1158
Anal. Chem. 1982, 5 4 , 1158-1163
Radiochemical Determination of Technetium-99 J. H. Kaye," J. A. Merrlll, R. R. Klnnlson, M. S. Raplds, and N. E. Ballou Pacific Northwest Laboratory, Richland, Washington 99352
A radlochemlcai method has been developed for determlnatlon of "Tc by which both the chemical yield and the yieidcorrected amount of "Tc In a sample may be determined. The chemical yield estlmatlon is based upon counting of lowenergy internal-conversion electrons from the added isotopic tracer 07mTc. Uncertalntles In the "Tc and yleld estlmates depend on the amount of 07mTc tracer used, the amount of '%In the sample, the fraction of tlme spent In counting with an added absorber, and the total countlng perlod. For a 2000 mln total counting period wlth 50% of It spent in countlng with added absorber, with 20 counts mln-' of 07Tc tracer added originally to the sample, and wlth 50% chemlcai yleld the chemlcal yield uncertalnty (relative standard devlation) Is 2.3 % and the detection llmlt Is approxlmately 44 mBq of '%.
Sensitive methods are needed for evaluating the potential hazard due to release of 99Tc to the environment from nuclear power plants, nuclear fuel reprocessing operations, and other sources. Technetium-99 is of concern because it has a relatively high fission yield (6.1% from 235U),has a long half-life (2.13 X lo5years), may be concentrated in plants and animals, and may be released to the environment in significant quantities as the volatile heptaoxide, Tc207,or as the very soluble and highly mobile pertechnetate ion, TcO,. During the conversion of uranium oxide resulting from reprocessing of reactor-irradiated uranium into UF6 prior to reenrichment at a gaseous diffusion plant, any technetium present in this oxide will also be transformed into the volatile fluoride. The most widely used method for radiochemical determination of 99Tc is by ,6 counting of a purified sample. The detection limit of this method is calculated to be about 3 mBq for a 2000 min counting period, assuming a 50% chemical yield and measurement with a low-background, anticoincidenceshielded GM detector. A serious problem with this method, however, has been the lack of a stable isotopic tracer for use in determination of the chemical yield (1). Similarly, no radioactive isotopic tracer has been identified which would not interfere with measurement of 99Tc. One approach has been to use another element, such as rhenium, as a stand-in for technetium. However, this introduces limitations on the chemical separation steps which can be used if one is to be sure that the two elements truly behave identically. Another approach has been to calibrate a standardized radiochemical procedure for its radiochemical yield by means of tracers such as 9 5 m T(half-life ~ 61 days) or 9 9 m T(half-life ~ 6.02 h). The accuracy of this method is seriously limited since reproducibility of chemical yields may be poor, especially if high radiochemical purity is required. In this paper it will be shown that these problems can be circumvented by use of the 90-day isomer, 97mT~, as an isotopic tracer. When isotopically enriched %Ru is irradiated with thermal neutrons, 9 7 R is ~ formed, which decays with a 2.88 day half-life. Although nearly all of this decays to the long-lived ground state of 9 7 T(half-life ~ 2.6 X lo6 years), 0.0385% of the (2). ~ decay events lead to formation of the 90-day isomer g 7 m T This isomer decays by emission of a 96.5-keV y-ray in an M4-type transition which is almost entirely internally converted. Consequently, electrons with energies of 75 or 94 keV 0003-2700/82/0354-1158$01.25/0
(96.5 keV minus the electron binding energy) are given off. The basis for the method which will be described lies in the fact that these relatively low energy electrons may be absorbed by an aluminum absorber placed over the sample, whereas only part of the higher energy ,6 radiation from 99Tc(E,,, = 293 keV) is absorbed. By taking two counts, one with the absorber and one without, it is thereby possible to distinguish the counting events from the two isotopes and determine the chemical yield as well as the amount of 99Tc in the sample. In the discussion which follows, the development of this method and its application to the analysis of 99Tc in environmental samples will be described. EXPERIMENTAL SECTION Chemical Separation, Purification, and Counting Procedures. Procedures for the separation, purification, and electrodeposition of technetium from environmental vegetation samples have been published earlier ( 3 ) . A method for the preparation of 97mTcisotopic tracer is included in this reference. Recently prepared tracer contains both 9 7 T(Tl ~ 2.6 X lo6 years) and 