special criteria were applied for the identification of partially resolved multiplets. T h e background curve was then subtracted from the smoothed data curve and the resulting background-sub-

5.3 Peak analysis All peak m a x i m a were identified by a zeroslope criterion and peak energies were calculated using a high- and a low-energy standard. Peak areas were evaluated in two ways. In the first method, in the case of singlets, the straight sums method of COV~r.L(5~ was used. All the counts between successive zeros in the background-subtracted smoothed data were simply summed. However, with multiplets, a modified Covell method was used. A Gaussian fit was m a d e to each peak in a multiplet and the contribution to any one peak from a neighboring peak was evaluated. T h e n individual peak areas within the multiplet were determined subject to the condition that the sum of these individual areas equal the total muldplet area determined by the Covell method. I n the second method of peak area evaluation, developed b y HAMAWI(~), the peak area distributions were characterized by a Gaussian with an energy dependent F W H M obtained by leastsquares fitting the F W H M of the strongest peaks in the actual data. Singlets, doublets, triplets and multiplets were handled separately, with the three strongest peaks in a multiplet treated as a triplet. I f the peak was not deformed, both methods of area determination gave the same result. I f the peak was a strong singlet the straight sums method was more accurate. I f the peak was weak or was a multiplet, the H a m a w i method was more accurate. Expressions for the standard deviations of the peak areas were derived and it was found that the expression for the error in the H a m a w i method was more accurate. T h e peak areas and corresponding standard deviations of those isotopes of interest in a particular analysis were selected and stored for later computations. 5.4 Elemental concentrations T h e mass of each element in each unknown biological sample was calculated separately for each measurable photopeak of each isotope in question. I f an isotope had more than one photopeak, weighted averages were used. Corrections were m a d e for overlapping peak interferences and provision was m a d e for weighted

350

D. SvL Linekin

averaging in cases where recounts were made. T h e results are expressed in terms of micrograms of trace element per gram of lyophilized tissue or blood. Standard deviations were propagated throughout the calculations and a final composite percentage standard deviation based upon all accountable sources of error is given for each elemental concentration. In all calculations dealing with the propagation of errors or weighted averaging, the conventions of BEElZS{:) were followed. 6. A P P L I C A T I O N S T O B I O L O G I C A L TISSUE T h e method described in this paper has been successfully applied in the measurement of the concentrations of several trace elements in various types of biological tissue, both normal and diseased. Among the elements analyzed were Au, Br, Na, Se, Cr, Cs, Ag, lZb, Sn, Zn, Hg, Sb, Sc, Co and Fe. Biological samples studied include normal h u m a n whole blood, pathological h u m a n lung, liver, heart, kidney and transplanted mouse tumors. A typical g a m m a ray spectrum of neutron activated lung tissue 9 days after activation is shown in Fig. 4. This lung sample was taken from a m a n who had pulmonary fibrosis. Elemental concentrations in Fg per gram of lyophilized tissue for this sample are shown in Table 2.

BfSZ Sets

B OZ

g5

Zn

snlrTm

$c4s • ~ ~1~o s

8

sr z

Se'm

Sb t24X5

ere2

C,S~

z

6fezs¢44

#

gbg4

e*eli

Aul'~l

!

o.~

[ o.~

i o-z

i I o.~ 0-4

I I o.~ 0-6

I o.r

I l o-a o 9

Energy, Fla.

4. G a m m a

F.esw Co~ °

ray

t I ~.o ~.~

~z

J Cc~O 4 ~-3

-' ~.4

' ~.5

MeV

spectrum

of

neutron

activated diseased lung tissue, 9 days after activation.

