Anal. Chem. 1983, 5 5 , 155-157
the data is not always possible due to transformation of the experimental data. Our proposed method is even more valuable in cases where some chromatographic separation of analyte and internal standard (using highly labeled compounds or high resolution capillary columns) occurs. This effect destroys the validity of the basic IDMS equation and subsequent calculation procedures based on this formula. Indeed the ion overlap is only partial, and no estimation of the interferences can be obtained by measuring pure product and/or internal standard separately. I t is obvious that the presented polynomial regression analysis with model-testing requires a reasonable computational facility (12). Modern mass spectrometers, however, all incorporate powerful computer systems which can handle this problem very easily. The above summarized regression analysis by multiple polynomials is worked out in a computer program, RAMP, which is available on request in FORTRAN IV or HPLBASIC.
4 h
7
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42
3
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c W
. +
2
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D
1
W 42
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2 U
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7
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-1
x
E 0 .r
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155
OY 0
= 1 . 3 9 t 5.34
Y = 2.38 10-1
-1.89
-4
2.33
lo-'
r
10-1 X t 1.02
= 0.99,
X
X2
23.3
7.0
X/Y theoretical
Flgure 2. Difference between calculated and given mole ratios for a
linear and polynomial regression line, constructed from the data in ref describing an IDMS assay for y-aminobutyric acid (GABA) with [2,2-*H,]GABA as internal standard.
4,
labeled, product is present. Also the setup of analyses with internal standards with low mass increment is facilitated. In contrast with the calibration methods based on expressions or approximations of the basic IDMS equation, no initial estimates of the amount of unlabeled product and/or influence of naturally occurring isotopes are necessary. Regardless the validity of the "theoretical" imodel used, this step greatly reduces the accuracy of such calibration procedures, because of the experimental error involved in determining these small abundances. Also, proper statistical handling of
LITERATURE CITED (1) Pickup, J. F.; McPherson, K. Anal. Chem. 1976, 4 8 , 1885. (2) Schoeller, D. A. Blomed. Mass Spectrom. 1976, 3 , 265. (3) Van Langenhove, A.; Costello, C. E.; Biller, J. E.; Biemann, K.; Browne, T. R. Clin. Chim. Acta 1981, 175,263. (4) Colby, B. N.; McCaman, M. W. Biomed. Mass Spectrom. 1979, 6. 225. (5) Bush, E. D.; Trager, W. F. Blomed. Mass Spectrom. 1981, 8 , 211. (8) Min, 8.H.; Garland, W. A.; Khoo, K. C.; Torres, G. S., Biomed. Mass Spectrom. 1978, 5 , 692. (7) Gambert, P.; Lallemant, C.; Archambault, A,; Maume, B. F.; Padieu, P. J. Chromatogr. 1979, 1 , 162. (8) Siekmann, L. "Quantltatlve Mass Spectrometry in Life Sciences 11"; De Leenheer, A., Roncuccl, R., Van Peteghem, C., Eds.; Elsevier: Amsterdam, 1978; p 3. (9) Jonckheere, J. A.; Thlenpont, L. M. R.; De Leenheer, A. P. Biomed. M s s Spectrom, 1980, 7 , 582. (IO) Schwarz, L. M. Anal. Chem. 1979, 51, 6. (11) VanArendonk, M. D.; Skoaerboe. R. K.: Grant, C. C. Anal. Chem. 1981, 53, 2349. (12) Mendenhall, W. "The Deslgn and Analysis of Experiments"; Wadsworth: Belmont, 1968; Chapter 7.
RECEIVED for review April 12,1982. Resubmitted August 16, 1982. September 24,1982. This work was supported through a N.F.S.R. grant to J.J. and a research contract from F.G.W.O. (No. 3.0011.81).