9 7 m T(TI ~ - 90 days). Irradiation o/ 1 i m g of isotopically enriched %du(97.92% enrichment, obtained from Oak Ridge National Laboratory) in a thermal neutron flux of about 1014n cm-2 s for 100 h yields a total of about 4 X lo5 counts min-' of IvlTcat 100 days after the end of irradiation under the counting conditions described below. It is necessary to allow the activation product 9 7 Rt o~ decay for several half-lives ( T I j 2= 2.88d) prior to chemical separation in order to permit the technetium activities to be formed. The separation procedure for vegetation and soil samples basically involves the addition of tracer vmTc solution to the dried sample, followed by ashing of vegetation, fusion of the ash with sodium peroxide in a Zr crucible,precipitation of hydroxides, ion exchange and solvent extraction separations, and electrodeposition. This procedure is a modification of that described by Foti, Delucchi, and Akamian (4). For soil samples, a sodium peroxide fusion method is used to dissolve the sample. A sodium peroxide to soil ratio of at least 5:l appears necessary to ensure complete fusion of the sample. Aqueous samples are adjusted to 2 M with ",OH, directly loaded onto a strong anion-exchange resin column (e.g., Bio-Rad AGl-X$ 100-200mesh) followed by elution with 6 M "OB, cation exchange and solvent extraction separations, and electrodeposition. The electrodeposited sample is a 5 mm diameter circle on a metal cathode. Iridium cathodes were used for our measurements, but other metals could be used as well. Two counting measurements are taken of the electrodeposited sample. For the first count, only a thin (0.83 mg/cm2) protective Mylar cover is placed over the sample, and the sample is counted for loo0 min. Then an aluminum absorber 14 mg/cm2 thick is placed over the sample to absorb 9 7 m electrons T~ and a second 1000-min count is taken. The method for determining the chemical yield and amount of 99Tcin the sample from the counting data will be discussed in detail in a later section. With this fusion procedure and after subsequent dissolution of the melt with HC1 there has been little or no residue remaining for the cases of the vegetation and soil samples analyzed to date, and any residue that has survived attack by molten sodium peroxide is unlikely to contain a significant amount of V c . Tracer studies using the y emitter 96mTcwere done with both vegetation and soil samples to verify that negligible loss of tracer occurred during the charring, ashing, fusion, and dissolution steps. Use of the powerful oxidizing agent sodium peroxide for the fusion step ensured that whatever its initial oxidation state might be, the technetium would end up in the +7 valence state and that 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982 0 1159
exchange with the s7mTctracer would occur. For water samples it was assumed that the V c would be in the soluble Tc04- anionic form and that therefore it would exchange with the s 7 m T tracer ~ which was also added in this form. Each iridium cathode was counted before and after electrodeposition, and again after dissolution and removal of the technetium deposit for those samples which were recycled through further decontamination chemistry steps. A finite but not significant amount of activity remained on the iridium cathodes in most cases. This was not removable with nitric acid but disappeared when the cathodes were cleaned with metal polishing compound. In order to verify the completeness of decontamination and reproducibility of the counting procedure, we recycled a set of measured samples through an additional purification procedure and counted again. The recycle procedure was as follows: The counted sample was dissolved by dropping 10 pL of concentrated HNO, directly onto the deposit. After 20 min, the liquid was transferred to a tiny quartz test tube (see Figure 4 of ref 3). An additional four 50-pL alnquots of a solution containing 200 pL of concentrated NH40H and 10 pL of HzOzwere used to leach any remaining technetium from the cathode. These solutions were also transferred to the test tube. The technetium sample was purified by two solvent extractions of the tetraphenylarsonium complex, as described in steps 2-10 of ref 3. The final solution, which contained the technetium in 100 pL of concentrated ",OH, was transferred into a 30-mL glass vial. The test tube was rinsed twice 'with 100-pLaliquots of distilled water, and these were also transferred to the vial. Then 2.6 g of NH4Clwas added, the solution was diluted to 20 mL with distilled water and transferred to a 'Teflon electroplating cell. The technetium was electrodeposited onto an iridium cathode, which was then counted