ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
189
Parametric Neutron Activation Analysis of Samples Generating Complex y-ray Spectra P. F. Schmidt” Bell Telephone Laboratories, Incorporated, Allentown, Pennsylvania
18 103
J. E. Riley, Jr. Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey
D. J. McMillan Northern-Tracor, Inc., Middleton, Wisconsin
The accuracy of multielement quantitative determinations by parametric neutron activation analysis using Fe-Ru flux monitors is compared with that obtained by relative measurements using Standard Reference Materials (SRMs) from NBS. Agreement is good. Isotopes with a large ratio of resonance integrallthermal cross section require empirical correction factors. Deviation of the resonance flux in the reactor reflectors from a 1 / E distribution appears responsible for at least part of the necessary corrections. The correction factors were found invariant from reactor to reactor. Molybdenum and certain rare earths can be determined by parametric counting in the presence of comparable amounts of uranium; interference by fission products is eliminated by calculations including the fission yields.
Previous S t a t e of the Art. Conventional neutron activation analysis (NAA) uses relative measurements, Le., standards for each element to be determined. These should be co-irradiated with the unknown sample, and later be measured in the same geometry as the unknown sample, and should have not too different count rates for the individual isotopes to be determined. This implies extensive work on the preparation of the standards before and after irradiation in order t o achieve comparable activities in identical geometries for a multiplicity of isotopes. Parametric counting avoids most of the work on standards. The concentrations of the various isotopes present are derived from the photopeak areas via the use of the pertinent nuclear constants, employing an equation which also contains the intensities of the thermal and resonance fluxes (or the fast flux for (n,p) reactions) in the reactor a t the time and location of the irradiation. Light elements are mostly activated by the thermal flux, but many of t h e heavier elements have large cross sections for resonance activation. Parametric counting is thus critically dependent on t h e accuracy of the nuclear constants and on the accuracy of flux determination. The fluxes are measured by including monitors (such as iron and ruthenium) in the irradiation. T h e nuclear constants for most isotopes have now been established with fair to very good accuracy. Except for some elements, such as gold or cobalt, the accuracy to be expected from parametric counting is still not quite as good as can be obtained from relative measurements. However, the speed and ease of the method make it very attractive in any kind of survey work involving a large number of samples. T h u s the main obstacle to applying parametric counting consisted in finding a convenient and reliable method for measuring the 0003-2700/79/0351-0189$01 . O O / O
thermal, resonance, and fast fluxes a t the location and time of irradiation. T h e flux in a reactor is not constant but depends on the power level, motion of control rods, and the locations and age of fuel elements. An accepted way of determining the thermal and resonance fluxes consists in making cadmium-difference and cadmium-ratio measurements on very dilute gold alloys. A bare sample is activated by both the thermal and the resonance fluxes, a cadmium-shielded sample only by the latter. T h e method, however, has its difficulties (possible non-uniformity of the gold alloy, possible leakage of thermal neutrons through cracks in the cadmium, absorption of epithermal neutrons in the cadmium, flux depression in the vicinity of the cadmium capsule). T h e very fact that parametric counting has so far not enjoyed any popularity in spite of its obvious conveniences testifies to the practical difficulties encountered with flux determinations via cadmium measurements on gold, especially for users without direct access to a research reactor. In a recent publication ( I ) we described a convenient new method of determining the resonance-to-thermal flux ratio, F , in a nuclear reactor by co-irradiation of Fe-Ru monitors. The advantages of using the activities of three isotopes of the same element, Ru, for the calculation of the epithermal flux, as compared to any method using isotopes of different elements, were outlined in Reference 1. This flux determination, together with precise photopeak area and detector efficiency determinations, enabled us to apply parametric counting successfully to instrumental (i.e., nondestructive) NAA of trace elements in silicon and other high purity materials with relatively simple y-ray spectra. While the accuracy of these determinations was adequate for the purpose a t hand, a rigorous determination of the accuracy achieved by measurements on SRMs had not yet been carried out. Original Purpose of T h i s Work. The purpose of the present investigation was to apply the parametric counting approach t o the analysis of samples yielding complex y-ray spectra after irradiation, as well as to compare the concentrations calculated in this manner to those obtained by relative measurements. The “Trace Elements in Gelatin” Reference Materials, recently fabricated and described by Kodak (2, 3) are rather ideal for this purpose, since each of t h e three gelatins, T E G A, B, and C, contains up to 25 elements a t the nominal 50-ppm level and the y-ray spectra are not dominated by any one element, because of a fortuitous choice of the components. We participated in the cooperative effort initiated by Kodak to characterize these Reference Materials, and the results are reported here. By making relative measurements with SRMs, and using parametric counting on aliquots of the same sample, an excellent opportunity was C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
afforded t o evaluate the accuracy achievable with parametric counting while also evaluating its performance in analyzing extremely complicated y-ray spectra. A wide array of analytical techniques ( 2 , 3 )by researchers both in the United States and abroad was employed in the characterization of the “Trace Elements in Gelatin” coordinated by Kodak. In general, good agreement resulted with the nominal concentrations expected from the method of fabrication-gelling from aqueous solutions doped with measured amounts of impurities. However, the uncertainty of the values obtained by many different techniques a t many different laboratories resulted in relative standard deviations which are decidedly too large for many applications. Our own work on the T E G materials was limited to one technique, NAA, and the relative measurements with SRMs covered only a fraction of the elements contained in the TEGs, but for the elements covered we were able to obtain significantly smaller relative standard deviations. During the course of this work it was noticed that the concentrations obtained by parametric counting were consistently too low for both isotopes of antimony, provided that the antimony concentrations in the TEG-50 samples were as stated by Kodak. Silver, arsenic, molybdenum, and uranium appeared to follow the same pattern, but in the case of Ag and As we deemed our determinations somewhat uncertain because of interferences, uranium was contained in only one sample (TEG-50A), and the nuclear constants for molybdenum are not very well established as yet. As a first step toward resolving this question, we irradiated the N B S SRM 364, having a certified antimony content, together with Fe-Ru monitors a t the University of Missouri light water reactor and evaluated the antimony concentration both by relative measurements and by parametric counting using the Fe-Ru flux data. Parametric counting yielded antimony concentrations too low by the same factor as seen in the T E G samples. Setting the concentration of each certified element in SRM 364 = l o o % , the following percentages were found: jgFe: 99.17 f 2.770; 6oCo: 95.17 k 2 . 