Anal. Chem. 1988, 60, 1851-1855
1851
Determination of Lead-2 10 in Admixture with Bismuth-2 10 and Polonium-2 10 in Quenched Samples by Liquid Scintillation Counting J e a n Simon Blais and William D. Marshall* Department of Food Science and Agricultural Chemistry,'Macdonald College, 21 111 Lakeshore Road, Ste. Anne de Bellevue, Quebec, Canada H9X 1CO
An aqueous acldlc mixture of 'lOPb, 'l0Bi, and 210Po,In secular equlllbrlum, was separated Into Its components by uslng a series of sequentlal complexometrlc extractions. A multlchannel pulse height analyzer was used to determlne the effects of varylng amounts of a chemlcal quenchlng agent on the llquld sclntlllatlon spectrum of each of the separated fractions. The 210Pbfraction was free of contamhating 'l0Bi. A mathematlcal algorithm was developed to predict the percent of 210Pbactivlty (0.61 % of the lnltlal '"BI activity) In the bismuth fractlon by uslng Cerenkov countlng, whlch Is specific for 210BI. The approach taken was to relate the slope of the h e a r relatlonshlp between the logarlthmlcally transformed theoretical decay curve for pure "OBI and the transformed theoretkal decay curve for 'loBI, 'OW mlxiures to the percent of contamlnatlng 'loPb In the mlxture. Equatlons were also developed (1) to predict the llquld sclntlllatlon counting efflclency of 'low (relative to an unquenched sample) by relating the level of quenching to a spectral quench parameter [SPQ(E)] as determlned by counting an external standard (226Ra) and (2) to correct for the presence of 210B1counts In the 'lOPb window by relatlng the 210B1count rate In the 'loPb window to the ratlo of the count rate In a 210B1Interferencefree window over the SPQ(E). When comblned, these expressions resulted In an algorlthm that can be used to correct for varlable amounts of chemlcal quenching and for the presence of varlable amounts of 210B1In the 210Pb-contalnlngsample. The approach Is llmlted to moderately quenched samples in which there Is no splll over of 'loPo actlvlty Into the 210Bl-speclflc window.
Lead-210, a naturally occurring radioisotope from the uranium-238 series, decays (half-life, 22.0 years) to the stable 206Pbisotope via two daughter radioisotopes 210Biand zlOPo. Because the daughter radionuclides have relatively short half-lives (5.01 days and 138.4 days, respectively), they rapidly reach a secular equilibrium with their parent radioisotope. Historically interferences from 210Biand 210Poemissions have complicated the direct radioanalysis of 210Pbby proportional gas-ionization detectors. If 210Pbis known to be in secular equilibrium with zlOBiand 210Po,one or both daughters are isolated and counted and the amount of zloPb originally present is extrapolated from the known decomposition rates (1-3). If secular equilibrium conditions cannot be assured, more elaborate separation conditions are employed to isolate the lead-210 from its decay products, and after a waiting time sufficient for an appreciable growth of 210Biand/or 210Po,the zloPb activity is extrapolated from the daughter activities. The possibility of the direct determination of lead-210 by liquid scintillation counting (LSC) was demonstrated by Fairman and Sedlet (4) who observed two completely resolved peaks (210Pband zlOPo)superimposed on a broad energy continuum of noBi. The low-energy peak resulted from 0003-2700/88/0380-1851$01.50/0
conversion electrons, y-rays, and soft fl particles from zloPb decay and was positioned on the low-energy portion of the 21"13ispectrum (approximately 23% of the 210Bispectrum was detected within the zloPb spectral region). Lead-210 was counted virtually quantitatively despite the fact that 81% of the fl emission had a maximum energy of only 15 keV. Fairman and Sedlet considered that the emission of particles from zloPb decay to zlOBi*and of internal conversion electrons from zlOBi*decay to the zlOBiground state were not discriminated by the instrumental electronics and consequently were recorded as a single event with a summed energy (Bparticle energy plus approximately 30 keV corresponding to the conversion electrons) sufficient for detection at virtually 100% efficiency. These authors demonstrated that knowing the degree of overlap of the zlOBispectrum