Curve smoothing can show up peaks in statistically poor spectra and smaller peaks in the vicinity of large ones. Comparison with normalized Gaussian curves eliminates statistical fluctuations and Compton edges, but not Gaussianshaped peaks like backscatter peaks. Results have been found to be better than those obtained by the application of Covell’s method with similar width of the calibrated peak fraction. However, it should be noted that the width is fixed by the proposed method, while Covell’s method leaves the choice of the most suitable width open. The method is applied in this department for the simultaneous determination of trace elements in biological material and the program (Algol 60) is available on request.
Table V. Evaluation of Neutron-Irradiated Hair Sample Peak fraction calibrated by points of inflection Width. Area. Energy, Channel channels counts MeV Comment 7.18 3135 13.59 0.237 Backscatter 6.48 4321 41.79 Annihilation (80Br, 0.511 C4Cu, 38Cl,z4Na) 80Br 53.55 5.17 312 0.625 66Mn, 2;Mg 0.847 76.38 8.19 2602 Z4Na 10.43 33835 1.37 130.07 38C1 10.99 24601 1.65 158.87 3 8 ~ 1 2.17 13.29 25986 212.92
ACKNOWLEDGMEYT The authors thank Professor Gilbert Forbes for his support and encouragement during the course of this study. They also thank the Scottish Research Reactor Centre and the Computing Department of Glasgow University for the use of equipment and facilities.
third order approximation curve are used. The shape of the peak fractions is checked with normalized Gaussian curves. The peak fraction method was chosen, because it permits work without a catalog of standard spectra and gives better results than spectrum stripping in the lower energy region of composite spectra.
RECEIVED for review August 10,1967. Accepted July 1, 1968. -
Direct Determination of Lead-210 by Liquid ScintiI lation Counting I
William D. Fairman and Jacob Sedlet Argonne National Laboratory, 9700 South Cuss Ace., Argonne, Ill. 60439
The soft betas, 15 and 61 keV end-point energies, the internal conversion electrons, and unconverted gamma rays from z10Pb are efficiently detected in a liquid scintillation counting system. The overall counting efficiency (cpm/dpm) is 97-98%. The background counting rate in the 210Pb window is 20 cpm for polyethylene vials and 40 cpm for low-potassium glass vials. It is possible to determine 0.98 t 0.58 pCi of separated 21oPb at the 95% confidence level in a 100minute sample counting period, with polyethylene vials. The Z10Pb can also be measured in the presence of its daughter activities, ZlOBi and 2lOPo. Both the 3lOBi beta (1.160 MeV end-point energy) and the 210Po alpha (5.305 MeV) are counted at essentially 100% efficiency. There is no interference between the 210Pb and Z1OPo spectral counting regions. The 210Bi beta spectrum overlaps the zloPb and 2l0Pospectral regions; 23 and 50% of the ZlOBi beta particles appear in the lead and polonium regions, respectively. However, an energy range covering approximately 27% of the bismuth spectrum is completely free of zloPb and zlOPo activity. It is thus possible to determine the concentration of each component of a lead-bismuth-polonium-210 mixture.
THEDETERMINATION of 2loPb is important in such fields as radium toxicity studies, atmospheric tracer studies, and uranium mining operations. The direct measurement of this radionuclide has always been difficult because of the low energy of its beta particles. If the zloPb in a sample is in secular equilibrium with its daughters, zlOBiand zloPo,one or both daughters can be separated and measured immediately to determine the amount of 2loPb ( I , 2). If, however, there is doubt as to whether secular equilibrium has been established, then it is necessary t o purify the lead from its decay products and wait for the 210Bior z1OPo to grow in significantly, before (1) E. S. Ferri and H. Christiansen, Piibl. Health Repts., 82, 828 (1967). (2) C. W. Sill and C. P. Willis, ANAL.CHEM., 37, 1661 (1965). 2004
ANALYTICAL CHEMISTRY
the zloPb can be determined from the daughter activities This growth period results in a delay of days to weeks before measurements can be completed. Current procedures have used one of the above indirect methods for determining zloPbconcentrations because of the ease of measurement of the energetic beta (1.160 MeV endpoint energy) from zlOBior the alpha (5.305 MeV) from ZlOPo, as contrasted with the previously believed difficulty of measuring the disintegrations from zloPb. Lead-210 decays by the combined emission of beta and gamma-rays and internal conversion electrons to the ground state of *loBi(6). Nineteen per cent of the beta emission is directly to the ground state with a maximum beta energy of 61 keV. Eighty-one per cent of the beta emission is t o the 0.04652-MeV level of zlOBiwith a maximum beta energy of 15 keV. The 0.04652-MeV level is highly converted in its rapid (99 50 30 210Po 55-100 100 4 4 2 1OBi 35-556 27 5 6 LLD = Lower level discriminator setting. ULD = Upper level discriminator setting. *Window setting used for ZlOBi when a mixture of the three isotopes is to be counted.