as before with and withlout the aluminum absorber. Detectors. The detectors used for these measurements are low-background Geiger-Muller counters, each consisting of a primary detector 1.2 cm high, 7.4 cm wide, and 9.0 cm long, surrounded on the sides and top by an anticoincidence (AC) detector. The primary detectors have a sensitive area of 113 mm2 which is covered by a gold-coated Mylar window. Two of these primary/AC detector pairs were placed inside a 10-cm thick lead cave for these measurements. Counting gas (98.7% helium, 1.3% isobutane) was passed through the counters connected in series at a flow rate of about 0.16 cm3s-l. Lucite holders were fabricated for each detector which allowed the iridum cathode samples, mounted in Lucite trays, to be slid into the counting position. The background obtained with these detectors is approximately 0.15 counts rnin". Calibration of Detectors. Two different methods were used for determination of the constants used in the calculations to be described later. In both cases the primary ssTc standard was a ~ from Amerssolution containing 2.3 X lo6 Bq of s s T obtained ham-Searle Corp., Arlington Heights, IL. Dilutions of this primary standard were made as necessary with 1 M NH40H. For the measurements reported in Tables I and 11, calibration of the detectors with V c was achieved by evaporation of lW-pL aliquots of a known dilution of the s s T standard ~ onto iridium cathode plates. A special Teflon cell was made for this purpose which has a 5.1 mm diameter hole, the same size as that of the electrodeposition cell. The iridium plate was clamped between this cell and a second piece of Teflon. A small cross was scribed onto each iridium plate to facilitate centering. Three separate s 9 T ~ standards were prepared and counted, each containing approximately 630 mBq of ssT'c. The wmTcstandards were prepared in a similar manner, except that only 10 p L of solution was used. If a larger amount was evaporated a measurable reduction in counting rate occurred, presumably due to self-absorption of some of the low energy electrons by impurities in the evaporated deposit. A better calibration procedure has recently been developed which is an improvement in that both the s S T and ~ s7mT~ standards are prepared by electrodeposition in the same manner as the samples. A 100-pL aliquot of a known dilution of the primary %Tc standard was electrodeposited onto an iridium cathode, and this cathode was counted with and without the ~ added to the aluminum absorber. A known amount of s 7 Twas plating solution after electrodeposition and this solution was
loonh
I VALUE 14 mgicm' USED I
THICKNESS OF ALUMINUM ABSORBER img/crn?
Flgure 1. Aluminum absorption curves for ""'Tc and "Tc.
analyzed by mass spectrometry after separation and purification T ~ the of the technetium (3). From the resulting s 7 T ~ / 9 9ratio, amount of s 7 Tadded ~ after plating and the amount of %Tc originally added to the plating solution, the plating yield and the amount of wTc deposited on the cathode were determined. The counting rates with and without absorber divided by the activity ~ on the cathode gave the constants a. and a14 of S g Tdeposited which are used in the computations to be discussed later. In a similar manner an aliquot of Q7mTc + s 7 Tsolution ~ containing a known weight of s 7 T ~ determined , by isotope dilution mass spectrometry with addition of a known amount of V c , was electrodeposited and counted with and without the aluminum absorber. A known amount of T c was then added to the plating solution and after purification the plating yield was again determined by isotope dilution mass spectrometry. Thus the s7mT~ counting rate for a given quantity of 9 7 m T +~9 7 Tsolution ~ was established. It was necessary to use a relatively high activity level in~order to achieve an acceptable uncertainty value for of s 7 m T the counting rate of the LnmTc with the aluminum absorber. Much smaller activity levels of wmTcwere actually added to the samples. Measurements on Environmental Samples. Several vegetation samples were collected from areas on the Hanford Reservation of the Department of Energy suspected of containing low levels of ssTc. Six of these samples were analyzed for 99Tc content. One sample of preatomic age alfalfa, assumed to contain no *TC,was also analyzed. One reagent blank was run through the procedure. To each sample 60 counts min-l of s7mTcisotopic tracer was added. The technetium fraction from each sample was separated, purified, electroplated,and counted. Then each deposit was dissolved, the technetium was further purified by means of the recycle steps previously described, and each was again electrodeposited and