Desorption of Riadon from Activated Carbon into a Liquid Scintillator Howard M. Prichard" and Koenraad Marien The University of Texas School of Public Health, P.O. Box 20186, Houston, Texas 77025
Activated carbon has long been noted for its ability to adsorb radon from the surrounding air and has often been exploited in measurement techniques requiring radon concentration. Radon entrained on the carbon can be measured directly by y spectrometry or can be removed from heated carbon for counting in an a scintillation cell. The latter approach ( I ) is far more sensitive than y counting but does involve a considerable amount of sampke manipulation. As part of an effort to develop a passive integrating radon sampler based on activated carbon adsorption, we sought a counting method that would have a level of sensitivity approaching that of the a scintillation cell while involving minimal sample treatment. If radon could be reproducibly removed from activated carbon by simple desorption in a solvent such as toluene, then liquid scintillation counting 'would be a promising 0003-2700/83/0355-0155$0 1.50/0
analytical method for large-scale applications such as personal dosimetry in uranium mines. Other potential applications include the analysis of activated carbon radon flux detectors or radon entrained on low temperature activated carbon traps. From theoretical considerations, it is reasonable to expect that the general approach might be applicable to the analysis of other radioactive noble gases, such as 133Xeand 86Kr.
EXPERIMENTAL SECTION Initial desorption experiments were performed with sufficiently high radon concentrations to permit analysis by conventional y spectrometry. Ten-gram lots of a large grained, low density activated carbon (Nuchar WVL 8x30 Mesh) were rinsed in methyl alcohol to remove fines and then dried at 110 O C for 24 h. The prepared carbon was exposed to radon gas, sealed in 22-mL glass liquid scintillation vials, and held for at least 3 h to allow for 0 1982 American Chemical Soclety
156
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983
Table I. Observed and Expected Radon Counts following Toluene Desorption sample
a
obsd 3970 * 70“ 3820 * 60 3570 5 60 2830 i 60 2980 5 60 2 0 6 0 i 50 1620 t 40 2040i; 50
expected 4040 t 50 4500 i; 50 3410 t 50 2900 i 40 3190 i; 40 1940 i 30 1650 i 30 2030 i; 30
Table 11. Desorption of Radon from 2-g Carbon Samples
O/E
0.98 * 0.85 i 1.05 i 0.98 t 0.94 t 1.07 i 0.99 * 1.00 t
n 4 3 3 3 3 3 3 4
0.02 0.02 0.02 0.02 0.02 0.03 0.03 0.03
std dev
of variation
coefficient
13 4 12 18 27 6 3 8
2.97 0.72 5.54 3.33 5.18 1.64 1.14 5.32
“ Standard deviations due to counting errors alone are < 2 cpm.
One standard deviation.
daughter ingrowth. Each vial was then placed in a Styrofoam jig and the 0.295-MeV and 0.352-MeV y lines of 214Pbwere counted with a Ge(Li) detector and a 4096 channel analyzer. The carbon was then transferred to an 80-mL separatory funnel connected to a 60-mL flask containing a known volume of reagent grade toluene. When the stopcock was opened, the toluene flowed down onto the carbon, producing an exothermic outgassing reaction. The evolved gas passed upward through the toluene, affording an opportunity for any radon in the gas to be transferred to the liquid (2). after a few seconds of gentle shaking the toluene passed entirely into the funnel and the stopcock was closed. The gas in the flask was sampled with a syringe to verify that a negligible fraction of radon had escaped to the flask. Subsequent analysis with an 01 scintillation cell showed that this fraction was less than 0.5%. After a wait of at least 2 h for desorption, the funnel was shaken and inverted, a syringe attached below the stopcock, and the free liquid portion removed. The toluene was transferred to a 22-mL glass liquid scintillation vial for y counting after a 3-h delay for the ingrowth of radon daughters. Care was taken to repeat the counting geometry used with the original carbon sample, and the resulting net counts at 0.295 and 0.352 MeV were decay corrected to the counting time of the sample before desorption. (If the toluene is to be counted by liquid scintillation, 1 or 2 mL of concentrated fluor solution must be added to the vial.) The repeatability of the desorption procedure was tested under relaxed conditions to test the degree of care actually required for effective counting. Two gram samples of carbon were placed in glass scintillation vials sealed with thick rubber septa into which glass diffusion tubes (3) 1cm long with an internal diameter of 0.265 cm had been placed. Sets of three or four such vials were capped and placed in an atmosphere of 50-200 pCi/L of radon222. The vials were simultaneously uncapped, exposed for a predetermined number of hours, and then simultaneouslycapped. It was assumed that the sampling rate of the diffusion tubes had been selected so that after an exposure of 24 h, the “volume” of air sampled diffusively would be much less than the effective capacity of the carbon. At a convenient later time, the vials were uncapped, the stoppers removed, and 20 mL of toluene liquid scintillator was smoothly poured into the vial. The vials were quickly capped, held for 6 h or more for chemical and radiological equilibrium, and then counted with a commerical liquid scintillation counter. The observed variability within batches represents the combined effects of variations in adsorption, desorption, handling loss, and counting error.