TABLE

2. E l e m e n t a l

c o n c e n t r a t i o n s ' of

neutron

activated diseased lung tissue (pg per gram of lyophilized tissue) Element

Concentration

Se Fe Hg Cr Au Br Sb Ag Sc Zn Co Na

1.47 674 24.4 5.53 0.0185 17.8 0-356 0.552 0.0713 10-3 1.25 159

7. D I S C U S S I O N T h e advantages of the pseudo-biological matrix standard approach to multielement activation analysis are several fold. First, the standard takes up only {--} of the rabbit space. Second, very little time and effort is required to prepare the standard for irradiation. Third, the standard need only be counted once. Fourth, computer time is minimized. Finally, as more accurate relative K-values are obtained, the resulting accuracy of the concentrations of the unknown elements is improved. The advantages of the neutron flux monitor wire standard approach are even greater. First, the entire reactor rabbit space is used for unknown samples. Second, the Ge(Li) detector time is devoted entirely to the analysis of unknown samples. Third, there are six to eight standards for each irradiation, the mean of which is more accurate than any single one. Fourth, by counting in the integral mode with a NaI well detector, the statistics are improved to such a degree that the standard deviation of the mean specific activities of the "standard", a*, are obtainable to less than one percent. Finally, the technique may be easily extended to the study of new elements, simply by experimentally determining appropriate relative Kvalues. A question which might be raised about this approach is whether the K-values are really constants. From (3) we see that the nuclear constantsfi, No,f~ and M are certainly constant. However, e, and ~ may change.

Multielement instrumental neutron activation analysis of biological tissue

The total photopeak efficiency, e, may be checked in two ways. First, periodic counts with a ~°Co reference standard permit us to monitor the geometry and the electronics of the spectrometer system. Second, by periodically counting an isotope such as l ~ L a which has several photopeaks throughout the entire energy range of interest, it is possible to maintain a relative constancy check on the intrinsic photopeak efficiency at different energies. Changes in e during this investigation have been negligible. I f changes are made near the rabbit position of the reactor, the reactor neutron spectrum may be affected. As a result, or, the effective activation cross section of an element may change. I f the change in o is different from the change in cr*, then errors are introduced. As Girardi points out, a cadmium ratio measurement for cobalt can be done periodically to check this possibility. For well thermalized neutrons, small changes in the epithermal contribution do not produce an significant effect on most elements. As long as the cadmium ratio remains above about 40, which is the case at the M.I.T. Reactor, this is not a problem. T h e burial of lower energy photopeaks in the Compton background of higher energy photopeaks is a severe limitation with our existing counting equipment. However, with improved counting efficiency and resolution along with sophisticated anti-coincidence electronics, it should be possible to improve the accuracy of measurements of those elements presently being studied as well as measure such elements as Ba, Cs, Cd, Ta, Hf, La, Zr, Ni, M n and Ca in m a n y biological samples. By judicious selection of activation, decay and counting times the relative sensitivity of either short- or long-halflife elements may be improved. For example, short-half-life elements such as C1, A1, K, Cu and Mg could be measured in many biological samples by choosing short activation and counting times and a minimum decay time. On the other hand, better measurements of long-

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half-life elements such as Co, Zn, Se, Ag and Cs will result if these times are long. It should be noted that although this technique was developed for trace element measurements in biological samples, there is no reason why it could not be successfully employed with non-biological samples as well. 8. C O N C L U S I O N In conclusion, a detailed description of a multielement instrumental neutron activation analysis approach to routine trace element measurements in biological samples has been presented. This approach, using a single comparator standard and data processing by computer, should also be useful in studies of nonbiological samples. Acknowledgements--The author wishes to thank G. L.

BROW~ELL for his helpful advice and J. F. B.,a~cavs for his technical assistance. This work was supported in part by U.S.P.H.S. Research Grant No. 5R01GMlI173. REFERENCES I. COOPER l~. D., LINEKIN D. M . and BROWNELL

G. L. Nuclear Activation Techniques in the Life Sciences, pp. 65-80. Vienna (1967).

2. l.a~EKm D. M., BALCrUsJ. F., COOPERR. D. and BROW~rELL G. L. Modern Trends in Activation Analysis, Vol. 1, pp. 110-113. NBS Special Publication 312, U.S. Government Printing Office, Washington, D.C. (1969). 3. GIg.~a~I F., Guzzx G. and PAu~J. Analyt. Chem. 37~ 1085 (1965). 4. ~ E R T., INotvx,~ T. and R.mMUSSEN N. C. GAMANL, A Computer Program Applying Fourier Transforms to the Analysis of Gamma Spectral Data. Dept. of Nucl. Engng, M.I.T., MITNE-97 (1968). 5. COVXLLD. F. Analyt, Chem. 31, 1785 (1959). 6. H ~ n w I J. N. Investigation of Elemental Analysis using Neutron Capture Gamma-Ray Spectra. Dept. ofNucl. Engng, M.I.T., MITNE107 (1969). 7. BEEgSY. Introduction to the Theory of Err'or. AddisonWesley, Reading, Mass. (1962).