counted. The chemical yield and the quantity of wTc in each sample after yield correction were calculated from the counting data as described in the following section. Eight soil samples were run from two locations on the Hanford Reservation, four samples from each location, at nominal depths of 5, 15, 25, and 35 cm. One sample of Ritzville-type soil, which did not contain V c , was also run. To each of these samples 6.3 counts min-' of wmTcwas added. These samples were not recycled. Calculations. The measurement method involves taking two counts, one with only 0.83 mg cm-2 of Mylar covering the sample, and a second with an additional amount of aluminum foil thick enough to stop the low-energy electrons from 97"Tc (energiesabout 75 and 94 keV) but thin enough so that a significant fraction of the fl particles from T c (E- = 293 keV) are transmitted. Figure 1shows the relative counting rates for 97mT~ and q c as a function of thickness of aluminum absorber. An absorber thickness of 14 mg/cm2 of aluminum was chosen because this is sufficient to absorb the electrons from s 7 m T and ~ yet permits 24% of the 99Tc activity to pass through. This thickness was obtained by use of three layers of household aluminum foil. The net counting rates with and without the aluminum absorber can be related to the amounts of g9Tcand s 7 m Tin~ a sample by No = N14
=
+ kobo
U~XY
a14xy
'
k14b14Y
(1) (2)
1180
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
where No = the net counting rate with Mylar absorber, N 1 4 = the net counting rate with 14 mg/cm2 aluminum Mylar absorber, x = the amount of 99Tcoriginally in the sample (in mBq), y = the chemical yield, expressed as a fraction, a. = the counting rate for a standard containing only SBTc, in units of counts m i d per mBq of q c , with only a Mylar absorber over the standard, a 1 4 = the counting rate for a standard containing only q c , in units of counts m i d per mBq of geTc, with Mylar and 14 mg cm2 aluminum absorbers over the standard, bo = the counting rate in~an amount equal to that for a standard containing only 9 7 m T added to each sample, in units of counts m i d , when counted with only a Mylar absorber over the standard, b14 = the counting rate for a standard containing only 97mTcin an amount equal to that added to each sample, in units of counts m i d , when counted with Mylar and 14 mg cm-2 aluminum absorbers over the standard, ko = a decay correction factor used to correct for decay of the 87mTc standard to the time the sample was counted, for the count taken with only a Mylar absorber over the sample, and k 1 4 = a decay standard correction factor used to correct for decay of the 97mT~ to the time the sample was counted, for the count taken with both Mylar and 14 mg cm-2 aluminum absorbers over the sample. Equations 1 and 2 can be solved for the unknown quantities x and y, which leads to the following equations.
+
x =
- b0k814
b14k14N0 a d 1 4
-
(3)
(4) The effect of uncertainties in the quantities on the right-hand side of eq 3 and 4 is now considered. Uncertainties in No and N 1 4 arise from the statistics of the counting process and variations in the detector backgrounds. Uncertainties in the "constants" ao,~ 1 4 bo, , and b14 likewise occur due to counting Statistics because these values are derived from counting data on the standards. Uncertainties in ko and k 1 4 can be the result of an uncertainty or~in the decay time intervals. in the decay constant for 9 7 m T With use of the standard procedures for calculating propagation of errors, the variance in x due to uncertainties in the eight quantities on the right-hand side of eq 3 can be expressed as
In eq 5 the uncertainties in each parameter are given by the value of u for the parameter and the partial derivatives of x with respect to each parameter are given as
ax _ ah0
-bd14 a 8 1 4
-a d o
Equation 14 can be derived from eq 5 through 13. g~~:No?(aobi4k14 - a14boko)~/(afl14 + (g,2Ni42 + ~ , , , 2 N 0 2 ) ( b o k f 1 1 4 - b i 4 k 1 4 N o ) ~ / ( a o N l r Ni42(g!+,2b02 + gb:k02) + No2(gkl,2bi42 + gb1,2k142)
ox2 = (UN:NI~' a14No)~
+
( a f l 1 4 - a14NO)'
(14) Similarly, the variance in the chemical yield, ,:TL to give the result shown in equ 15. =
+
%4
(a814
ax -_ ab0
- b0k814)
- a14N0)2
-k814 a d 1 4
- ala,
(11) (12)
+ (ai42aa,2 + ao2ga,,2)(bokf114 - a o b 1 4 k i 4 ) ~+ (afli, - a i 4 N o ) 2 ( a o 2 b 1 4 2 g k 1 ~ + a 0 ~ k 1 4 ~ % , , 2/)( a i 4 b o k o - a o b 1 4 k d 4
( g ~ , 2 a i 4 ~ g ~ ~ : a o ~ ) ( a i 4 b o k o- a o b 1 4 k i 4 ) ~ / ( a i 4 b o k o aobi4k14)~
b14ki4N0)~/(al.&oko
+ ai42b02gk,2 +
(15)
