NaI(T1) scintillator. The detector resolution was 3.3 k.e.v. (FWHbI) and 4.8 k.e.v. (FWHXI) a t 122 and 1333 k.e.v., respectively. The new drifting technique should make available Ge(Li) detectors of excellent resolution with several times the volume of this detector. ACKNOWLEDGMENT
The authors acknowledge the kind assistance of R. E. LIcCracken and the crew of the Livermore LPTR nuclear reactor for help in obtaining irradiations. LITERATURE CITED
(1) Ewan, G. T., Tavendale, A. J., Can. J . Phys. 42, 2286 (1964).
( 2 ) Girardi, F., Guzzi, G., Pauly, J., Radiochim. Acta 4 (Z), 105 (1965). (3) Goulding, F. S., Lawrence Radiation Laboratory Rept. UCRL-11302 (1964). ( 4 ) Gouldinp, F. S.. Hansen. W. L..
Lawrence-Radiation Laboratory Rept: UCRL-11261 (1964). 15) Gouldinrr, F. S.,Lsndis. 13.. “Proceedings of Conference on Instriimentation
Techniques in Nuclear Pulse Analvsis. Alonterky, April 1963,” NAS-gRC Publication 1184. ( 6 ) Hansen. W. L.. Jarrett. B. Y..Lawrence Radiation Laboratory ‘ Rept. UCRG11589 (1564).
( 7 ) Hawkings, R. C., Edwards, W. J., McLeod, E. M., “Tables of Gamma Rays from The Decav of Radionuclides,” Vol. 1 and 2, CRDC-1007, .4tomic Energy of Canada Ltd., Chalk River, Ontario, 1961. ( 8 ) Heath, R. L., “Scintillation Spectrom-
etry Gamma-Ray Spectrum Catalog,” 1’01.I and I1 (IDO-16880-1). (91 Lvon. W. S.. ed.. “Guide to Activatioi; Analysis,” ]‘an Nostrand, Princeton, Pi. J., 1964. (10) Malm, H. L., Tavendale, A. J., Fowler, I. L., Can. J . Phys. 43, 1173 f 1 !3fi.i~ ,. ~
(11) Miner, C. E., Lawrence Radiation Laboratory Rept. UCRG11946, (1965). (12) Nuclear Data Sheets, K. Way, et al.,
Printing and Publishing Office, National Academy of Sciences-National Research Council, Washington, D. C. (13) Prussin, S. G., Harris, J. A., Hollander, J. hl., ANAL. CHEM. 37, 1127 (1965).
RECEIVEDfor review January 24, 1966. Accepted hlarch 28, 1966. This work was performed iinder the auspices of the United States Atomic Energy Commission Contract No. W-7405-eng-48.
Reactor Neutron Activation Ana lysis Instrumental Sensitivities in Six Matrix Materials HERBERT P. YULE’ General Atomic Division/General Dynamics Corporafion, San Diego, Calif.
b Radioactivation of major, and sometimes- minor, sample constituents often precludes the possibility of instrumental neutron activation analysis for desired trace elements-even though their anticipated concentrations are well above known detection limits. Knowledge of sample composition may be inadequate for accurate prediction of feasibility of the analysis. One way to overcome this difficulty is to assemble data on elemental detection limits in various matrices. This paper presents experimentally determined matrix sensitivities of over 65 elements in six matrix materials commonly encountered in activation analysis. These results enable the analyst to determine immediately the feasibility of a particular analysis in one of these materials, and it seems likely that extrapolation to similar materials should be both easy and quite successful.