7 7 ~ ; 51Cr: 106.14 f 2.3%; 65Zn: 115.54 (?)%; 12%b: 53.15 0.2% ; ‘%b: 53.33 f 0.170.Clearly, an uncertainty in the true antimony concentrations in the T E G samples cannot be blamed for the anomalous antimony results. Since each of the isotopes showing this anomalous behavior: 121Sband lZ3Sb,75As,loSAg,and 238U,has a large ratio R of resonance integral/ thermal cross section, it seemed logical to assume that the ruthenium monitor indicated an artificially large resonance flux, thus leading to artificially low isotopic concentrations. In order to clarify this question, a double-pronged approach was chosen. On the one hand, milligram quantities of the metallic elements were co-irradiated with Fe-Ru monitors in the highly thermal flux a t Georgia Institute of Technology; on the other, cadmium-ratio and -difference measurements on well characterized Al(Au) flux wires a t the University of Missouri light water reactor were combined with flux determinations by the Fe-Ru monitors in the same irradiation. The resultant two sets of flux data were used for concentration calculations of co-irradiated Reference Materials. T h e measurements a t Georgia Institute of Technology would be little affected by uncertainties in the magnitude of the resonance integrals because of the low resonance flux; the measurements a t the University of Missouri would show up errors in the magnitude of the resonance flux calculated from the ruthenium data. Both sets of measurements provided interesting results. Other Investigations. A well known type of interference in NAA comes about when fissionable isotopes like 2s35Uare present: the fission fragments add t o the concentration of
*
*
some other radioactive species stemming from (n,y) reactions. We found a simple way t o overcome this problem by means of parametric counting. Finally, the natural abundance of j8Fe was not known accurately before, since it is very low. Co-irradiation of a natural iron foil with a Fe203sample highly enriched in j8Fe offered a simple and very accurate way to determine the natural abundance of jBFe. This method is, of course, applicable to any isotope that becomes radioactive upon neutron capture and is obtainable in enriched form. The j8Fe results are described in an accompanying Aid in this Journal ( 4 ) .
EXPERIMENTAL The spectrometry system employed a t Allentown has been described before ( I ) . The system a t Bell Labs, Murray Hill, consisted of a Tennecomp TP-5000 pulse height analyzing system, a Northern Scientific (Model 623), 100-MHz ADC, and a 60 cm3 coaxial lithium drifted germanium detector. Detector efficiency as a function of energy was established by means of a certified multiple isotopes point source from the National Bureau of Standards, the same source as used at Allentown for this purpose. The counting system at University of Wisconsin (UoW) consisted of a computer-based Tracor-Northern TN-11 multichannel analyzer, a Canberra model 2010 spectroscopic amplifier, and a 30 cm3 lithium-drifted germanium detector. An automatic pneumatic tube-sample changer controlled by the TN-11 is also incorporated into the system, so that samples may be counted with the facility unattended. Detector efficiency was measured over the range 122 to 1407 keV by means of a calibrated lj2Eu source, and extended to 2754 keV by measurements on the 24Na photopeaks. Irradiation Conditions and Counting Procedures for the TEG Samples. These varied at the three locations depending on the delivery time from the reactor and on the method of investigation. A t the University of Wisconsin, samples could be measured immediately after irradiation, and a short irradiation of 60 s, followed by 300s acquisition was used for the short-lived 52v,66Cu).In addition, a 15-min irradiation followed isotopes (%Al, by decay times of one and of three days for the longer-lived isotope was used. All samples were counted as solids at a distance of 5 cm from the detector, except for the longest decay time when they were counted a t contact. Bell Labs, Murray Hill, is too far from a reactor to measure very short lived isotopes but close enough to measure isotopes like 56Mnor 64Cuwith time to spare. All samples were dissolved and counted at a distance of 15 cm from the detector in a rigidly fixed geometry. Aliquots of the solutions were transported to Allentown where they were counted at a distance of 4.6 cm from the detector, also in a rigidly fixed geometry. Allentown is too far from any reactor for reliable measurements even on j6Mn or “Cu for the given level of activity generated. The maximum duration of irradiation is set by the nature of the gelatin sample, i.e., 20 min at a flux of about 8 X 1013n cm * s (charring, pressure buildup during irradiation). Both solid and liquid samples were measured at Allentown, and a conversion factor for the different geometry was determined for the samples counted as solids. The same geometry that was used for counting TEG samples after dissolution was also used for measuring SRM 364 and 127B, as well as the iron and gold monitors, and the Sb, As, Ag, MCJ, Ga, and Se samples discussed later. Ruthenium is exceedingly difficult to dissolve, and it was counted only as a solid. However, the method of determining the epithermal-to-thermal flux ratio from the activity of the three ruthenium isotopes (’-Ru, ‘”Ru, lo51iu)does not depend on knowing the amount of ruthenium present, so that an inaccurate geometry factor is unimportant for the epithermal flux determination. Evaluation of the spectra at Bell Labs, Murray Hill, was limited to those isotopes also covered by co-irradiated SRMs, and these measurements did not require counting times longer than 3 h. Evaluation at Allentown comprised all isotopes detected, Le., also very long-lived species, and counting times up to 18 h were employed. Both at Murray Hill and at Allentoun, samples were counted immediately after receipt and then again after about 2 and after about 10 days. SRMs co-irradiated with the TEG samples were z85B (aluminum alloy), 2364 (high carbon steel),
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2 , FEBRUARY 1979
Table I.