A programmable calculator was found convenient to use for numerical solutions of the equations. Data entered included the net counting rates and uncertainties with and without the aluminum absorber, the times these counts were taken, and the quantity of sample analyzed. Output included the chemical yield ~ and its uncertainty and the yield-corrected amount of g g Tper unit quantity of sample and its uncertainty. Other programs were written to calculate the constants ao, bo, a14,and b14 and their uncertainties and to calculate net counting rates and uncertainties from raw counting data. Empirically it has been observed that in addition t o the uncertainty due to counting statistics there is an additional uncertainty caused by random virations in the background of the detectors. This variation is approximately 0.02 counts m i d for these detectors and may be related to cosmic ray flux variations. To take this into account, we added the value 0.02 counts m i d to the counting statistical uncertainties uN0and oNl,. RESULTS AND DISCUSSION Results of analyses for 9 9 Tin ~ environmental vegetation samples are given in Table I. Results from measurements both before and after recycling are given. Prior to recycling, tracer ~ was 30-40 counts mi& in the count rate of the 9 7 m T these samples after correction for chemical yield. After recycling (which was unavoidably delayed), the count rate of the tracer after correction for chemical yield had decreased to 6-11 counts min-l due to decay. Very low but measurable levels of 99Tcwere found in all of the contemporary environmental vegetation samples. No detectable amounts of %Tc were found in the preatomic age alfalfa sample or in the reagent blank, using the detection limit criteria given by Currie (6). With one exception (sample 5) there was agreement within statistical errors between measurements before and after recycling, although these errors were admittedly rather high in some cases. Sample 5 may not have been completely purified for the first measurement or some other unidentified error may have caused the value to be approximately 36% lower after recycling. Results of analysis for 99Tcin environmental soil samples are given in Table 11. Only the first sample listed gave a positive result, 7.0 f 1.5 mBq/g of dry soil. This sample was later measured by isotope dilution mass spectrometry (3)and a value of 7.7 1 mBq was obtained. Chemical yields ranged from 27 to 64% for these samples. Estimation of the Critical Value and Detection Limit. If certain assumptions are made about the efficiency of the counters, their background, and the counting periods with and without the aluminum absorber, it is possible to use eq 3-4 and 14-15 to estimate the critical level, detection limit, and the uncertainty in the chemical yield for given amounts of g7mTctracer added and for different counting times with and without absorber. The overall uncertainty in the 99Tcvalue and the chemical yield uncertainty may also be calculated as a function of the amount of 99Tcin the sample. Results of
*
_ ax - N0(b14k14N0
can be derived
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
1181
____ Table I. Results of Analyses for 99Tcin Environmental Vegetation Samples
sample no.
sample type
1
asparagus, aerial portion cottonwood, leaves cottonwood, bark cottonwood, wood willow, leaves Canadian thistle alfalfa (preatomic) reagent blanka
2 3 4
5 6 7 8 a
% chemical yield before after recycle recycle
99Tcactivity, mBg/g before recycle after recycle
wt of
sample, g 14.88
13.7 + 0.7
13.0 + 1.0
96
1
65r 3
15.00 15.02 10.00 15.04 10.69 14.75
3.7 f 0.5 4.4 + 0.6 3.0 f 0.9 16.1 f 0.8 489 + 34 1.0 f 0.8 0.9 f 0.8
4.4 * 1.0 6.1 c 1.1 2.8 f 2.0 10.3 i: 1.1
76+ 1 93+ 1 69f 1 96 + 1 37 f 3 43 f 1 57 + 1
47 i 4 40+3 28f 3 36i: 2
15.00
5.4 i 5.3 1.7 f 1.4
f
82 3
23
*3
Reagents added equivalent to those used for a 15-g sample.
Table 11. Results of Analyses for 99Tcin Environmental Soil Samples wt of % sample, 99Tcactivity, chemical sample sample mBq/g yield no. t Y Pe g 7 . O i 1.5 41 i: 4 1 soil, 5 c m , 12.8 site A 1.3 + 1.5 2 soil, 1 5 c m . 9.2 50 f 3 site A 3 soil, 25 cm, 20.6 -0.6 i: 1.4 27 f 4 site A 4 soil, 35 cm, 30.1 0.2 f 0.5 62 * 4 site A . 5 soil, !5 cm, 9.9 -0.1 i 1.7 45 i: 4 site B 0.6 i: 1.6 41 i 3 6 soil, '15 cm, 10.7 site B 0.4 i: 0.6 47 f 4 7 soil, 25 cm, 30.2 site B 0.4 f 0.4 64 + 4 8 soil, 35 cm, 30.6 site B 31.0 0.4 ?: 0.8 33 i: 4 9 soil, Ritzvilletvae
70
- DETECTION LIMIT m' -E t
2
DETECTION LIMIT L N C E R T A I N T Y IN CHEMICAL YIELD
\ \
60-
V E L D UNCERTAINTY
5020 cpm
z
'
____
-
40-
f
'
T:
Scpm
20cem
J
Tc
30-
/
---___02
04
06
/
08
FRACTION OF TIME COUNTED W I T H A L U M I N U M ABSORBER
Figure 3. Detection limit and uncertainty in chemical yield for '9c as a function of percentage of time sample is counted with aluminum absorber.