RESULTS AND DISCUSSION The outcomes of eight desorptions of 10-g carbon samples are shown in Table I. The expected counts were computed on the assumption of total desorption of radon from the carbon to the toluene, followed by the establishment of full solution equilibrium between the air and toluene in the closed vessel. The expected count rate is
C’ =
mean (cpm)“ 444 498 21 7 532 515 391 281 158
CLV’ L V + v,
(1)
Where C is the count rate before desorption, C’is the count rate in the recovered toluene fraction, V is the volume of the toluene injected, V’ is the volume of the recovered toluene fraction, and V , is the residual air volume in the vessel. The
-
Table 111. Backgrounds, Counting Efficiences, and Lower Limits of Detection for Several Counting Windows
window open 32Ppreset 3H/32P preset 14C/32P preset optimum
background, efficiency, MDTA,“ cpm cpm/pCi pCi 58.4 19.2 15.6 12.4 12.9
10.60 8.73 8.12 1.06 8.52
0.365 0.254 0.229 0.252 0.213
a For a 60 min count with the combined probability of type I and type I1 errors = 0.05.
constant “L”is the coefficient for the partition of radon between equal volumes of air and toluene (4). With L = 13 and V , = 50 mL, the expected count rate reduces to C’ = CV’/(V 3.8) (2)
+
where V and V’are in milliliters. The actual y counts observed in the recovered toluene fractions are consistent with this simple model. The average ratio of observed to expected counts is seen to be 0.984 f 0.065 for the eight runs in Table I. If the low value in entry 2 can be ascribed to experimental error, this becomes 0.999 f 0.044. Table I1 shows the results of the tests made under less restricted conditions. Counting was performed with 2 g of carbon residing on the bottom of the vial. I t is seen that if there are losses of radon before the vial is capped and losses of pulses due to the presence of the carbon, they are a t least consistent.
SENSITIVITY Once radon is dissolved in a liquid scintillator, the a and /3 emissions of the radon series can be counted with almost complete efficiency. For every picocurie of radon-222 present, three 01 and two hard p are emitted a t equilibrium, providing an expected count rate of 11.1counts min-l pCi-l. Some counts are missed due to energy losses in the vial walls and actual count rates of 10.5 to 10.6 counts min-l pCi-l are observed in an open counting window. As the upper and lower discriminators are varied to narrow the window, both the background count rate and the counting efficiency are reduced. An optimum setting is one that minimizes the lower limit of detection, here taken to be the minimum detectable true activity (MDTA) as defined by Altshuler and Pasternack (5). Table I11 shows background count rates, counting efficiencies, and MDTA’s for a number of window settings on a commerical liquid scintillation counter. I t is to be noted that the preset window for 32Pin the presence of 3H produced an MDTA nearly as low as the experimentally determined optimum window. Backgrounds and counting efficiencies will vary somewhat from machine to machine, and the efficiencies of the preset windows will vary somewhat with the degree of quench in the scintillation solution.
Anal. Chem. 1903, 5 5 , 157-160
Registry No. Carbon, 7440-44-0; radon, 10043-92-2;toluene, 108-88-3. LITERATURE C I T E D (1) Lucas, H. F. Rev Sci. Instrum. 1957,28, 60. (2) Prichard. H. M. Heafih Phys ., In press. (3) Palmes, E. D.; Gunniso8nl,A. F. Am. Ind. Hyg. Assoc. J. 1973, 3 4 , 78.
157
(4) Weigel, F. Chem.-Ztg. 1978, 102, 287-299. (5) Altshuler, B.; Pasternack, B. Health Phys . 1983, 9 , 293.
RECEIVED for review July 30, 1982. Accepted September 29, 1982. This work was supported in part by-Grant No. ES01742-02 from the Institute Of Health Sciences.