P
of reactor neutron activation analysis often requires a careful evaluation of how the desired results may best be obtained. Past experience with the material to be analyzed may obviate the need for a n evaluation, or knowledge of sample composition may be complete enough to make the evaluation quite simple. For nonroutine analyses, however, this knowledge is often lacking. Lukens ( 3 ) has suggested the problem be approached by determining, for a number of matrix materials commonly LANNING
818
ANALYTICAL CHEMISTRY
encountered in activation analysis studies, the minimum instrumentallydetectable amounts of those elements having good neutron activation analysis sensitivities. This paper presents experimental results of 72 elements in each of six different matrices: whole blood, urine, milk, tap water, “pure” water, and polyethylene. EXPERIMENTAL
Experiments were conducted along the lines set forth by Lukens ( 4 , 5 ) . Several samples of each matrix material were weighed and encapsulated in polyethylene vials obtained from Olympic Plastics Corp., Los Angeles, Calif. Either 2/5-dram or 2-dram vials were used, depending on sample size. (Polyethylene vials are hereafter referred to as vials.) Three samples were irradiated in the rotary specimen rack of the General Atomic TRIGA Mark I nuclear reactor (thermal-neutron flux, 1.8 X lo1* n/cm.%ec.) for 30 minutes, and counted after delays of 1 hour, 1 day, and 8 days. The activated sample to be counted after a delay of 1 hour was transferred (and weighed) into a fresh, unirradiated vial, eliminating Ar4I activity (from argon in the air in the original vial). Two samples were irradiated using the pneumatic tube to transport them in and out of the reactor core (6, 7 ) , where the thermal-neutron flux was 4.3 x lo1? n/cm.*-sec. The first sample was irradiated for 15 seconds and counted for 18 seconds, allowing 15 seconds to elapse between
irradiation end and count start. The 15-second irradiation feature described above is not described by Lukens; it is merely an extension of his calculation method to these conditions. The remaining sample was irradiated for 1 minute, allowed to decay 1 minute, and counted for 1 minute. Counting was done with systems described in references (6-8) using gain settings of 15 k.e.v./channel to emphasize gammaray energies above 500 k.e.v., and 3.75 k.e.v./channel for energies below 500 k.e.v. Thus, each of the samples was counted twice for each delay time. The samples irradiated in the pneumatic-tube were counted only once, since they were expected to decay rapidly. A second count on one of these samples could be too late to approximate the spectrum at its designated time. Counts at the other gain setting were taken with fresh samples in separate experiments. The matrix materials studied were whole blood, urine, milk, local tap water, “pure” water, and polyethylene vials. The whole blood was treated with a little sodium heparin as an anticoagulant, but was otherwise typical whole blood. The urine sample was collected during mid-day, and no attempt was made t o collect the urine a t different times to average any diurnal effect ( 2 ) . Tap water was local San Diego water just as it came from the faucet. Milk was a commercially available local brand, stated by the dairy to be ho1 Present address, Activation Analysis Research Laboratory, Texas A&M University, College Station, Texas 77843.
mogenized and have 400 USP units of vitamin D added. “Pure” water was distilled a t this laboratory, and then run through a demineralizing column. Select’ion of sample size was important-so that the sample would not be too radioactive to count in the standard geomet-y, nor contain too little activity. The first step in determining optimum sample size was to learn as much as possible about the sample composition. Rather complete data on biological samples are available ( 2 ) ,but information on other samples was largely unavailable, and had to be determined in preliminary experiments. For each element known in the sample, the photopeak yields listed in references (6) and ( 7 ) mere adjusted for the weight of a given element in 1 ml. of total sample, for irradiation time, decay time, and, for the samples irradiated in the rotary rack, the lower neutron flux (6). Summing these corrected yields over all anticipated isotopes gave the expected total photopeak counting rate. The sample size was then adjusted so that the expected total photopeak counting rate was in the range of 5000 to 10,000 c.p.m. This procedure usually meant milligram-size samples for the pneumatic-tube irradiations, and three different-sized samples irradiated in the rotary rack-a few milligrams to be transferred to a “cold” polyvial and counted 1 hour after irradiation end, 0.01 to 1 gram to be counted 24 hours afterward, and several-gram samples to be counted 8 days afterward. Discussion and calculatioa of argon, nitrogen, and oxygen are omitted from this study because most samples are contaminated with these elements. ANALYSIS
OF
THE DATA
T h e first step is t o identify the components in the gamma-ray spectra, and calculate the amounts of these elements present in the sample. This calculation compares photopeak areas in the present sample with those listed in reference (6)-accounting, of course, for any different experimental conditions (flux differences, different irradiztion periods, and counting geometries). This approach, although not as accurate as the approach utilizing comparator standards, is sufficiently accurate for the present purposes. The upper-limit calculations are done by the General Atomic IBh/i-7044 computer, following the scheme of Lukens (4). The raw data are corrected for dead-time counting losses and converted to counts per minute. Provision is also made to correct for geometry differences for samples too radioactive to count in the standard position or for samples larger than the defined 1-ml. volume (4). For those spectra a t gains of about 3.75 k.e.v. per channel, the corrected counts per channel are summed into five ranges: 0-100, 100-200, 200-300, 300-400, and 400-500 k.e.v. Similarly, those spectra taken a t a gain setting of 15 k.e.v. per channel are
Concentrations of Identified Components (in .p.p.m.) Table I. Approximate .. .