A
\ \ \' \'
2 1
OW
WIN COUNTS
/ 5
10
--
/ /
/
0 /WCERTAI\TV N CYEMICAL YIELD
5 c
i
\
\
J
I
L OlO
L
d
100
, ,
1
1
m!
8
000
,
I 1 1 8 / I 8 .
10
000
1
1
# l ! l # J 100 000
TC I N SAMPLE m841
Figure 4. Uncertalnties in measurement of 'qc and of chemicai yield vs. amount of "Tc in sample, 5 counts min-' added tracer.
A M O U h T O F B l m T c T R A C E R ADDED lcpmi
Figure 2. Detection limit and uncertainty in chemical yield for 'qc as a function of amount of ""'Tc tracer added.
these calculations are shown in Figures 2-4. Uncertainties in the constants a,,, ,al4, bo, and bI4 have been included. Definitions of the critical level and detection limit are given by Currie (6). The critical level, L,,is defined as the value which must be exceeded to yield the decision "detected", with an error risk of 5%. :It is given by
L, = 1 . 6 4 5 ~ ~
1
I
A M O U N T OF
(16)
where u, is the uncertainty in the 99Tcvalue for the case in which there is no 98Tc in the sample. Equations 3 and 14 were solved for this case and the value for u, from eq 14 was substituted for uc in eq 16. The detection limit is given by Ld
= 1 . 6 4 5 ( ~+ ,
ud)
(17)
where again 5% risk is assumed. The value CTd is defined as the uncertainty in the estimate for the case in which the amount of geTcin the sample is equal to the detection limit. This value is not known beforehand (in fact this is the value
1162
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
to be determined) so as a first approximation it is assumed that the uncertainty is independent of the amount of 9gTc present. Ld
75
1.645(2~~,,)
\'"'I
35
'"I " ' I
'
'
I
' ' I
'
2 o o'p m #lrnTC
(18)
This turns out to be a fairly good approximation, since u does not change very rapidly as a function of the amount of 99Tc present. An iterative calculation procedure was then used to get better estimates of Ld and bd in which the amount of *Tc corresponding to Ld was used as input to equations 3 and 14 to come up with successively better estimates of Ld. After the first try Ld was estimated from eq 17. The assumptions used for these cases are as follows: (1)detector background 0.15 f 0.006 counts min-l; (2) chemical yield 50%; (3) ko = k14 = 1; (4) counting intervals are 1000 min with and without absorber, for Figures 2 and 4-6; ( 5 ) constant a. is 2.798 X counts min-' 99Tc/mBq g g Tand ~ a14 is 6.208 X counts min-l BeTc/mBq T c ; (6) the constant bo was set equal to the amount of 97mTc(in counts m i d ) added to the sample, and b14 was set to 4.218 X bo; (7) The values for the uncertainties in the constants were uw = 2.1 X lo4, a,,, = 1.0 x "bo = 8.1 x and abl, = 1.6 X (8) uncertainties in ko and k14 were assumed to be zero. Effects of Counting Time on Critical Value, Detection Limit, and Chemical Yield Uncertainty. Figure 2 shows the calculation results for equal 1000- and 4000-min counting tracer ~ added. intervals as a function of the amount of 9 7 m T The detection limit varies slowly over the region from about 5 counts min-l to about 100 counts min-l added tracer, with minima of about 44 mBq and 35 mBq, respectively. In both cases the uncertainty in the chemical yield decreases if the tracer ~ added is increased. For the case of amount of 9 7 m T two 1000-min counting intervals the uncertainty in the chemical yield estimate is about 10% for 4 counts min-l, 5% for 8 counts min-l, and 2% for 25 counts min-l added tracer. Results for the critical values as calculated from eq 14 and 16 ranged from 19 to 44 mBq for two 1000-min counting intervals and from 16 to 28 mBq for two 4000-min counting intervals as the quantity of tracer was increased from 3 counts min-l to IOOO counts m i d . Effect of Fraction of Time Sample is Counted with