2006
ANALYTICAL CHEMISTRY
RELATIVE
P U L S E HEIGHT( Beckman log energy scale)
Figure 4. Liquid scintillation spectra of *loPb,*IoBi,and *loPo at gain = 50.0 zloPb(Total counts = 6.17 X lo4) zloBi(Total counts = 6.17 X lo4) 0 zlOPo(Total counts = 6.00 X lo4)
X
+
possible to count either the separated zloPbalone or in conjunction with its daughter activities. The above percentages were observed to be independent of the amount of activity up t o at least lo4dpm. Counting rates that exceed the capability of the instrument may alter this percentage distribution. In considering the accuracy and sensitivity of determining R a D E F by liquid scintillation counting, it is useful to distinguish two cases: (1) *lOPbis separated from its decay products and counted for the purpose of determining the zloPb alone; (2) a mixture of the three radionuclides is counted for the purpose of determining the concentrations of each independently (the degree of radioactive equilibrium may or may not be known). When separated zloPb is counted with the efficiency and background counting rates in Table 11, the minimum detectable level of activity, calculated according to the method of Currie (8), is 0.98 0.58 pCi of *loPbat the 95% confidence level in a 100-minute sample counting period, with polyethylene vials. The background is also measured for 100 minutes. As the 21uBigrows in, a correction must be made if the determination is based on 210Pbalone. Figure 5 presents the decay curve for zlaPband the growthdecay curves for zlOBi and zlOPo. The curves were computer plotted from computer derived tables, using the CORD and CORD-PLOT computer programs ( 9 , 10). The halflives used in the computations were: zlaPb = 22.0 years, ZlOBi = 5.0 days, and 210Po= 138.4 days. As can be seen from Figure 5 , 210Bigrows in quite rapidly with a 5.0-day halflife. If the 2lnPb is counted within less than 2 hours after separation, only 1.15% or less of the zlOBi will have grown into the counting sample. This amount of *lOBiis equivalent to less than 0.3% interference in the *loPbcounting region. Even if as much as 7 hours (3.96% *lnBi ingrowth) have elapsed since the lead separation, there is still less than 1% (8) L. A. Currie, ANAL.CHEM., 40, 586 (1968). (9) W. D. Fairman, S. A. Tyler, M. H. Dipert, and J. Sedlet in “Radioisotope Sample Measurement Techniques in Medicine and Biology,” Proceedings of a Symposium, Vienna, May 24-28,
1965, International Atomic Energy -~ Agency, Vienna, Austria, 1965; pp 187-209. (10) W. D. Fairman, S. A. Tyler, M. Dipert, and J. Sedlet, Trans. Am. Nircl. SOC.,9, 99 (1966).
x
@“L.’-
0450
02,
,250
Id00
1J50
d
!!@O
1650
I800
TIMEIX
R E L A T I V E P U L S E H E i G H T I B e r i m m n log t n i r q r ~ c o i e )
Figure 5. Decay curve for zloPband growth-decay curves for zlOBi and 210Po X zloPb 0 zlOBi n ZlaPo interference in the counting of the zlOPb. If longer times elapse before counting can be completed, then the counting technique described next can be applied t o correct for the zlOBi activity. However, one can take advantage of the daughter growth and count all the activity, use the appropriate growth factor, and obtain increased sensitivity in this way. When the sample is counted 5 days after separation, the minimum detectable level is 0.79 + 0.48 pCi; after 15 days 0.62 ==I 0.38 pCi can be detected; and after secular equilibrium has been reached for both 210Biand zloPo,0.39 =I= 0.24 pCi is detectable. All errors are at the 95% confidence limit. If all three isotopes are to be determined, o r if separation procedures or separation time are not available, a rapid, but somewhat less accurate, determination of the zlOPb, zlOBi, and zlOPocontent of a sample can be made in a single liquid scintillation measurement : Total dpm flOBi
=
(”
- ”)
=
DSi
(1)
Ebu)
Total dpm zlOPo=
(cc - Bc) - (DSi x E,,,) E,,,
(3)
where E
=
C B
= =
D
=
counting efficiency = counts per min/disintegration per min total counting rate in counts per min background counting rate in counts per rnin disintegration rate in disintegrations per min
and u
b
= =
c
=
Pb Bi Po
= = =
zloPbtotal spectral counting region zlOBispectral counting region between zloPbspectral upper limit and ZlOPo spectral lower limit Z T o total spectral counting region as it pertains t o zloPb as it pertains to zlaBi as it pertains to 210Po
*
The minimum detectable level of activity for zlOBiis 1.94 1.21 pCi at the 95p7, confidence level in a 100-minute sample counting period, when polyethylene vials are used and counting is made in the 35-55 window. Therefore, this amount of zlOBimust be present in a sample mixture before a zlOBicorrection can be applied t o that mixture.