191
Concentrations Obtained by Parametric Counting from Irradiations at Reactors with Different F Ratios irrad. position: graphite reflector, irrad. position: D,O reflector, irradiation irradiaiion a t University of Missouri = F ratio = 7.17% a t Georgia Tech, F ratio = 1.38% - integral/thermal isotope TEG 50A TEG 50B TEG 50A TEG 50B cross section 0.386 55.44 i 4.9 40.0 i 0.18 48.52 c 9.8 39.5 i 1.7 I9"g 43.03 i 1.6 0.816 43.78 f 4.4 31.89 I5.5 31.79 i 3.7 '"Hg 0.685 302.50 -t 1.6 125.7 i 1.8 290.75 * 1 2 . 3 108.1 i 0.68 24 Naa 0.566 51.5 i 0.56 37.52 i 0.77 40.93 * 0.50 "Cr 48.2 i 1.6 2.42 56.72 i 1.8 44.5 I 3.5 "Zn 1.286 47.52 t 0.90 45.81 i 0.79 "CO 7.73 42.71 i 0.39 72Ga 41.81 i 0.75 10.89 26.16 f 0.28 24.9 i 0.33 'lumAg 32.16 i 4.5 9.17 30.84 * 2.9 35.4 i 2.1 Se 32.0 i 0.32 interfered 62.20 i 0.06 58.94 i 0.10 17.44 interfered lbAsb 31.86 27.54 i 0.36 27.3 * 0.13 28.72 i 1.4 lz2Sb 26.0 i 1.2 30.28 28.38 i 1.0 28.20 c 1 . 2 28.9 i 0.07 29.2 i 0.93 I2'Sb 2~9U 100.74 31.3 i. 0.35 32.0 i 0.03 (239NP) 35.86 39.8 f 3.1 9 9 ~ ~ c 41.6 i 0.11 'j
a Probably nonuniform. zi5U;see text.
All photopeaks interfered.
Calculated by hand from the total 9 9 M-~9 9 Mdue ~ to fission of
and 1571 (orchard leaves). The TEG samples were equilibrated for 2 days in a controlled atmosphere clean room, 2 1 h 1 "C. 40 f 5% RH, before irradiation, as suggested by Kodak (2). The nuclear constants used in the Absolute Counting calculations are listed in Table VI1 for all isotopes determined in this manner. RESULTS M u l t i p l e T r a c e E l e m e n t D e t e r m i n a t i o n by Absolute Counting. Table I shows concentrations of impurities in TEG 50A and 50B as obtained by irradiation a t the Georgia Institute of Technology heavy water moderated reactor, and a t the University of Missouri (UoM) light water moderated reactor. Because of the different moderators, the F ratio (epithermal/thermal flux) is quite different at the two reactors. This ratio is shown in Table I, next to the irradiation facility, as determined by the Fe-Ru method. In spite of the large difference in F ratio, the concentrations calculated for isotopes with such different resonance integral/thermal cross section ratios as '03Hg (= 0.816) and 238U ( = 100.74) are in good agreement for irradiations a t the two reactors. This strongly suggests that the Fe-Ru method ought to be quite reliable. Table I1 shows the factors by which the elemental concentrations for Sb, As, Ag, Mo, Ga, Se, and Au were found too low in the irradiation of milligram quantities of these elements a t the Georgia Tech heavy water reactor (in the most thermal location, a t a flux of 5.7 X 10" n cm s '). These factors turned out to be identical to those which would have to be used to make the concentration of these elements, as determined by parametric counting, coincide with the true concentrations in the Kodak T E G or NBS SRM samples. In calculating these factors, the small correction for self-shielding of thermal neutrons has been taken into account according t o the equations given by Zweifel (5). Self-shielding of the approximately 1.4% epithermal flux detected by the ruthenium monitor has not been taken into account, though this would obviously decrease the magnitude of the correction. This self-shielding of the small resonance flux was disregarded because the ruthenium resonances occur a t higher energies than the large resonances of the other isotopes. If the discrepancy is, in fact, due to the other isotopes not "seeing" the resonance neutrons still present a t the higher Ru resonance levels, then it appears questionable whether such a correction should indeed be applied. Incidentally, calculations showed that while such self-shielding would reduce the magnitude of the discrepancy, it would certainly not eliminate it (except for Au).
Table 11. Factors by Which the Amounts of the Metallic Elements, Irradiated in a Highly Thermal Flux of the Graphite Reflector of the Georgia Tech Heavy Water Reactor, Were Found Too Small When Using the Flux Data of the Co-irradiated Fe-Ru Flux Monitors for the Calculationa elements antimony ( I2'Sb or '"Sb) arsenic ( ' b A ~ ) gold ( "'Au) molybdenum (99Mo(Tc)) silver ( IIUmAg) gallium ("Ga) selenium (-'Se)
multiply by 1.923 1.652 1.119 1.802 1.535 1.300 1.413
a The calculated concentrations of the isotopes have to be multiplied by the factors indicated in order to arrive at the correct concentrations. These correction factors were found t o be independent of the reactor neutron spectrum in which the samples were irradiated, see Tables I , 111, IV, and V.