Element Na A1
c1
K
Mn
co cLl
Zn Br
Ag
Whole blood 1400 140 2400
Urine 1900 100 2600 ...
N
...
...
... ... ...
2.3 ...
...
-
... ...
Milk
=
57 4
620 -100 780 70
-
110 ...
...
...
...
... ... ... ...
1
...
0.014 pg.
For each spectrum, the computer calculates limits for each element listed in Lukens’ table, provided the raw data cover the appropriate energy range. When the computer finishes calculating upper limits from all the spectra of a
...
1.9
...
summed by 100-k.e.v. units, starting a t 300 k.e.v. Sums are taken over those channels which most nearly approximate these energy ranges; interpolation to provide sums over exactly defined energy ranges is regarded as an unnecessary refinement. An example of the upper-limit calculation is appropriate. Suppose one wiqhes to calculate an upper limit for lutetium, wing the spectrum taken on a gain setting of 3.75 k.e.v. per channel after a delay of 8 days. Lukens’ Table 4 (4)gives the following information: (1) The most sensitive means of determining lutetium a t this delay is through the 0.208-n1.e.v. gamma ray emitted by Lu-177 (half life 6.8 days). This procedure assumes that the radioactivities in the matrix do not emit gamma rays which interfere with the chosen gamma ray of 0.208 m.e.v. To overcome this problem would require that detailed information on many gamma-ray spectra of reactor induced radioactivities be included as input information t o the program. The computer could then decide on the proper gamma ray to use as a comparison basis. This refinement has not been included because the necessary data were not available in a form compatible with the needs of this program. Reactor thermal neutron products have over 400 detectable peaks, using NaI(T1) detectors. For each peak, information would be required on peak energy, intensity, boundaries, etc. (2) If the number of counts in the spectrum between 0.2 and 0.3 m.e.v. is 1000 counts, then the upper limit to the missing peak’s size is 100 counts. (3) The amount of lutetium corresponding to 100 c.p.m. is 0.28 pg. Suppose that our spectrum contains 500 counts between 0.2 and 0.3 m.e.v. The upper limit to the lutetium concentration is
O!oX 0.28 X 0.1 1000
water
...
...
1.0
Matrix Tap
“Pure” water lo b 6000 6000 2000 100 N 0.03 ,.. ... ... Na 0.009 3 100 60 300 1000 Nb 0.3 0.05 0.5 2 0.2 6 Nd 11 5 10 200 5 500 Ne 0.02 0.04 400 10,000 2000 8000 Ni 0.006 0,004 9 2 60 30 os 40 20 50 600 1000 1000 P 10,000 40 10,000 60,000 200,000 200,000 Pb 0.05 0.07 0.3 0.5 0.4 3 Pd 0.008 0.002 3 2 20 8 Pr 0.04 0.02 0.4 2 2 3 Pt 0.3 0.1 0.5 2 9 7 Rb 0,002 0.001 0.1 1 0.03 4 Re 0.03 0.2 0.09 1 0.03 4 Rh 0.09 0.05 0.2 0.8 4 2 Ru 50 30 300 3000 6000 3000 S 0.005 0.007 0,003 0.03 0.2 0.08 Sb 0.002 0,0009 0.003 0.01 0.01 0.04 sc 0.09 0.2 0.5 1 4 0.2 Se 10 16 7 200 400 500 Si 0,0008 0,0005 0.005 0.04 0.07 0.04 Sm 0.2 0.1 20 200 5 300 Sn 0.02 0.009 3 20 2 40 Sr 0.04 0.03 0,009 0.02 0.1 0.2 Ta 0.02 0.01 0.03 0.1 0.2 0.4 Tb 0.08 0.03 0.4 1 2 3 Te 2 5 5 72 2 200 Ti 30,000 20,000 40,000 100,000 300,000 600,000 T1 0.2 0.1 0.2 5 6 2 Tm 0.02 0.01 0.03 0.2 0.6 0.8 v 0,001 0.003 0.2 0.6 0.1 2 W 0.3 1 0.2 4 15 8 Xe 1 0.3 1 i_ n_ 3 6 Y 0.02 0.01 0.08 0.4 0.03 0.2 Yb 2000 80 500 100 0.3 0.4 Zn 20,000 13,000 4000 800 4 2 Zr Three dots indicate a known component. b High matrix activation makes nitrogen undetectable a t any concentration. t . .