Adsorber on Detection Limit and Chemical Yield Uncertainty. With a fiied time interval to count a given sample, Figure 3 illustrates how the detection limit and yield uncertainty vary with the fraction of the time the sample is counted with the aluminum absorber, for 5,20, and 150 counts min-' of cT"@ ' tracer. The detection limit is minimized if the sample is counted for most of the time with the aluminum absorber. The uncertainty in the chemical yield is minimized if the sample is counted with the aluminum absorber about half of the available time. It is important to remember that the tracer shown in Figure 3 assume curves for quantities of 97mT~ a chemical yield of 50% and do not take into account decay of the 9 7 m T tracer. ~ Thus, if 10 counts min-l of tracer were originally added to a sample and if it were counted 90 days later (one half-life of wmTc)and if the chemical yield was 50%, the curves in Figure 3 for the 5 counts min-l case should be used to determine the fraction of the time the sample should be counted with the aluminum absorber. For this case the sample should be counted with the aluminum absorber about two-thirds of the time (i.e., for 1333 min) which would reduce the uncertainty in the chemical yield to less than 8% and give a detection limit of 44 mBq of 99Tc. Effect of Amount of 99Tc in Sample on Uncertainty Estimates. Equations 3-4 and 14-15 may be used to estimate both the uncertainty in the 99Tcvalue and the uncertainty in the chemical yield as a function of the amount of 9gTcin the sample, given a fixed quantity of added 9 7 m tracer T ~ and specified counting periods with and without absorber. These
i /
5L
-
0!0
'
JlvCERTAINTY IN CHEMICAL Y I E L D
0
/
I,,#.l 10 000
I # / #1 ' 1 " ' '100 '
1
000
,
, ,
I
1
i
1 1
100 000
A M O U l T O F Y T c IN SAMPLEImBql
Flgure 5. Uncertaintles in measurement of g g T and ~ of chemical yield vs. amount of "Tc in sample, 20 counts min-' added tracer. 154 cpm *ImTc
30
A M O U N T OF Tc IN SAMPLE mBql
Flgure 6. Uncertainties in measurement of "Tc and of chemical yield vs. amount of "Tc in sample, 150 counts min-' added tracer.
results are given in Figures 4, 5, and 6 for cases in which 5, 20, and 150 counts min-l of 9 7 m T are ~ added to the sample, respectively, and the counting period for each measurement with and without absorber is 1000 min. A chemical yield of 50% is assumed. In all cases as the quantity of 99Tcin the sample becomes larger the uncertainty in the 99Tcmeasurement goes through a minimum and then increases. The uncertainty in the chemical yield starts out fairly low, and then increases and becomes the predominant source of uncertainty, merging with the ggTcmeasurement uncertainty curve for samples containing large amounts of %Tc. It is expected that ~ uncertainty curve expressed in units the g g Tmeasurement of percent relative standard deviation (% RSD) will be large for small quantities of 99Tc in the sample. It is not so obvious, however, that the uncertainty in the %Tc values will increase if there is a large amount of 99Tc in the sample. In this ~ rate is relatively high and circumstance the g g Tcounting therefore the uncertainty in the 99Tc counting rate is similarly high, making it difficult to detect a relatively small perturbation due to the isotopic tracer. One would expect this effect to be largest when small amounts of isotopic tracer are added. This is indeed the case, as can be seen from Figures 4-6. For 5 counts min-l 9 7 m T tracer ~ added and 1000 mBq g g Tin~the sample the uncertainty in the determination of %Tc is 22%, but the uncertainty decreases to just over 7 % for 20 counts m i d tracer added and to under 4% for 150 counts min-l tracer added.