The correction factors determined for %b and lz4Sb,'6As, IlornAg.%Mo(Tc),'*Ga, and have been used to correct the concentrations of these isotopes in the following Tables 111-V. I t is worth noting that cadmium measurements on Al(Au) flux wires carried out in the same location a t Georgia Tech indicated an epithermal flux 2 orders of magnitude smaller than derived from the ruthenium measurements (6). T h e comparison between concentrations obtained by parametric counting (at BTL Allentown and a t UoW) and those obtained by relative measurements using SRlCls is given in Tables 111-V. Not all isotopes detected and determined by parametric counting are listed; for a complete list of the components of the TEG samples, the reader is referred to Refs. (2,3). T h e data in Tables 11-V need some explanation. Access to the UOW reactor enabled determination of very short-lived isotopes such as i2V and 28A1, but use of loaned equipment limited the acquisition time and hence the determination of very long lived isotopes with correspondingly low count rates. The situation a t B T L Allentown was exactly the reverse of that a t UoW, while the evaluation of isotopes a t Murray Hill WBS limited by the number of SRMs that could be co-irradiated and co-evaluated with the T E G samples in the given number of runs executed. The meaning of the standard deviation is also different for the relative measurements a t BTL Murray Hill and for the
192
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2 , FEBRUARY 1979
Table 111. Intercomparison of Concentrations Obtained for TEG 50A by Parametric Counting ( a and b ) , and by Relative Measurements Using NBS Standard Reference Materials (c), to the Values Quoted by Kodak (Average of Several Techniques)
"Cr 6SZn "GaC ll0rnAgC 75SeC 56Mn 76
As
122SbC 124SbC y9Mo(T C ) ~ 2'yU(Np) 1 1 6 r n 1 ~
ratio of resonance ( b ) Univ. of integralitherma1 ( c ) comparison t o cross section NBS SRMs (JER) (a) Allentown (PFS) Wisconsin (DJM) 51.5 i 6.0 43.0 i 8.0 0.566 (51) 56.7 i 1.6 2.42 48.6 t 2.3 55.5 i 1.4 58.6 i 0.6 7.73 47.4 t 0.5 40.15b 55.26 i 9 10.89 50.0 + 0.8 56.5 i 1 0 9.17 47.8 i 2 1.28 50.3 i 1.4 interfered interfered 17.44 53.1 i 4.1 53.36 I 0.88 59.6 i 0.5 31.86 54.7 i: 2.4 55.59 t 0.93 74.4 i 1.3 30.28 54.7 t 2.4 55.44b 35.86 31.3b 100.74 45.3 i 1.2 13.45
a Note: Samples of all three runs shown were irradiated in graphite reflector positions. rection factors from Table I1 have been applied in the calculation.
( d ) average of several techniques, Kodak ( 2 , 3) 56 i 9 53 i 4 57 -L ? 46 t 5 38 i 5 52+ 7 48 i 8 53 i 4 53 i 4 55 i 7 49 i ? 51 i ?
Single determination.
Cor-
Table IV. Intercomparison of Concentrations Obtained on TEG 50B by Parametric Counting ( a and b ) , and by Relative Measurements Using NBS Standard Reference Materials (c), t o the Values Quoted by Kodak ( d ) (Average of Several Techniques)u
( a ) Allentown (PFS) 51Cr 65Zn 24 Na j6Mn "'Hg 2u3Hg
40.9
ratio of resonance integral/thermal cross section
( c ) comparison to NBS SRMs ( J E R )
( d ) average of several techniques, Kodak ( 2 , 3)
51.3 + 8.5
0.566 49.9 t 3.0 49 i 4 2.42 49.9 i 2.2 50: 5 290.8 f 12.3 327 i 2 0.685 378 i 1 3 328 i 45 43.9 i 1 1.28 49.9 i 1.5 50 i 3 60.5 i 9 0.386 56 i 5 55.4 i 4.9 43.0 i 1.6 42 i 1 5 0.816 56 i 5 6oco 47.5 f 0.9 52.4 i 2.7 2.016 47.8 i 4.9 50i 3 %SeC 45.5 f 4.5 52.3 7.4 9.17 35 i 9 76Asb,C 97.3 i 0.1 99.1 i 0.5 17.44 99.6 i 4.3 95 i 1 0 I22SbC 55.2 i. 1.4 63.1 + 0.5 31.86 53.4 f 5.2 50 i 6 124SbC 54.6 i 1.0 61.5 f 3.0 30.28 53.4 i 5.2 50i 6 28A1 4 9 k 11 0.662 54 + 7 Note: Samples of all three runs shown were irradiated in graphite reflector positions. All 7 6 Alines ~ interfered, the 657-KeV line by the 657.7 line of 'lUrnAg,an unintended contaminant i TEG50B. Correction factors from Table I1 have been applied in the calculation. i
0.5
( b ) Univ. of Wisconsin (DJM)
Table V. Intercomparison of Concentrations Obtained on TEG 5 0 C by Parametric Counting (a and b ) , and by Relative Measurements Using NBS Standard Reference Materials (c), to the Values Quoted by Kodak ( d ) (Average of Several Techniques)a ( d ) average of ratio of resonance several ( a ) Allentown ( b ) Univ. of integral/thermal ( c ) comparison t o techniques (PFS) Wisconsin (DJM) cross section NBS SRMs ( J E R ) Kodak ( 2 , 3) "Cr 40.0 i 2.5 47? 5 0.566 49.1 i 2.1 49 i 4 '12K 49.0b 57 i 1 2 0.931 48.2 + 1.4 64 f 1 7 8 2 i 20 1.046 50+? 86Rb 47.1 + 0.5 54.8 i 0.8 53.7 f 3 10.89 6 0 t 10 liUrnAgC 52.8 t 1.0 35.86 6 0 i 10 99M~(T~)C 72GaC 52.0 + 0.8 49.0 t 0.5 7.73 46.5 f 1.0 51 i ? 24Na 89.5 t 0.1 100 i 1 0.685 116 i 1 120 * 34 1.28 45.7 i 1.2 481 3 56Mn 40i 2 52v 64.5 i 0.7 0.594 491 3 6'cu 1.356 56.8 t 2.3 491 4 1 1 6 r n 1 ~ 50.8 -t 1.1 13.45 49 t ? a Note: Samples of all three runs shown were irradiated in grlphite reflector positions. rection factors from Table I1 have been applied in the calculation.