in the zloPb and zlOPoregions, it was possible to determine the three radioisotopes simultaneously by liquid scintillation counting. Ideally, correction for 210Bicounts in the zloPbor in the 210Poregions would be estimated from the count rate measured in the 210Bi-specific spectral region. In practice however, chemical quenching of the scintillation process may modify the energy distributions complicating the correction procedures. These practical difficulties may explain the paucity of information on the radioanalysis of zloPb and progeny by LSC. An alternate approach to zloPbquantitation, which could be applied to severely quenched samples, was also to be investigated. In these cases zloPb can be determined indirectly by examining the temporal changes in the activity of its first daughter (210Bi),which would be selectively determined by Cerenkov counting. The Cerenkov process is apparently unaffected by chemical quenching (5). Although any transparent medium can be used as a generator (6), there are at least two variables to be considered in the choice of a solvent: (a) the efficiency of the Cerenkov process is proportional to the index of refraction of the solvent (6)and (b) the resulting Cerenkov ultraviolet photons are susceptible to color quenching. Since it was desirable to analyze different physicochemical forms of zloPb by this technique, both aqueous and organic generators were to be investigated. Despite the disadvantage that LSC suffers from an instrumental background, which is somewhat higher than other counting techniques, it is very rapid; it may be automated and may be used to count 210Pbemissions directly and quantitatively, advantages that increase experimental precision. In this paper an LSC calibration technique is presented that permits the determination of zloPb levels in moderately quenched samples containing nonsecular equilibrium quantities of zlOBiand zlOPo. MATERIALS AND METHODS Separation of 210Po,210Bi,and 21"Po. The sequential extraction procedure is presented in Figure 1. A 100-pL sample of lad-210, bismuth-210, and p010ni~m-210,in secular equilibrium in 2 M HNOB(0.03 MBq total),was diluted in 2.5 mL of 20% (v/v) acetic acid (fraction 1). The resulting solution was extracted 0 1988 American Chemical Society
1852
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
-
F1 k10Pb2+, 2106i3+,210P04+I(in 20%CH3C02H) I Dithizone (Dz) in Benzene
Aaueous Phase
Oroanic Phase
3 [21opb2+)
210Bi:Dz + 21opo:Dz x
Aaueous Phase
6;l
Oroanic Pha
b l 0 M HNO?
A- -
-
Aaueous Phase
F fraction
I-- [ F4
Oroanic Phase
Dithizone
Figure 1. Flow chart of procedures for the separation of "'Pb 210Bi from 2 1 0 ~ o .
from
twice with 1.5 mL of 4 mM diphenylthiocarbazone (dithizone, Dz, Fisher Scientific Co.) in benzene. After the solution was shaken for 5 min, centrifugation (1OOOg) hastened phase separation. The aqueous phase (fraction 2) was conserved for analysis. The organic washes were combined and back extracted twice with 1.5 mL of 0.5 M HN03 containing 4% (w/v) sodium bromide. The aqueous back extracts were combined (fraction 3) and conserved for analysis. The organic phase was then back extracted twice with 1.5 mL of 10 M HN03 the aqueous phases were again combined (fraction 4). Activity in Separate Fractions. Small aliquots of fractions 1 to 4, in 7-mL plastic scintillation vials, were diluted with 5 mL of scintillation cocktail (Universol, Amersham, Oakville, ON) or 5 mL of distilled deionized water (for Cerenkov counting). Liquid scintillation spectra and total counts (corrected for background and color quenching if any) in each fraction were recorded after 0, 2.1, 4.2, 8.3, 12.0, 16.0, 21.9, and 27.9 days. Apparatus. Scintillation counting and Cerenkov counting were performed with a Rack Beta, Model 1219, liquid scintillation counter (LKB-Wallac, Turku, Finland) equipped with a multichannel analyzer providing logarithmic analog to digital conversion to 1024 channels. All counting was performed in 7-mL polyethylene vials. Reagents and Standards. Aqueous 210pbstandard (in secular equilibrium with and 210po) in 2 M HN03 (48.9 kBq Z1?Pb/g solution) was purchased from Amersham International, Ltd., Aylesbury U.K. Organic solvents were Distilled in Glass Grade