796
Anal. Chem. 1983, 55, 796-797
The experimental procedure was checked with several other drugs including heroin, methaqualone, PCP, quinine, methamphetamine, barbiturates, procaine, benzocaine, tetracaine, lignocaine, butacaine, and methapyrilene. None of these drugs gave a positive test. However, the study of possible interfering compounds was by no means exhaustive, and because the
reaction is generally applicable to organic bases, interfering substances may be encountered in the future. Registry No. Cocaine, 50-36-2. RECEIVED for review December 10, 1982. Accepted January 13, 1983.
Determination of Bromine in Blood Serum by Eplthermai Neutron Activation Analysis Zeev 6. Aifassl" Department of Nuclear Engineering, Ben-Gurion Universl@ of the Negev, Beer-Sheva 84 12 1, P.O.B. 653, Israel
Nathan Lavi Soreq Nuclear Research Center, Yavne, Israel
The determination of trace amounts of elements by neutron activation analysis (NAA) has been a standard technique for many years. Usually, the amount of the produced radionuclides has been measured by y-ray spectrometry either without any chemical treatment, i.e., instrumentally, or after chemical separation. In some cases ( I ) , there are advantages to measure the activated nuclides by their emission of X-rays due to electron capture or internal conversion processes. The main advantage of X-ray spectrometry is that many elements do not emit X-rays, thus reducing the background; this is in addition to the small number of lines in the characteristic X-ray spectrum of each element and the direct correlation between the element and the energies of its X-rays. Shenberg et al. (2) were the first to recommend X-ray spectrometry for the nondestructive determination of bromine. Rapaport et al. (3) determined the bromine content of blood serum by neutron activation analysis followed by X-ray spectrometry reducing the background due to the 0-electrons by a magnetic field ( 4 , 5 ) . The advantage of X-ray spectrometry is that in y-ray spectrometry the Compton effect of the more abundant 38Cland 24Nainterferes with the measurement of the shortlived *@Br(17.8 min) and force the use of longer lived 82mBr (35.3 h) after long decay (6-8). Nakahara et al. (9) determined Br by the 617-keV y line of the 17.8 min mBr in instrumental neutron activation analysis (INAA);however, they do not give their spectra while other works were not able to detect Br in short-time irradiation followed by short decay ( 1 0 , I I ) . Another way to get rid of the interferences of the more abundant elements in biological tissues, Na and C1, is by using epithermal neutrons. Guinn and Miller (12) determine the bromine content in biological material by using Cd-lined irradiation and by measuring the short-lived 79mBrfrom the 79Br(n,n')79mBrreaction. Epithermal neutron activation analysis is advantageous in the case when the desired element has a nuclide with a high I / u o (resonance activation integral/thermal neutron cross section) ratio while the major interfering activities in the sample have a lower ratio. This is the case for Br in a biological matrix, since Br has a ratio of 16.6 while the ratios for C1 and Na are 0.39 and 0.58, respectively (13). If a sample is irradiated within a suitable cadmium cover, the thermal neutrons are greatly attenuated and only neutrons with energies higher than about 0.4 eV will contribute significantlyto the activation process. Na ions can be removed also by the use of chemical separation, as by HAP (hydrated antimony pentaoxide), or bromine can be oxidized and extracted by organic solvents. However, these are destructive methods. This work was done to measure the detection limit of Br in this method and to compare with the other methods of 0003-2700/83/0355-0796$01.50/0
NAA. For a representative biological sample we chose blood serum.