parametric counting measurements a t BTL Allentown and a t the University of Wisconsin. T h e standard deviation has its usual analytical meaning a t BTL Murray Hill. At the two other locations, it refers only to the counting statistics of a single sample measured at three consecutive decay times. The systematic errors inherent in the nuclear constants are riot
Single determination.
Cor-
included since these errors are largely unknown. If they were included, the standard deviation for the parametric counting would definitely become larger than shown in t h e tables. There is, however, agreement between parametric counting results a t Allentown and a t the University of Wisconsin, and also between results for irradiations a t Atlanta and a t the
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
University of Missouri. This indicates that the standard deviations obtained for irradiation of several batches of the same sample and evaluated by parametric counting would be very similar to the ones shown here in the tables. As already pointed out, the correction factors listed in Table I1 have been used to correct the concentrations of "'Sb and lZ4Sb.76As, IlomAg,99Mo(Tc),':'Ga, and %e. Iione of the other data have been corrected in any way. Discrepancies between values obtained for sodium should not be taken too seriously. in view of the ubiquitous nature of the element and that it is not an intentionally added impurity in the T E G samples but contained in the gelatin. We also had some indication that a few other elements were not completely uniformly distributed, but those deviations were minor. C o m p a r a t i v e F l u x Determinations. An irradiation run was carried out with the pneumatic tube facility at the University of Missouri light water reactor, in the same irradiation position which had been used for the TEG samples (the so-called "front-row" position). A Fe-Ru monitor couple. a bare -41 (Au = 0.127%) flux wire. 15 mg of NBS SRM 127B SRM 953 were eo-irradiated. and 51.60 mg of Al(Co = 0.1167~) immediately followed by irradiation of another A1 (Au = 0.127%) flux wire under 20-mil cadmium shielding (length of the capsule '2.0cm: inner diameter 2 m m ) . T h e concentration of gold in the .41(Au) flux wire was determined both by atomic absorption spectrometry and by relative neutron activation measurements vs. a dilute gold solution (volume 2 cm.') of known concentration and using the most thermal flux available a t the Georgia Tech heavy water reactor; both measurements agreed. giving the gold concentration = 0.127%. SRM 127b is a lead-tin based alloy containing small amounts of antimony, and very small amounts of arsenic. Unfortunately. it carries only a provisional certification but still appeared as the most suitable SRM for the particular purpose. T h e thermal and epithermal fluxes were determined both from the Fe-Ru monitors and from the cadmium-ratio and -difference measurements on the Al(Au) flux wires. These flux data were then used to calculate the concentration of Sb and As in S R M 127B, and of Co in SR1.I 953. The results are shown in Table TI. I t is immediately obvious that the two sets of flux data are not in agreement with each other. Some of this discrepancy might he due to the difficulties noted before-some absorption of epithermal neutrons, possible leakage of thermal neutrons into the cadmium shield-but some more fundamental cause appears more likely. T h e uncorrected antimony value is again found too low by approximatel). a factor of 2 from the Fe-Ru data. and approximately 3 0 7 ~too low from the gold data. For Co the two sets of data are in better agreement. though the Fe-Ru data give the better fit. T h e uncorrected arsenic value, too, is too low by more than a factor of 2. and the As value from the gold data is not much higher. We suspect that "100 ppm" is really only a nominal concentration, and that the true value is close to 73 ppm, the corrected value from the Fe-Ru flux data.
193
Table VI. Comparison of Fluxes and Concentrations Obtained by the Fe-Ru Method with Those Obtained by Cadmium Ratio and Cadmium Difference Measurements o n Golda Thermal and epithermal flux from the Fe-Ru data
o
~ 6.215 ~X
from Au-AuiCd data Concentration of cobalt in NBS SRM 953 Al(Co) flux wire from Fe-Ru data from Au-AutCd data Concentration of antimony in NBS SRM 127 B from Fe-Ru data (both "'Sb and "'Sb) from Au-AuiCd data from SRM 127B spec. Concentration of arsenic in NBS SRM 127 B: from Fe-Ru data from Au-AuiCd data from SRM 127 B spc.
othemal= cm2s - ' ;
l O I 3 n~
oepl= 7.356
cm-l s-'; 0.004%
7.733 x 1 0 ' ' n
oepl= 2.742%
Co = 97.4 value Co = 88.4 value
i
0.8% of NBS
i
0.8% of NBS
Sb = 2095 ppm; = 4029 ppm if empirical correction factor is used. Sb :3038 ppm; Sb = 4300 ppm.