unless otherwise noted and all chemicals were ACS Reagent Grade or better. Chemical Quenching of 21"Pband of zlOBiLSC Spectra. Aliquots corresponding to 26000, to 2600, or to 700 cpm (counts per minute) of 210Biactivity (integrated counts in channels 49-400) were removed from fraction 3. Carbon tetrachloride quenching agent was injected into the test ,l0Bi aliquot (diluted with scintillation cocktail) via a pinhole that had been bored in the cap of the scintillation vial. The hole was sealed with a Teflon-coated silicon disk that was pressure fitted into the inside of a second oversized cap from a Reacti-vial. The Reacti-vial cap, counting vial cap assembly provided a leak-tightseal. For studies with 21'%i, volumes of CC14 were added in 12 sequential 2-pL additions followed by two sequential 3-pL additions. After each addition the contents of the vial were thoroughly mixed, set aside for 2 min, and then counted. Activity in the 210Bicounting window, channels 401-424 or 401-446 or 401-464 or 401-488, and in the 210Pbcounting window (channels 49-400) were determined. The counting time was adjusted so as to decrease the theoretical error in the zloPbwindow to less than 1%. The experiment was replicated three times for each counting level. Three 100-pL aliquots of freshly isolated zlOPb,diluted with scintillation cocktail, were counted (windows 49-400) in the presence of CC4, which had been added in sequential 4-pL increments to a total of 32 pL. After each addition the sample was thoroughly mixed and allowed to stand for 5 min prior to counting. The activity of the isolated zloPbwas determined indirectly by monitoring the growth of 210Biactivity in this fraction. Three
I
I
2
I
l
4
,
,
l
6
l
8
CHANNEL X lo-* Figure 2. Liquid scintillation energy spectra of (1)lead-bismuth-poIonium-210 secular equilibrium mixture, (2)lead-2 10 fraction, (3) bismuth-210 fraction, and (4) polonium-210 fraction
100-pL aliquots of the isolated zloPb solution were diluted to 5 mL of H,O and zlOBiactivity was determined periodically over 14 days by using standardized Cerenkov counting. Determination of zlOBiby Cerenkov Counting. The following solvents were investigated as generators: methanol, acetonitrile, hexane, tetrahydrofuran, chloroform, carbon tetrachloride, cyclohexanol, bromoform, diiodomethane, dimethyl sulfoxide, and sucrose solutions (040% (w/v)). Aliquots (5 pL) of ,loBi standard (in secular equilibrium with *l0Pb)in aqueous 2 M HNOBwere diluted with 5 mL of test solvent. Complete solubilization of the cationic radioisotopes in organic solvents was assured by adding 1pL of aqueous sodium diethyldithiocarbamate (DEDTC) (10 mg of DEDTC/mL) to the spiked solution. After thorough mixing the samples were set aside for 12 h prior to counting.
RESULTS AND DISCUSSION The separation of 210Pb,210Bi,and 210Po(Figure 2) was based on the fact that bismuth (7,8) and polonium (9) can be extracted with dithizone from aqueous solution (pH 2) whereas lead is extracted efficiently only from alkaline solutions (10). A secular equilibrium mixture of the three isotopes freshly spiked into 20% (v/v) acetic acid was extracted twice with benzene-dithizone to remove Bi3+ and Po4+. The selective back extraction of Bi3+ into the more acidic medium (0.5 M HNOB)was facilitated by the presence of bromide ion (7).Polonium has been recovered from moderate (pH 0.2-5.0) (9) to high acidity (10 M HNO,) (11)demonstrating the high stability of the Po-dithizone complex. A complexometric reextraction of the 210Pbfraction 3 weeks after purification left an appreciable proportion of the 210Biin the acetic acid solution. Thus, in less acidic media (20% acetic acid vs 2 M HN03),zloPbdecay results in unextractable physicochemical forms of zlOBi. Spectral energy distributions of the emissions from the lead-bismuth-polonium-210 secular equilibrium mixture (fraction l),lead-210 (fraction 2), bismuth-210 (fraction 3), and polonium-210 (fraction 4) are presented in Figure 2. The 1024 channels of the multichannel analyzer were automatically summed together so that eight channels corresponded to one dot column in this figure. Individual fractions were counted for sufficient time (different for each fraction) to provide a full scale display of its spectral energy distribution. The region