EXPERIMENTAL SECTION Blood serum was prepared by centrifugation of blood samples. A 0.2-mL sample of the serum was transferred by a micropipet to a very thin Mylar sheet (0.006 mm) and left to dry in an under-vacuum desiccator containing P20s,till complete dryness. After dryness, the samples were covered with another piece of Mylar sheet and were introduced to polyethylene bag which was heat-sealed. Standards of bromine were prepared in the same way using dilute solutions of NH4Br. The samples were irradiated in the pneumatic tube of the IRR-1 reactor. For irradiation with epithermal neutrons, the polyethylene bag was inserted into a 0.5 mm thick cadmium box. After irradiation, the samples were taken from the plastic bag and measured directly. The y-rays of the activated samples were measured with a calibrated 100 cm3 Ge(Li) detector connected to Cannbera 30 multichannel analyzer. The energy resolution (fwhm) of the system was 2.5 keV for 1332 keV. The samples were counted for 60-900 s after 5-10 min of cooling time necessary for the transfer of the irradiated sample to the counting room. The irradiation and counting times for the Cd-lined irradiated samples were usually longer than for the thermal neutron activated samples due to the higher activity of the second ones. The X-ray spectra of the activated samples were measured with a Si(Li) detector of 100 mm2area, 4 mm depth, and resolution (fwhm) of 320 eV for 6.4 keV (Fe k X-rays). A 1.5-mm plastic absorber was used to reduce interference from fi decay electrons for both y-ray and X-ray measurements.
RESULTS AND DISCUSSION Figure 1 shows the spectra obtained from the irradiated blood serum, irradiated with a Cd cover (Epithermal Neutrons) and without a Cd cover (Reactor Neutrons), in the range of 400-1500 keV. As can be seen in Figure la, the Compton effect of the 24Nacovers all the peaks when using reactor neutrons. Figure l b shows that when using only epithermal neutrons, bromine is seen very clearly and its amount can be measured from several peaks which increases the accuracy. Figure 2 shows the X-ray spectrum measured after irradiation with reactor neutrons (2a) and with epithermal neutrons (2b). For reactor neutrons, the peaks of Br are masked with the activity of electrons due to 24Na and $%l. It is possible to remove this interference by magnets (3-5). However, Figure 2b shows that by use of epithermal neutrons, bromine can be determined quite easily and the use of a magnet is not necessary. A calibration curve for 0.1-1.5 pg of bromine, irradiating with epithermal neutrons and measuring the 617-keV activity, 0 1983 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
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Table I. Determination of Bromine in Human Blood Serum by Epithermal Neutron Activation followed by y-Ray (618keV 80Br)and X-ray Spectrometry (K, t Kp of 8oBr) 7-w X-ray amt of Br, amt of Br, sample pg/mL sample pg/mL 1 2
3 4 5 6 7
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6.45 6.40 6.85 6.96 6.70 6.60 6.80 6.40 6.83 6.66 f 0.21
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6.40 6.51 6.86 6.80 6.90 6.80
av
6.71
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2
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ENERGY (kev)
Figure 1. y R a y spectra of irradiated human blood serum: (a) 0.2 mL irradiated without Cd for 1 rnin and counted for 1 min after a cooling time of 10 min; (b) 0.2 mL irradiated under Cd for 20 min and Counted for 5 min after a cooling time of 10 min.
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Figure 2. X-ray spectra of irradiated human blood serum: (a) 0.2 mL irradiated for 1 min without Cd and counted for 60 s after a cooling time of 10 min; (b) 0.2 mL lrradlated under Cd for 20 min counted for 10 mln after a cooling time of 10 min.
was found to be linear with the amount of bromine. The sensitivity obtained from this curve is 19931 f 150 counta/pg Br (20 rnin irradiation, 5 min cooling, and 15 rnin counting). An almost identical line is obtained for the K a KO X-rays where the sensitivity is 20432 f 180 counts/pg of Br. When counting is done without the 1.5 mm absorber, the sensitivity increases to 39 940 f 350 counts/Br, but however the ratio of peak to background is worse. Table I gives the amount of bromine measured both by y-ray and X-ray spectrometry. The average value, 6.66 f 0.21 pg/mL is in a good agreement with the published data (3, 14-16). The detection limit of a radionuclide is usually defined as the smallest photopeak which can be detected with a certain confidence above the background continuum. Currie (17) has shown the detection limit for the case of radioactivity to be given by the equation
+
LD(counts) = 2.71
+4 . 6 5 6
where pB is the true mean of the blank. The minimum detected mass is given by MD = LD/K where K is the sensitivity factor (counts per milligram).