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
Figures 4-6 show that for the above cases of 5,20 and 150 counts m i d added tracer and two 1000-min counting intervals for each (i.e., with and without absorber), the minimum values in the uncertainty in measurement of ggTcwill be 17.4, 7.5, and 3.3% RSD, respectively. The corresponding detection limits from Figure 2 are 47, 44, and 53 mBq. If a high 99Tc activity level is anticipated for a given sample, it is best to tracer, ~ even though add a relatively high activity level of 9 7 m T one must tolerate a somewhat higher detection limit as a to~add should, consequence. Decisions on how much 9 7 m T if possible, be based on available knowledge about each individual sample. Additional Sources of Uncertainty. In addition to the sources of uncertainty discussed so far, other factors may also have to be considered. As mentioned earlier, one of these is counting geometry. It is very important that the sample be located in a reproducible position with respect to the detector. A convenient way of doing this is to scribe a mark such as a circle or an X on each cathode to aid in centering it in the electrodeposition apparatus and in the counter sample holding tray. For the 99Tc results reported in Tables I and 11, the standards were measured by evaporation of aliquots directly onto the cathodes,whereas samples were electrodeposited from NH&l solution. The possible errors introduced in doing this are probably not very :significant for the case of ggTc,since the mass of the evaporated deposit is so small that negligible self-absorption of the 0 radiation should occur. However, for ~ counting rate per microliter was the case of Q 7 m aT lower obtained when 100 pL (of solution was evaporated compared with evaporation of 10 ML of the same solution. This is probably due to self-absorption of some of the very low-energy internal conversion electrons in the former case. Any selfabsorption for the case of the 10-pL solution has been neglected. Any errors introduced by possible self-absorption occurring in the electrodeposited sample or by nonuniform deposition of the technt?tiumactivities on the cathode surface have likewise been ignored. The second method for calibration of the detectors, discussed earlier, which involves preparation of electrodeposited standards of 997'c and 9 7 m T ~ reduces , some of these uncertainties since standards and samples are plated in the same manner. However, a niethod'such as isotope dilution mass spectrometry is then required to accurately determine the plating yield for the standards. The effect of uncertainties in the constant ao, aI4,bo, and b14 was not included in the results in Tables I and 11. However, eq 14 and 15 allow these to be included. The effect of these uncertainties was included for Figures 2-6, as mentioned
1163
earlier. These uncertainties are minimized by use of relatively standards ~ for determination of high activity %Tc and 9 7 m T ao, a14 and bO/b14. An aliquot of the wmTcsolution containing a much smaller amount of g7mTccan then be added to the samples, and the counting rate of this aliquot gives the constant bo. CONCLUSIONS The counting method for 99Tc described in this paper provides adequate sensitivity for many applications and is relatively inexpensive in terms of equipment required and labor. It is also compatible with the more sensitive isotope dilution mass spectrometric (IDMS) technique. If the 99Tc level in a sample is too low to give a positive result by the counting method, the sample may be dissolved from the Ir cathode and further purified as described in ref 3, mainly to remove Mo which interferes with the mass spectrometric . purified solution may then be measurement of 9 7 T ~The directly loaded onto a Re filament (3), or the resin bead technique of Anderson and Walker may be used (7). Detection limits of about 0.06 mBq are achievable by IDMS. A mixture of recently prepared tracer giving the desired 9 7 m T counting ~ rate and dead (i.e., decayed) tracer, such that the total quantity is ~(5-10) X g, is added initially to such of 9 7 T ~9 7 m T samples. Ideally, one could add the 9 7 m T isotopic ~ tracer in sufficient quantity to allow measurement of the chemical yield with an acceptable uncertainty and then allow the tracer to decay to a nonsignificant level and recount the samples. This would allow the lowest detection limit to be achieved.
+
LITERATURE CITED Anders, E. "The Radiochemistry of Technetium"; National Academy of Sciences-National Research Council: Washinaton. DC, 1960: NASNS-3021, pp 22-24. Lederer, C. M., Shirley, V. S., Eds. "Table of Isotopes", 7th ed.; Wiiey: New York, 1978; p 410. Kaye, J. H.; Rapids, M. S.;Bailou, N. E. "Determination of Picogram Levels of Technetium-99 by Isotope Dilution Mass Spectrometry"; Proceedings of Third International Conference on Nuclear Methods in Environmental and Energy Research, Columbia, MO; CONF-771072; 1977; pp 21 1-224; available from NTIS, Springfield, VA. Foti, S.; Delucchi, E.; Akarnian, V. Anal. Cbim. Acta 1972, 6 0 , 269. Bevlngton, P. R. "Data Reduction and Error Analysis for the Physical Sciences"; McGraw-Hili: New York, 1969; p 59. Currle, L. A. Anal. Chem. W88, 4 0 , 586-593. Anderson, T. J.; Walker, R. L. Anal. Chem. 1980, 52, 709-713.
RECEIVED for review February 17, 1981. Resubmitted February 1, 1982. Accepted March 18, 1982. The authors are grateful for support; of this work by the Pollutant Characterization and Safety Research Division of the Department of Energy under DOE Contract No. DE-AC06-76RLO 1830.