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
between channels 801 and 1024 has not been included because no energetic emissions above background were detected in this high-energy region. The characteristics of fraction 1were as previously described ( 4 ) ;two peaks separated by a ramping energy continuum. The lead-210 peak was relatively broad (low-energy spectral region) reflecting the multienergetic character of these emissions. Scintillations, reflecting the monoenergetic a particles from polonium-210, were recorded as a relatively sharp peak in the high-energy region. The bismuth-210 continuum overlapped the two peaks. The spectrum of fraction 2, containing z10Pb2+,demonstrated the high selectivity of the first step of the extraction sequence since no bismuth or polonium emissions were recorded. Emissions of lead-210 were recorded between channels 49 and 400. The spectrum of the third fraction indicated that z10Bi3+was selectively liberated in 0.5 M HN03 containing 4% NaBr. Since the bismuth-210 continuum (channels 49-784) completely overlapped the polonium-210 spectrum, it was not possible to assess quantitatively the amount of 210Poin this fraction. The spectral shape in the zlOPoregion of fraction 3 did not reflect the presence of appreciable amounts of this contaminant. The spectrum of fraction 4 contained small quantities of emissions from residual bismuth-210 coextractant. If necessary, residual bismuth-210 could have been removed from the organic phase (prior to removal of 210Po) by scrubbing with 0.5 M HC1 (12). A method for the determination of the interfering 210Pbin the 210Bifraction (fraction 3) was required. The approach taken was to use Cerenkov counting and to compare the theoretical decay curve with time for a pure 210Biemitter with the decay curves for mixtures of 210Biand 210Pbin which the latter nuclide contributed 1-25% of the total absolute activity. An equation (13)that describes the theoretical decay curve of a daughter radioisotope (zlOBi)in a sample that includes both parent (zlOPb)and daughter $OBi) activities at time = 0 served as a model
AD = [(XDAP,O)/(XD- hp)l[(exp(-Xpt)- exp(-A~t))l + AD,Oexp(-hDt) (1)
0 0
= OlAD,0 + h A P , 0
(11)
A2 = aZAD,O + b A P , 0
(111)
Al
where Al is the activity at time t = 1 and Az is the activity at time t = 2, al is the fraction of AD,^ remaining a t time t = 1 [i.e. exp(-XDt)], and bl is the fraction of Ap,Oconverted to A D at time t = 1
bl =
AD - Xp)l[exp(-Xpt) - exp(-bt)l
The determination of 210Biabsolute activity by Cerenkov counting is necessary to solve equations I1 and 111. To avoid uncertainties associated with the activity of the standard, an alternate approach was taken. From eq I, the theoretical decay curves for 210Bidoped with varying activities of 210Pbwere determined. Regression analyses of the logarithmically transformed theoretical decay curves of mixtures of 210Biwith zloPb (in which the absolute zloPb activity accounted for 1, 5,10,15, 20, or 25% of the Cerenkov count rate of 210Biat t = 0) on the transformed theoretical decay curve for pure 210Bi at different times indicated a perfect linear correlation (0.999998 r < 1)over the range of ? P b studied. In addition, the slopes of these six linear relationships were found to be
’
SQP = 177
SQP = 134
; I
Channel Number Figure 3. Effects of increasing chemical quenching on ‘IoPb, 210Bi, and ‘‘OPo liquid scintillation spectra. Vertical bars denote the 210Pb (channels 49-400) and the “%i (channels 401-449) counting windows. ? . 1
I
40Y
bismuth-210 A polonium-21 0
cz 20 ;
j
where AD is the activity of the daughter radioisotope at time t, A P , is ~ the activity of the parent radioisotope at time 0, AD,o is the activity of the daughter radioisotope at time 0, Xp is the decay constant for the parent radioisotope (210Pb),and XD is the decay constant for the daughter radioisotope (210Bi). The two unknown parameters (Ap,oand AD,o)would be determined by solving the following simultaneous equations:
1853
,
,
160
,
,
,
240
,
,
,, ,
SQP(E) 320
,
,
,
,
,
,
I
4
400
Effects of increasing chemical quenchin [decreasing SQP(€)]on the relative counting efficiency of 210Pb, 8Bi, and 210Po. Figure 4.