As = 44.4 ppm; = 73.3 ppm if empirical correction factor is used. As = 55.6 ppm; As = "100" z ? ppm.
The University of Missouri light water reactor was used for this run. Irradiation position: front row of the graphite reflector.
OTHER INVESTIGATIONS
production of Io3Ru and 99Moare known, we only needed to convert. on a proportional basis, the number of l ' J B Ratoms ~ detected (at end of irradiation) into the number of %Mo atoms present a t end of irradiation but due to fission. Subtraction of this number of 99Mofrom the total number of 99;\.10atoms present then gives the number of %lo atoms due to activation by the (n.i.1 reaction on 98Mo. and hence the molybdenum concentration in the sample. It should be noted that this approach would still be valid in the presence of ruthenium in the original sample. In this case one would use only the fraction of ".'Ru in excess over the amount present in natural ruthenium as determined by the concentration calculated from "Ru and lniRu. T h e method could also be applied to the determination of certain rare earths which are likewise formed with high yield as fission fragments of '35U. As a check on our calculation, we calculated the amount of "W which in the given reactor spectrum (as determined with the Fe-Ru monitor) would have caused the observed number of '''.''Ru atoms by fission. The amount of *38Upresent was then obtained from the natural abundances of 235Uand ""C and compared to the 239Npactivity detected in the TEG 50A sample. T h e two calculations checked within 26'3, the '('.'Ru branch giving the higher value. This is probably as good an agreement as can be expected for a calculation of this type, given also the uncertainty in the abundance of 235Uin the actual sample.
D e t e r m i n a t i o n of M o l y b d e n u m i n t h e P r e s e n c e of U r a n i u m . Because of the fission reaction on 2:'sU (which has a very large cross section, 280 b) producing fission fragments decaying to Yghloand Y g m Tthe ~ , determination of Mo by neutron activation in the presence of uranium has been a problem in the past. Parametric counting provides a simple way to overcome this difficulty. In our particular example, analysis of TEG 50 A, we observed that only ""'Ru was present after irradiation; the "Ru and ""Ru isotopes were missing or below our limit o f detection. Since the fission yields for
DISCUSSION Tables 111-V compare the concentrations in the three T E G materials as obtained by parametric counting with those obtained by relative measurements using SRMs, and with those quoted by Kodak (2. 3 ) . It must, of course, be remembered that the nuclear constants of the various isotopes have not all been established with the same degree of accuracy. Some of the discrepancies in Tables 111-V undoubtedly arise from the use of incorrect nuclear constants. T h e large discrepancy between the concentrations calculated for mercury
~
~
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Table VII. Nuclear Constants Used in Calculating the TEG Impurity Concentrations as Given in Columns (a) and (b) of Tables 11-IV halflife & absolute thermal cross section, resonance integral, isotope & y-ray intensity of y-ray, % % abundance barns barns energy 19'Au 64.7 hb 100.0 98.8E + 00 1.540E + 03 95.50 411.8 keV 0.84 675.9 j9Fe 1080 h 0.31 1.14E + 00 1.20E + 00 1099.2 56.1 1291.6 43.51 97R~ 5.51 2.08E - 0 1 4.80E + 00 69.0 h 215.7 85.30 325.1 10.76 '03Ru 955.2 h 31.61 1.29E + 00 4.30E + 00 497.1 86.80 5.47 266.3 m 18.58 4.47E - O l a 5.8E + OOa 724.2 49.00 469.4 17.50 676.3 16.66 316.6 12.25 5'Cr 667 h 4.35 1.59E + 01 9.00E + 00 320.1 9.90 "Zn 245.0 d 48.89 7.15E - 0 1 1.73E + 00 1115.5 50.60 "Ga 14.2 h 40.00 4.40E + 00 3.40E + 01 834.1 95.80 2201.6 25.60 629.9 26.20 15.00 2507.6 8.65 2490.9 5.50 600.9 10.30 894.2 66.2 h-6.0 h 99Mo(Tc ) 23.78 1.45~5- 01 5.20E + 00 88.00 140.5 12.20 739.5 4.30 777.9 239U 23.54 m 99.27 2.733 + 00 2.75E t 02 '3yNp 56.52 h 228.1 keV 10.70 1 1 6 r n 1 ~ 54.0 m 1.57E + 02 95.72 2.11E + 03 24.20 416.9 818.7 11.30 1293.5 82.50 2112.2 15.30 24Na 15.0 h 100 5.40E - 0 1 3.70E - 01 1368.6 100 100 2754.0 I9'Hg 65.0 h 0.15 3.19E + 03 1.23E + 0 3 77.4 19.10 191.4 0.50 1130.0 h '"'Hg 29.80 4.90E + 00 4.00E + 00 279.2 81.00 6OCO 1928.3 d 100 3.723 + 01 7.50E + 0 1 1173.2 99.88 1332.5 100.00 "A1 138.6 s 2.34E - 0 1 100 1.55E - 0 1 1779.0 100 42 K 12.4 h 1.45E + 00 6.70 1.35E + 00 1524.7 18.30 0.31 312.8 6010.0 h 48.18 4.50E + 00 4.90E + 01 llurnAg 94.30 657.7 744.3 4.88 884.7 73.40 937.5 35.20 1384.3 25.20 75Se 2900.0 h 0.90 5.18E + 0 1 4.75E + 02 136.0 57.20 264.7 60.85 279.5 24.95 121.1 17.65 56Mn 154.8 m 100 1.33E + 01 1.7E 4 01 846.8 98.80 2113.0 15.25 1810.7 28.40 l6As 26.3 h 100 4.30E + 00 7.50E + 0 1 559.1 42.90 657.0 5.80
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Table VII. (Continued) isotope & y r a y energy
halflife & absolute intensity of 7-ray, %
% abundance
1228.5 2096.2 12’Sb 564.1 692.8 1140.5 124Sb 602.7 1690.9 722.8 645.8 86Rb 1076.8 64cu 1345.8
thermal cross section, barns
resonance integral, barns
1.10 0.56 57.25 6.56E t 00 65.76 h 71.20 3.98 0.86 4.263 t 00 42.75 1450.0 h 97.90 48.10 10.74 7.38 449.0h 72.17 4.60E - 01 8.76 12.8h 69.09 4.50E + 00 0.48 W U 306.0 s 30.91 2.16E + 00 1039.2 9.00 833.5 0.34 52v 225.7 s 99.75 4.883 + 00 1434.0 100 46sc 83.7 d 100.00 2.60E + 01 889.3 100.00 1120.5 100.00 These values for the thermal and resonance cross sections make the Ru concentration calculated with those calculated from 9 7 Rand ~ lo3Rufor any reactor irradiation. h = hour, m = minute, d = from the activities of lg7Hgand *03Hgalmost certainly belongs in this class in view of the small resonance integral to thermal cross section ratio for both isotopes, since even a grossly inaccurate F ratio would not greatly affect the concentrations calculated. Other discrepancies are more baffling, like those for the two antimony isotopes lz2Sband 124Sb,for i6As, llOmAg, and 239U. All of these isotopes have large resonance integrallthermal cross section