Figure 6. Effect of setting time and subsequent mixing on 210Po(in “ O s ) liquid scintillation spectrum 210Poin 0.2 ml of 0.5N “ 0 3 and 10 ml liquid scintillation solution. Gain = 33.0 0 210Pospectrum after setting approximately 5 days X zlOPospectrum after shaking (0). This spectrum is comparable to the original 210Pospectrum before setting If the zlOPb, zlOBi,and zlOPoconcentrations are known, some of the history of the original sample can be determined. If one examines the ratios of these concentrations and relates these ratios t o growth-decay tables, it is possible in some cases to determine the age of the sample if secular equilibrium has not been reached and/or if the daughter activities are supported by the parent activities. Such information is useful in pollution studies and the study of transport mechanisms, such as occur in environmental systems ( I , 2, 11, 12) and in biological organisms (3, 11, 13, 14). I n previous experiments, the separated fractions had been taken up in 0.5N HNOI instead of 1N and were counted in glass vials. Under these conditions, the zlOPospectrum gradually shifted with time toward a lower energy position. If, after the spectral shift, the sample was shaken, the normal spectrum was again obtained, as shown in Figure 6, in which the “after shaking” spectrum is identical with that which had been obtained before any spectral shift had occurred. I n order t o determine if the activity was being gradually adsorbed on the walls of the glass counting vial, the sample solution was carefully transferred t o another vial after spectral shift had occurred. Essentially all of the activity was transferred t o the new vial. Although several days were required t o obtain the spectral shift depicted in Figure 6, subsequent shifts of the same magnitude occurred in less than a day after shaking. No change in degree of quenching during the spectral shift was evident from the external standard ratios and from the observation that only the z l o Pspectrum ~ shifted and not the zlOBi or the zlaPb. O n the other hand, samples in polyethylene counting vials did not appear to undergo this ZlOPo spectral shift phenomenon. Two separate processes are occurring in producing the (11) B. Loebel, H. Murth, and E. Oberhausen, Struhlent/zerupie, 131, 218 (1966); U. S. At. Energy Comm. Rept. ANL-TRANS458 (1967). (12) P. Kauranen and J. K. Meittinen in “Radioecological Concentration Processes,” Proceedings of an International Symposium, Stockholm, April 25-29, 1966, P. Aberg and F. P. Hungate, Eds., Pergamon Press, New York, N. Y.,1967, pp 275-80.