exactly correlated (r = -1) with the corresponding proportion of 210Pbinterferent at t = 0. Since only zlOBiemissions are detected, the slope represents the apparent decay constant for the 210Bi,zloPbmixture. In this case,the variable “slope” ( S ) was related to the variable “Pb interferent” (Pb) by the following equation:
S = (-9.99871 X 10-3)Pb + 0.99995 (IV) The slope of the transformed decay curve for Cerenkov radiations in fraction 3, when substituted into eq IV, indicated that the absolute activity of interfering zloPbwas 0.61 % of the initial zlOBicount rate, which was considered to be negligible. A third complexometric extraction of the initial secular equilibrium mixture (fraction 1)with benzene-dithizone resulted in removal of greater than 99.5% of the initial zloPb activity. The effects of varying amounts of chemical quenching on the liquid scintillation spectra of 210Pbor *l0Bior zlOPoare presented in Figure 3 and the relative counting efficiencies (from spectral integration) in each of the fractions for different quenching levels are recorded in Figure 4. Each quenching level was automatically quantified into a spectral quench parameter [SQP(E)J by the instrument. This parameter is defined as the end point at a constant percentage value of the external standard spectrum after correction for sample contribution and color quenching. It represents a channel number
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
below which 90% (or some other arbitrary value) of the 226Ra spectrum is detected. As indicated by Figure 4 increasing degrees of chemical quenching (reflected by decreasing SQP(E) values) resulted in a migration of the pulse distribution toward the lower portion of the spectra. The relative counting efficiencies of the strong p emissions from 210Biand the highly energetic 210Po (Y particles (Figure 4) were relatively less affected by quenching (>80% relative counting efficiency) in the presence of high levels of quenching agent (for comparison 3H emissions are counted with less than 30% efficiency when the SQP(E) approaches a value of 200). The relative counting efficiency for 'lOPb,which varied from 100 to 18% as the quantity of carbon tetrachloride quencher was increased, appeared to vary linearly with SQP(E) (for SQP(E) values between 452 and 320). After radiochemical purification, the absolute activity of the zloPb fraction was to be determined for accurate calibration. This was accomplished indirectly by monitoring the growth of 210Bi activity with time in this fraction (Cerenkov counting) and by solving eq I1 and 111. For the SQP(E) range 452-320, the absolute 210Pbcounting efficiency (&b, expressed as a percent) varied linearly ( r = 0.9880) as the inverse of SQP(E). Epb
= -5367.6/SQP(E)
106.4
(VI
In practice, before application of a correction term for the counting efficiency, it would be necessary to subtract zlOBi emissions from the zloPb spectrum. The effect of chemical quenching on the relative spectral interference of 210Biin the zloPb spectral region (channels 49-400) was determined by relating the integrated 210Biinterfering spectral region (zlOBicounts per minute, cpm, in channels 49-400) with the ratio of the counts per minute in a 210Bi-specificregion over the corresponding spectral quench parameter. Four 210Bi-specific regions were considered: channels 401-424, channels 401-446, channels 401-464, and channels 401-488. Correction parameters were optimized for counting zlOBiinterfering activity at high (11000-26000 cpm), intermediate (1100-2600 cpm), or low (300-700 cpm) levels in the zloPbspectral region. Aliquots (three replicates of each activity level) from fraction 3 were quenched with successively increasing quantities of carbon tetrachloride. Linear regressions of 210Bicounts per minute in the zloPbcounting window (channels 49-400) vs the ratio of counts per minute in the 210Bi-specificcounting window over the SQP(E) are presented