ratios, and for some of them the nuclear constants are known particularly well. Thus the most likely source of the errors ought to be an incorrect F ratio used in the calculation. This simple assumption, however, does not explain all of t h e facts, t o wit: (1) T h e relative standard deviation between the concentrations of ruthenium calculated from the activities of its three isotopes is reproducibly less than 1%, using the nuclear constants as listed in Table VII, and is independent of the actual composition of the neutron spectrum in which the ruthenium was irradiated (e.g., light vs. heavy water moderated reactors); in addition, the calculation yields the correct weight of t h e ruthenium, indicating t h a t the determination of the thermal flux via t h e iron monitor is also acceptable. These observations simply must mean that the ruthenium monitor correctly measures t h e F ratio as seen by the resonances of its three isotopes. (2) T h e concentrations of isotopes with low resonance integrallthermal cross section ratios are calculated correctly, proving again that the magnitude of the thermal flux assumed for t h e calculations is correct. (3) T h e relative error in t h e concentration calculation is larger for large ratios of resonance integral to thermal cross section; however, this relative error appears to be independent of t h e ratio of thermal to resonance flux present during irradiation, i.e., of the ratio of thermal to resonance activation. The ruthenium isotopes sample the resonance flux at much higher energies t h a n does gold with its low lying resonance a t 4.9 eV. I t seems probable that this fact somehow constitutes the reason for t h e observed discrepancies, perhaps because of deviations of the resonance flux from its theoretical 1 / E dependence. Such deviations from the 1/E shape have been noted by several authors (7-12), and a n epithermal flux distribution proportional t o l/E1+”’has been proposed. Bereznai et al. (12)suggest that m is a negative quantity inside
2.09E + 02
1.29E t 02
4.81E + 00 6.10E+ 00 2.30E + 00 2.90E t 00 1.20E + 01 from Io5Rucoincide day, s = second.
the core, and changes to a positive quantity in the (graphite) reflector. Ryves (9) states: “Well moderated systems containing hydrogen or deuterium have a nearly zero value of “m”, but for graphite moderated assemblies “m” may vary from -0.1 to +0.1 . . ., which will have a pronounced effect on the relative resonance integral measured in these geometries”. All of our measurements were carried out in the reflectors of the various reactors used, thus some deviation from a 1/E shape of the resonance flux was to be expected. Positive values of m would produce effects of the observed magnitude using the formalism suggested by Bereznai and MacMahon in a recent report (13);it should be noted, however, that negative values of m were observed in the reflector in t h a t work, contrary to the predictions in Reference 12. T h u s there does not seem to exist a t this time an unambiguous treatment to take these deviations from a 1 / E shape into account. From the practical point of view, however, application of empirical corrections factors appears to be adequate, a t least for irradiation positions a t core face or in a graphite reflector. Our results indicate t h a t these correction factors were invariant for t h e three different reactors used. CONCLUSIONS Parametric counting in conjunction with flux determinations by means of the Fe-Ru monitor couple gives concentration values for isotopes with not too large resonance integral/thermal cross section ratios, R , which are in fair agreement with those obtained by relative measurements using SRMs, and which are certainly adequate for survey work. At large values of R , systematic deviations of the calculated concentrations towards too low values are observed. “Large” refers to R values of about 10 or larger. T h e percentage by which the concentration is found too low depends on the individual isotope and appears t o increase with increasing values of R , though the largest deviation was observed for antimony with R = 31, not for uranium with R = 100. Our experience with isotopes with large R values has been that for a given isotope the concentration is always found too low by the same percentage, independent of the composition of the reactor flux in which the sample was irradiated. We do not understand the reason for this behavior, but we can arrive a t t h e approximately correct concentration values by multi-
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plication with an empirical factor. For antimony (both isotopes) this factor is 1.923, for gold it is 1.119. Values for 76As,'"Ag, "Mo(Tc), 72Ga,and 75Seare 1.652, 1.535, 1.802, 1.300, and 1.413 respectively. We carried out a flux determination run simultaneously utilizing Fe-Ru couples and cadmium measurements on gold. T h e resonance flux is definitely found lower from the gold data. Concentration calculations from the Fe-Ru flux data (and using empirical correction factors for isotopes with large R ratios) were better than on the basis of cadmium-ratio and -difference measurements on gold; however, our experience with cadmium measurements is far too limited for any sweeping statements. We do not claim that determination of the actual epithermal flux distribution via the high-lying resonances of the three ruthenium isotopes yields necessarily a more accurate value than can be obtained via the low-lying resonances of lS7Au. We have, however, developed an internally consistent approach which yields the correct concentration values for all elements. Parametric counting provides an easy way of determining molybdenum or certain rare earths in the presence of uranium, utilizing the known relative fission yields for the isotopes in question. ACKNOWLEDGMENT We are indebted t o Norman Holden and others a t Brookhaven National Laboratory and to Ronald Fleming of the National Bureau of Standards for very helpful discussions.