For a 0.2-mL sample irradiated for 20 min and counted for 5 min after a decay time of 5 min the detection limits for
determination of Br by y-ray and X-ray spectrometry is 37 and 46 ng/mL, respectively. Since y-ray measurement has a lower detection limit and X-ray measurement has a drawback of self-absorption, it is clear that for epithermal neutron activation it is preferable to use y-ray spectrometry. When the concentration of bromine is very low, too low to be detected by this method, the use of longer irradiation with thermal neutrons followed by a long decay and a long counting time can enable the measurement the bromine content. In this case, the 82mBris used. The advantage in this case is the higher flux (Cd ratio for bromine is about 7 ) and the longer intensity+% for 618 keV of @Br,compared to 66% for 554 keV and 41% for 619 keV of 82Br. Since the cross section is about the same, this can lead to an advantage of about 65 for 82Br. However, in this case, long irradiation and counting times are required, which is inconvenient for routine measurement. Registry No. Bromine, 7726-95-6; bromide, 24959-67-9.
LITERATURE CITED (1) Mantel, M.; Amiel, S. Anal. Chem. 1972, 44, 548. (2) Shenberg, C.; Gllat, J.; Finston, L. H. Anal. Chem. 1967, 3 9 , 780. (3) Rapaport, M. S.;Mantel, M.; Nothmann, R. Anal. Chem. 1979, 51, 1356. (4) Amlei, S.;Mantel, M.; Aifassi, 2. B. J . Radioanal. Chem. 1977, 3 7 , 189. (5) Mantel, M.; Alfassl, 2. B.; Amlel, S. Anal. Chem. 1978. 50, 441. (6) Stella, R.; Genova, N.; Di Casa, M. Radlochem. Radloanal. Len. 1977, 3 0 , 65. (7) Behne, D.; Diel, F. In "Nuclear Activation Technlques in the Life Sciences"; IAEA: Vienna, 1972; IAEA-SM-157, p 41. (8) Behne, D.; Jurgensen, H. J . Radloanal. Chem. 1978, 4 2 , 447. (9) Nakahara, H.; Nagame, Y.; Yashlzawa, Y.; Oda, H.; Gotoh, S.; Murakaml, Y. J . Radloanal. Chem. 1979, 5 4 , 183. ( I O ) Guzzl. G.; Pletra, R.; Sabionl, E. Comm. Eur. Communities, [Rep] EUR 1974, EUR 5282e. (11) Spyrou, N. M.; Fricker, M. E.; Robertson, R.; Gilboy, W. B. In Symposium on Nuclear Techniques In Comparative Studies of Food and Environmental Contamination"; IAEA: Vienna, 1973; IAEAISM175. (12) Guinn, V. P.; Miller, D. A. J . Radloanal. Chem. 1977, 3 7 , 313. (13) "Handbook on Nuclear Activatlon Cross-Section, Technlcal Reports No. 156"; IAEA: Vienna, 1974. (14) Altman, P. L., Dittmer, D. S., Ed. "Bloiogy Data Book"; Federation of Amerlcan Societles for Experimental Biology: Bethesda, MD, 1974; Vol. 3, p 1752. (15) Barrette, M.; Lamoureux, G.; Lebei, E.; Lecomte, R.; Paradis, P.; Monara, s. Nucl. Instrum. Methods 1976, 134, 189. (16) Rapaport, M. S.; Mantel, M.; Shenberg. C. Med. Phys. 1982, 9 , 194. (17) Curie, L. A. Anal. Chem. 1968, 4 0 , 586.
RECEIVED for review August 23, 1982. Accepted November 22, 1982.