in Table I. For SQP(E) values above 312, 210Bi counts per minute in channels 49-400 were highly correlated with the ratio of counts per minute in the 210Bi-specificwindow divided by SQP(E) (0.9857 < r < 0.9984). The standard error of estimate (a measure of the uncertainty associated with the dependent variable, 210Bi counts per minute in channels 49-400) was used to assess the precision of the model for the four 210Bi-specificcounting windows. As the activity of the sample was decreased, the relative standard error of estimate (standard error of estimate divided by the minimum counts per minute) increased only slightly. The narrowest 210Bi window resulted in the highest relative standard error of estimate, and as the activity of the sample was decreased, wider counting windows generally resulted in better precision. The widest window (401-488) often resulted in slightly higher uncertainty reflecting the spill-over of residual zlOPocounts per minute into the 210Bi-specificregion at high quenching levels. It was concluded that within the ranges of activity studied, the narrower windows did not appreciably decrease the precision of the counting. Below SQP(E) values of 312 there was an appreciable spill-over of zlOPoactivity into the zloPb counting window (channels 49-400) (Figure 4). However the correlation between 210Bicount rate in the zloPb window and 210Bicounts per minute in the 210Bi-specificwindow (divided by SQP(E)) is
Table I. Linear Regression of *loBiCounts per Minute (cpm) in Channels 49 to 400 vs cpm in a 210Bi-Specific Window Divided by SQP(E) std error
re1 std
of
error
counting range, cpm
268.9 242.8 257.6 305.5
f2.44 f2.21 f2.34 32.78
11000-26000
26 26 26 26
60.7 39.3 39.3 36.8
f5.51 f3.57 f3.57 f3.35
1100-2600
26 26 26 26
19.5 15.4 13.6 15.1
f6.51 f5.44 f4.54 f5.05
300-700
We:
corr coeff
(channels)
r
401-428 401-446 401-464 401-488
0.9981 0.9984 0.9982 0.9975
45 45 45 45
401-428 401-446 401-464 401-488
0.9917 0.9965 0.9965 0.9969
401-428 401-446 401-464 401-488
0.9857 0.9911 0.9931 0.9914
Dfb estimate" est: %
a 210Bi-specificcounting window. Degrees of freedom. Standard error of estimate (absolute uncertainty associated with 210Bicpm in channels 49-400). dStandard error of estimate divided by the minimum counts per minute (cpm) in the counting ranee.
predicated upon the lack of 210Pocounts per minute in this latter region. A practical limit for the optimum 210Bispecific region (channels 401-464) is therefore considered to be SQP(E) = 331 (Figure 3). The resulting linear equations for (i) determining the 210Pb counting efficiency and (ii) determining zlOBiinterference in the 2'oPb counting window (Wpb) were as follows: (i)
Epb
= (-5367.6/SQP(E))
+ 106.4
(VI)
where Epb
= zloPb counting efficiency ( % )
SQP(E) = spectral quench parameter and (ii)
wpb
where 300 cpm
= (648.7 W,i/SQP(E))
< 331
+ 89.9
(VII)
< WPb < 700 cpm in channels 401-464
A correction term for zlOBiinterference in the 210Pbcounting window ( w p b ) should be effected by subtracting Wpb from the total counts obtained in the zloPb counting window cc = C - w p b (VIII) where Cc is the 210Pbcounts per minute corrected for zlOBi interference and C is the total counts per minute in the lead window. Eq VIII then becomes Cc = C - [648.7w~i/SQP(E)+ 89.91 (IX) The corrected count should be compensated for the reduced counting efficiency due to quenching PbDpM = cc x 100/Epb (XI where PbDpM is the 210Pbactivity in the sample (disintegrations per minute) [C - (648.7Wsj/SQP(E) + 89.9) X 1001 (XI) P ~ D P=M -5367.6/SQP(E) + 106.4 The Cerenkov counting efficiency of zlOBi (in secular equilibrium with 210Pb) in various Cerenkov generators is presented in Table 11. In aqueous sucrose solutions, the counting efficiency, the average of three replicate determinations, increased only moderately with the index of refraction from 16.4 & 0.5 to 19.9 f 0.4%. In organic media, efficiency
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