(13) R. L. Blanchard, ibid., pp 281-96. (14) C . R. Hill, ibid., pp 297-302. VOL. 40, NO. 13, NOVEMBER 1968
2007
9t-
Figure 7. Effect of setting time on RaDEF (in HCI) liquid scintillation spectrum RaDEF in 0.2 ml of 1N HCI and 10 ml liquid scintillation solution. Gain = 33.0 X Immediately after preparation of sample 0 Four days fte r preparation of sample
spectral shifts. The first is a slow process, possibly radiocolloid formation and/or adsorption on silica particles from the glass vials, while the second is more rapid and may be adsorption of the colloidal polonium on the glass walls or settling out of the particles containing adsorbed polonium. If adsorption on the glass walls occurs, the bonds must be very weak, because the polonium is very easily transferred between vials. It is sufficient to note that 210Posamples preferably should be counted in polyethylene vials. If glass vials need t o be used, for example when the sample is to be prepared by evaporation in the counting vial, then the spectral shift effect may be minimized by using 1N H N 0 3 as the aqueous phase, as given in the routine procedure. Under these conditions, the ?loPopeak position shifts very slowly with time toward a lower energy position. Approximately 1% of the count-rate shifts out of the zlOPocounting window in 17 hours. Thus, if samples are allowed to stand for longer than a day before counting, or if the samples are counted for longer than a day, this spectral shift must be accounted for in making the measurements or else the samples should be shaken once or twice each counting day, because shaking will restore the normal energy spectrum, as seen in Figure 6. The chemical quenching action of acids on liquid scintillation solutions is well known and was observed in this work as described below. In the course of the spectral shift experiments, 0.2- and 0.5-ml volumes of R a D E F (0.5, 1.0, and 2.ON in H N 0 3 or HC1) were counted in 10 ml of scintillation solution. It was observed that with HNOI, spectral shift with time was limited to zlOPo. No phase separations were evident with any of these samples. On the other hand, when HCl was studied, with the same volumes and concentrations, quite different results were obtained. As expected, chemical quenching with HCl was quite severe, as evidenced by the external standard ratios and by strong shifting of the total (zloPb,210Bi,and zlOPo)spectrum toward the low energy side. With 0.5-ml volumes of the above HC1 concentrations, the pulse-height distribution was considerably lower than that obtained with the same volume and concentration of H N 0 3 , and phase separation occurred in the 1N and 2 N HC1 samples. With 0.2-ml volumes, at all HCI concentrations studied, the initial lower pulse-height distribution increased with time. The change was completed in 2-5 days, depending upon the acid concentration. This effect is shown in Figure 7, in which 2008
ANALYTICAL CHEMISTRY
the initial spectrum has a lower energy equivalent than the corresponding H N 0 3 acid sample; but the final spectrum, after spectral shift has occurred, has a higher energy equivalent sample. These spectral shift than the corresponding " 0 3 phenomena will be studied further. These spectral shifts again illustrate the fact that when doing liquid scintillation counting, one must be totally familiar with the sample matrix and its stability with time. This is particularly important when narrow window counting and/or lolig counting periods are being used. Large errors can occur if these effects are not considered when preparing and counting samples and standards. Other radioactive lead isotopes in the sample will interfere in the counting procedure and give high results if their presence is not taken into account. Once the lead has been separated and the various lead isotopes are no longer supported by their parents, a delay of three t o four hours in counting will permit >99% of both 2I4Pb (half-life of 26.8 minutes) and 211Pb (half-life of 36.1 minutes) to decay to noninterfering nuclides. If the lead has been separated from thorium-rich samples, then *12Pb (half-life of 10.64 hours) may be a serious interference. A three-day delay in counting would be required to permit >99% of the z12Pbto decay to stable *08Pb. The resulting lead-bismuth-polonium-210 mixture could then be assayed directly as described earlier or a second lead separation could be performed, if it is desired to count zlOPbalone. The presence of *12Pb in a zloPb sample is detectable from a liquid scintillation spectrum taken a few hours after separation because of the energetic radiations from its descendants, z12Bi(2.27 MeV beta) and zlzPo(8.79 MeV alpha). Therefore, it is believed that the 212Pbconcentration could be directly determined by the described liquid scintillation technique and a correction applied to the liOPbcounting rate in a manner similar t o the zlOBicorrection described earlier. Such a technique would eliminate the necessity of waiting several days for the decay of 212Pb. Recalling the decay scheme of 210Pb,one would not expect to obtain such a high detection efficiency for its emitted radiation, because 81 of the beta emission has a maximum energy of only 15 keV (an energy lower than the tritium beta). However, this beta decay leads t o a zlOBienergy level which is highly converted in its rapid decay to the ?loBiground state. The conversion electron is formed sufficiently rapid that it and the beta particle appear in coincidence and thus are counted simultaneously by the liquid scintillation spectrometer as a single event, with an energy equal to the sum of the two events. This summed energy, the beta particle energy plus approximately 30 keV for the conversion electron, is sufficiently high t o be detected at essentially 100% efficiency. The unconverted 46 keV gamma-rays are weak enough that most of them are totally absorbed in the liquid scintillation solution producing monoenergetic electrons which are counted with 100% efficiency (15). ACKNOWLEDGMENT The assistance of R. Fergus in the design of the circuit used to interface the liquid scintillation spectrometer to the pulseheight analyzer is gratefully acknowledged.
RECEIVEDfor review May 10, 1968. Accepted August 5 , 1968. Work was performed under the uspices of the U. S. Atomic Energy Commission. (15) D. L. Horrocks in "Progress in Nuclear Energy, Series IX, Analytical Chemistry, Volume 7," Pergamon Press, New York, 1966, pp 21-110,