Q
820
ANALYTICAL CHEMISTRY
ratelg between aluminum in the sample and in the polyvial. Upper limits for elements not detected in the matrix materials are given in Table 11. Results are quoted only to one significant figure, since the calculation is not exact, and results may be in error by factors of two to five (4). The results in Table I1 indicate that for the biological materials the minimum instrumentally-detectable amounts of many elements correlate with the sodium (and to a smaller extent, the chloride) content of the matrix This correlation is well known t’o those engaged in reactor neutron activation analysis work-indeed sodium seems to be ubiquitous. Comparing tap water and “pure” water, the measurable amounts of sodium and chloride (Table I) in tap water are effectively reduced, in the purified water, to quite low concentrat’ions, as indicated by the upper limits in Table 11. The silver and copper concentrations are probably also reduced. Yials show small concentrations of aluminum, sodium, and manganese. The vial upper limits are generally not as good as those for the “pure” water, since the water samples were heavier. A few hours after irradiation, the gamma-ray spect’ra of “pure” water and vials were quite similar, giving roughly the same sensitivity in micrograms, but not in parts per million. To achieve t’he best limits in a sample with a low yield of activities, transfer of the sample to a vial t’hat has not been irradiated may be required. It is of interest to examine the relationships between the instrumental sensitivities in the various matrices in Table 11, and the corresponding ideal matrix (interference-free) sensitivities, in which only the radioactivity of interest is produced by neutron irradiation. The best instrumental sensitivities are found for those matrix materials producing little or no activity, especially if the half lives of the radionuclides generated from the matrix are only an hour or less. Good instrumental sensitivities are obtained when the half life of the radionuclide sought is either much longer or much shorter than the half lives of the principal matrix activities. If the gamma ray of interest is hi her in energy than all, or most, of the gamma rays from the matrix activities, good instrumental senqitivities are obtained-if the sample is not too radioactive to be counted a t high geometry (about 307, or so). The poorest instrumental sensitivities are obtained when half lives and gammaray energies of the radionuclide sought and the matrix radionuclides are comparable. Under these circumstances, instrumental sensitivities are often several hundred or a thousand times the interference-free sensitivities.
CONCLUSIONS
The upper limits in Table I1 provide useful information on the feasibility of instrumental neutron activation analysis in six different matrix materials. Consider, for example, the element selenium, which is of considerable biological interest. According to Table 11, the instrumental limit of detection, under the conditions employed, is about 4 p.p.m. for blood, and hence the instrumental determination of selenium in subparts per million concentration range would require considerable additional effort ( I ) . One obvious way to improve the sensitivity is to employ longer irradiations to build up the amount of long-lived Se75; this approach may be prohibitively expensive, however. Table I1 supplies the answer to step two in the evaluation procedure: selenium will be difficult to determine instrumentally, in blood, in concentrations below about 4 p.p.m. Gold, on
the other hand, would be easily detectable in blood, even in the partsper-billion range. It must be emphasized that the limits in Table I1 are only approximate, and that they do not represent ultimate limits which cannot be improved. It is hoped that the data presented here will be interesting and useful to those engaged in nonroutine neutron activation analysis. Higher neutron fluxes and longer irradiations will give much improved limits for many elements. Other beneficial counting techniques will undoubtedly emerge as time passes. The approach used here may be modified to optimize results for particular matrices; in its present form it attempts to provide a general solution for any matrix.