Thanks are due to Richard Cashwell of the Nuclear Engineering Department a t the University of Wisconsin for performing the irradiations carried out there, and for permission to use his measuring equipment. Appreciation is expressed to L.D. Blitzer, B. E. Prescott, and D. L. Malm for quantitative analysis of the Al(Au) and the iron foil monitors, and to A. M. Mujsce for the thermogravimetric analysis of the gelatin samples. LITERATURE CITED P. F. Schmidt and D. J. McMillan, Anal. Chem., 48, 1962 (1976). D. H. Anderson, J. J. Murphy, and W. W. White, Anal. Chem.. 40, 116 (1 976). D. H. Anderson, J. J. Murphy, and W. W . White, Eastman Org. Chem. Bull., 49, l ( 1 9 7 7 ) . P. F. Schmidt and J. E. Riley, Jr., "Determination of the Natural Abundance of Iron-58 by Neutron Activation Analysis", Anal. Chem.,see paper in the Aids for Analytical Chemists section. P. F. Zweifel, Nucleonics, 18, ( l l ) , 174 (1960). R. Kirkland, Georgia Institute of Technology Research Reactor, private communication. K . W. Geiger and L. van der Zwan, Mefrologia, 2, 1 (1966). T. B. Ryves and E. B. Paul, J . Nucl. Energy, 22, 759 (1968). T. 8. Ryves, Metrologia, 5 , 110 (1969). P. Schumann and 0. Albert, Kernenergie, 8, 88 (1965). J. W . Connolly, A. Rose, and T. Wall, AAECITM 191 (1963). T. Bereznai, D. Baizs, and G. Keomley. J . Radioanal. Chem., 36,509 (1977). T. Bereznai and T. D. MacMahon, University of London Reactor Centre, Research Report, August 1977.
RECEIVED for review May 17, 1978. Accepted November 3, 1978.
Comparison of the Determination of Cobalamins in Ocean Waters by Radioisotope Dilution and Bioassay Techniques G. M. Sharma," Henry R. DuBois, Albert T. Pastore, and Stephen F. Bruno New York Ocean Science Laboratory, Montauk, New York
11954
Application of two radioisotope dilution techniques to the direct determination of cobalamins in 1 mL of seawater is described. These techniques give results which are approximately 4-10 times higher than the results obtained by the standard microbiological assays. The disparity in results obtained by the biological and isotopic methods is explained by suggesting that the later technique measures both biologically active and inactive cobalamins indiscriminately. The combined sensitivity range of the two isotopic methods described in this paper is 0.5-400 pg B,JmL. The accuracy ( X , k)of individual values at the 1.0 pg/mL level was found to lie in the range of 0.1-0.2 P9.
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T h e concentration of cobalamins in ocean waters is usually measured by microbiological techniques ( I ) . Although these techniques are highly sensitive (range: 0.05-3 ng B,,/L) they are technically tedious and time consuming. Furthermore, the precision of the bioassays depends largely upon the presence or absence of inhibitors in the samples being analyzed. Because of these drawbacks of the biological methods, there is need for a technique for the determination of cobalamins in seawater which should not only be rapid and highly sensitive but must also be immune to the presence of 0003-2700/79/0351-0196$01 OO/O
inhibitors in the medium. Several radioisotope dilution techniques developed for the determination of vitamin B12 in plasma have been shown to be endowed with these characteristics (2, 3). Recently one of these techniques has been used for the determination of benzyl alcohol extractable cobalamins from ocean waters ( 4 ) . In this paper, we report our experience and results obtained in applying two radioisotope dilution techniques to the direct determination of cobalamins in a small volume (1.0 mL) of ocean waters. The two isotopic methods tested for the determination of cobalamins in ocean waters are called competitive binding and sequential saturation analysis ( 5 , 6). The B,,-specific binder used in the experiments was the porcine intrinsic factor (IF). Comparison of these IF-based radioassays with a standard biological assay ( I ) revealed that the former techniques give substantially higher values for the concentration of cobalamins in ocean waters. One explanation of unexpectedly higher values obtained by isotopic methods would be that these techniques measure the sum of the concentration of all cobalamin and cobalamin-like molecules. The cobalamin-like molecules may have been produced by environmental degradation of various naturally produced BI2 molecules. EXPERIMENTAL Apparatus. Radioactivity was measured with a Beckman Gamma 8000 Counting System. This instrument has a factory c 1979 American Chemical Society