Table 11. Cerenkov Counting Efficiency of 210Biin Various Generators counting efficiencyb generator
index of refraction"
%
methanol acetonitrile hexane tetrahydrofuran chloroform carbon tetrachloride cyclohexanol dimethyl sulfoxide tetrachloroethylene methyl iodide bromoform diiodomethane sucrose 0% sucrose 20% sucrose 40% sucrose 60% sucrose 80%
1.329 1.344 1.375 1.405 1.446 1.460 1.464 1.477 1.505 1.538 1.598 1.742 1.333 1.364 1.400 1.442 1.490
18 f 2 20 f 1 21 f 2 0.03 22.3 22 f 3 21 f 5 24.3 f 0.3 25 f 3 18 f 1 14 i 4 16.3 f 0.5
5f1 16.4 f 0.5 17.4 f 0.5 17.9 f 0.5 19 f 2 19.9 0.4
*
" nD20values taken from ref 14. Average fl standard deviation for three replicate determinations. was a function not only of the index of refraction but also of the ultraviolet absorbing properties of the medium. The efficiency was appreciably lower in halogenated solvents than in nonhalogenated media of similar indices of refraction. The most efficient generators were dimethyl sulfoxide (25 f 3 % ) and cyclohexanol (24.3 f 0.3%). Several advantages accrue when the LSC calibration technique is used in conjunction with studies to determine the environmental fate of zloPb labeled organometallics (1.516). Reisolated substrate or its transformation products need not be free of contaminating zlOBi or zlOPocompounds and counting procedures may be performed at any convenient time postreisolation. However the technique is limited to moderately quenched extracts and to extracts that contain appreciable activity above the background of the counter. For more severely quenched samples [SQP(E) < 3311 oxidizing
1855
agents may be used to reduce the concentration of chemical quenchers (17). Given the very low background (1-2 cpm) of specially designed liquid scintillation counters, this technique may also find application in the determination of ambient levels of zloPbin environmental samples. Although not investigated experimentally, there is no reason a priori why quench parameters featured in comparable instruments from other manufacturers could not be used successfully in this calibration technique.
ACKNOWLEDGMENT It is a pleasure to acknowledge several helpful discussions with N. Barthakur. Registry No. zlOPb,14255-04-0; zlOBi,14331-79-4; zlOPo, 13981-52-7; methanol, 67-56-1; acetonitrile, 75-05-8; hexane, 110-54-3; tetrahydrofuran, 109-99-9; chloroform, 67-66-3; carbon tetrachloride, 56-23-5; cyclohexanol, 108-93-0; dimethyl sulfoxide, 67-68-5; tetrachloroethylene, 127-18-4; methyl iodide, 74-88-4; bromoform, 75-25-2; diiodomethane, 75-11-6; sucrose, 57-50-1. LITERATURE CITED (1) (2) (3) (4) (5) (6)
(7)
(8)
(9) (10) (11)
(12) (13) (14) (15)
Sill, C. W.; Wiliis, C. P. Anal. Chem. 1965, 3 7 , 1661. Ferri, E. S.; Chrlstlansen, H. Public Healfh Rep. 1867, 82, 828. Cohen, N.; Kneip, T. J. Health phys. 1969. 17, 125. Fairman, W. D.; Sedlet, J. Anal. Chem. 1968. 40, 2004. Robinson, J. R. Int. J. Appl. Radiat. hot. 1969. 20, 531. Johnson, M. K. Anal. Biochem. 1969, 29, 348. Young, R. S.;Leibowitz, A. Analyst (London) 1946, 71, 477. Laug, E. P. Anal. Chem. 1949, 21, 188. Bagnall, K. W.; Robertson, D. S. J. Chem. SOC. 1957. 509. Copper, S. S.;Hlbblts, J. 0. J . Am. Chem. SOC. 1853, 75, 5084. Ashizawa. T.; Haruyama, K.; Nagasawa, K.; Moritomo, Y. Bunzekl Kagaru 1964, 13, 11. Bouissiere, G.: Ferradini, G. Anal. Chim. Acta 1950, 4 , 610. Kirby, H. W. Anal. Chem. 1952, 2 4 , 1678. CRC Handbook of Chemistry and physics, 62nd ed.; Weast, R. C., Ed.; CRC Press: West Palm Beach, FL. 1981. Blais, J. S.; Marshall, W. D., submitted for publicationIn Appl. Organomet. Chem .
(16) Blais, J. S.; Marshall, W. D. J. Appl. Radiat. Isot., in press. (17) Horrocks, D. L. Applications of Li9uid Sclntiliation Counting; Academic: New York, 1974; Chapter VII.
RECEIVED for review December 18,1987. Accepted May 4, 1988. Financial support was from the Natural Science and Engineering Research Council of Canada (A6687 and G0316).