Screening Method for Aroclor 1254 in Whole Blood Shane S. Que Hee," Jerry A. Ward, M. Wilson Tabor, and Raymond R. Suskind The Kettering Laboratory, University of Cincinnati Medical Center, 3223 Eden Avenue, Cincinnati, Ohio 45267
Polychlorinated biphenyls (PCBs) hawe been utilized as nonflammable and heat resistant oils in such articles as electrical transformers, condensers, and paint since 1930 ( I ) . They are ubiquitous in the biosphere (2, 3 ) . They have achieved some notoriety in the "Yusho" episode in Japan in 1968 ( 3 ) and more recently in Taiwan in 1979 (4, 5 ) . As of November 1, 1979, use of PCBs in new heat transfer systems in US. factories manufacturing or processing food, drugs, and cosmetics was no longer authorized. Use in electromagnets, transformers, and heat transfer and hydraulic systems is permitted until July 1, 1984 (6). Defective fluorescent light ballasts also emit PCBs contributing t o office air pollution (7). Thus, occupational and environmental exposures to PCBs will still occur into the future, and determination of PCBs En workers' blood ( 8 , 9 )wiill continue to be a valuable measure of PCB exposure. The methods for separation and quantification of polychlorinated biphenyls in serum or whole blood usually involve alkaline hydrolysis, hexane extraction, aind silica gel column chromatography of the concentrate of the hexane extraction (4,9,10). These methods are generally time-consuming and tedious: the extractions are usually multiple; the many concentrative steps may lead to losses by volatilization and adsorption; any impurities, in the solvents also are concentrated, limiting sensitivity or confounding GC/MS identification. In addition, incomplete documentation of the gel type may lead to irreproducible separations from investigator to investigator. There is a need for a semiquantitative quick-screening method to estimate PClBs in blood or serum. Such a method would be valuable to (assess if further chromatography is necessary for quantification or GC/MS analysis. Aroclor 1254 was the PCB chosen for this study since it is probably the most ubiquitous PCB (9). EXPERIMENTAL S E C T I O N Optimized Procedure. All glassware including syringes and tubes to be utilized in collecting blood was soaked in chromic acid overnight and rinsed (five times) in order: Type I distilled water a~ defined by the U.S. EF'A (11),acetone (Fisher Pesticide Grade A-40), Type I distilled water, hexane (Fisher Pesticide Grade H-300). Blood was drawn through a polyethylene butterfly valve. The fist 70 mL was discarded to minimize contamination in the event of later GC/MS. Blood (approximately 10 mL) was collected in preweighed 50-mL Kimalx 14-930 10A tubes fitted with Teflonlined screw caps and containing 200 USP units of heparin in 0.2 mL of aqueous isotonic saline. The heparin-containingtubes were shaken gently as the blood was collected to prevent clotting. The samples were then placed in a polystyrene refrigerated box (4-5 "C) in which the samples were transported to the laboratory, where they were stored (at 4-5 "C until analysis. Each blood sample was allowed to equilibrate to room temperature. The total weight was then recordled. A known weight 0003-2700/83/0355-0157$0 1.50/0
(ca. 5.0 f 0.1 g) of potassium hydroxide pellets (Fisher P-250) was added. Absolute ethanol (2.5 mL) was added by a prerinsed pipet. The caps were screwed on tightly and the tubes shaken until all the potassium hydroxide was dissolved. The solution will become hot but should not foam. The solution was digested at 90 "C for 1h in a water bath. With the capless tubes still in the bath, the ethanol was evaporated by blowing nitrogen over the surface of the solution for 10 min using a Pasteur pipet connected by Teflon tubing to a cylinder of compressed nitrogen. The solution was removed from the bath and allowed to cool. Pesticide Grade hexane (10 mL) was added (pipet), the caps were screwed on tightly, and the tubes were vigorously shaken for at least 15 separate shakes. The layers were allowed to separate for at least 10 min, or until no opacity was evident. An aliquot (10 wL) was then injected into the gas chromatograph to complete the screening phase. The column used for gas chromatographic analyses was a 1.87 m X 6 mm 0.d. X 2 mm id. Pyrex column packed with 3% OV-101 on 100/120 mesh Chromosorb W-HP. A 63Nielectron capture (EC) detector was utilized in a Hewlett-Packard 5730-A gas chromatograph. The temperatures were 250 "C (injector), 203 "C (column), and 250 "C (detector). The flow of 95% argon/ methane was 25.0 k 1.5 mL/min. After 16 min the column temperature was raised to 250 "C for 30 min to allow elution of other blood compounds. A Hewlett-Packard Reporting Integrator (HP-3390A) was utilized to visualize and quantitate the peak areas. Preliminary examination of many blood samples revealed that the Aroclor 1254 pattern at low concentrations (