ment of General Atomic for programming the upper-limits calculations. LITERATURE CITED (1) Fleishman, D. kl., Guinn, V. P., Trans. A m . Xucl. Soc. 7, (2), 327 (1964). ( 2 ) Long, C. A., Ed., “Biochemists’ Handbook,]’ Van Nostrand, Princeton, N. J.,
ACKNOWLEDGMENT
1961. (3) Lukens, H. R., Jr., General Atomic Division/General Dynamics Corp., San Diego, Calif., private communication, 1965. (4) Lukens, H. ,R., Jr., Rept. GA-5896, General Atomic Division/General Dynamics Corp., San Diego, Calif., 1964. ( 5 ) Lukens, H. R., Jr., Trans. A m . A k c l . Soc. 8, ( l ) , 83 (1965). (6) Yule, H. P., ANAL.CHEM.37, 129 (1965). (7)7Yule,H. P., Lukens, If. R., Jr., Guinn, 1. P., Nucl. Instr. Methods 33. 277 ( 1965j. (8),Yule, H. P., Lukens, H. R., Jr., Guinn, 1. P., Rept. GA-5978,General Atomic Division/General Dynamics Corp., San Diego, Calif., 1964.
The author thanks Lavon Todt of the Mathematics and Computing Depart-
RECEIVED for review September 13, 1965. Accepted April 8, 1966.
Determination of Trace Amounts of Carbon, Oxygen, and Nitrogen in Metals by Spark Source Mass S pect rometry W. L. HARRINGTON, R. K. SKOGERBOE, and G. H. MORRISON Department o f Chemistry, Cornell University, Ithaca, N. Y.
b The simultaneous determination of trace amounts of C, 0,and N in metals by spark source mass spectrometry has been accomplished. Reduction of the instrument blank for these elements to < O S p.p.m. b y weight and prevention of blank fluctuations are the results of cryosorption pumping in the source chamber. The micro-sampling of the rf spark is shown to provide reliable localized concentrations which can b e used for impurity distribution studies. A sample scanning technique is suggested which gives a good estimate of bulk concentrations as well as an indication of sample homogeneity.
0
of the most critical problems in the analytical chemistry of metals has been the quantitative determination of the impurities carbon, oxygen, and nitrogen. Tolerance levels for these impurities in many metallurgical materials have been steadily reduced to the point where a concentration of 1 p.p.m. or less is of current interest. This has been generated by the fact that concentrations of these impurities in the SE
range of 1-10 p.p.m. exert significant effects on the physical properties of many metals. The importance of this problem has promoted extensive research on the determination of these elements by vacuum fusion ( 3 , 5 , 8 ) , inert gas fusion ( 3 , 4,8), emission spectrometric (3, 8, I S ) , nuclear activation ( I , 2, 8 ) , isotope dilution ( 8 ) , and wet chemical (8) methods. I n general, all of these methods are subject to the same primary limitations: the magnitude and inconsistency of the operating blank, the restriction, in all cases except vacuum fusion, to one element in a particular determination, losses due to gettering, nonquantitative recovery, etc., the lack of standards at low concentrational levels, and the large amount of sample required. The optimum range of measurement for these techniques generally lies above 10 p.p.m., but determinations in the 1-10 p.p.m. range can be achieved if sufficiently large samples are available to produce a signal significantly greater than the operating blank. Although the spark source mass spectrographic method of analysis has
shown the capability to minimize most problems of this type for metallic and some nonmetallic impurities, the limited published results on the determination of carbon, oxygen, and nitrogen by this method have essentially been negative. Some results have been reported by Roboz ( I I ) and Henry, (7) but precision, accuracy, and limits of detection have suffered, presumably due to inadequate blank control. Socha and Willardson (12) have shown reduction in residual gases in the source and collector sections of a mass spectrograph by N P flushing, cryogenic pumping, and cathodic etching; however, they do not report quantitative data on the determination of C , 0, and ?i using these techniques. The success in this laboratory of reducing hydrocarbon interferences as well as the C, 0, and N blank by cryosorption pumping (not to be confused with cryogenic pumping) in the chamber of a spark source mass spectrograph (6) has allowed this method to be extended to the analysis of metals for C, 0, and K over a wide concentration range. The method reported here allows the possibility of simultaneous determination of C, 0, and N (as well as VOL. 38, NO. 7, JUNE 1966
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