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
SE 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
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
821
all other impurities) in metal samples with a limit of detection better than 0.5 p.11.m. I n addition to smaller sample consumption, as compared with other techniques, this mass spectra-
removal by cryosorption pumping (6) ference in the values without argon presented the possibility that the inpurging. Thus, residual gases did not contribute to a discernible background strument blank for C, 0 , and N could when cryosorption pumping was used. be reduced by this method- For an Since most blank contamination probiron containing 2' P'P'm. C J 30 p.p.m. 0, and 10 p.p.m. N, mass ably exists as adsorbed gases on surfaces graphic method provides average bulk data with and without cryosurrounding the sparking sample, bakconcentrations or localized concentrasorption pumping were ~ 1 1 ing these surfaces while employing tions for distribution studies. calculations were referenced to N13S cryosorption pumping was tried. Again, standard iron sample #465 as exno reduction of C, 0, or X intensity EXPERIMENTAL plained in the following section. Withwas observed, leading to the conclusion Apparatus The mass spectroout cryosorption pumping, C, 0, and that cryosorption pumping had brought graphic facilities and parameters have N values were 55 f 14 p.p.m., 57 f 23 the blank down to a constant low level been previously described (6). With p.p.m., and 17 f 5 p.p.m., respectively. suitable for analysis of low concentrathe exception of magnetic field strength, Under identical sparking conditions, tion samples. An estimate of this the same parameters were used for all but with cryosorption pumping, C, 0, blank level will be presented after dismatrices. Cnless otherwise noted, all and N concentrations were reduced to cussion of analysis results. values are expressed in p.p.m. by weight 25 f 7 p.p.m., 30 f 7 p.p.m., and M a s s Spectrographic Data Reducto facilitate comparison with other 11 i 2 pap.m. T o find out more tion. Calculations of impurity conanalytical techniques. Experimental about the nature and magnitude of centrations from mass spectrographic conditions for inert gas fusion and comthe remaining blank, a dilution of data were referenced to a standard of bustion-chromatographic analyses are residual gases in the spark chamber each matrix type, Fe, Cu, and Ag. described in Tables I and 11,respectively. was attempted. The chamber was For each sample an esposure ratio Preliminary Blank Experiments evacuated to loW6 torr and then brought E,/Ei, where m and i refer to matrix Before attempting to analyze metals up to atmospheric pressure with purified and impurity, respectively, was defor low concentrations of C, 0, or N argon. This procedure was repeated termined from probability os. exposure by spark source mass spectrometry, three times, and finally the chamber curves as reported by McCrea (10). i t was necessary to reduce the instruwas evacuated to 2 X lo-' torr with Probability, in this contest, is a cumulament blank for these elements to a cryosorption pumping following the tive normal distribution function which constant value lower than the level expresses the percentage of light abestablished procedure for analysis. of t h e purest samples t o be investiValues for C, 0, and N of 26 f 4 p.p.m., sorbed during the densitometric process 25 i 8 p.p.m., and 12 i 3 p.p.m., regated. by the relation P = (OD,/OD,,, Prior observations of hydrocarbon 100, where O D , represents optical spectively, showed no significant difdensity a t peak maximum for mass m, and OD,,, >, , represents the saturation value of optical density for the same mass m. Since saturation absorption Table I. Experimental Conditions for Determination of Oxygen in Fe, Cu, Ag, varies with mass and energy of the inand W by Inert Gas Fusion cident ions, optical density a t saturation Analysis facility was determined for each mass under Leco 537-100 Furnace consideration. Plotting matrix line Leco gas chromatograph 589-400 Analyzer probability values on a probability Leco carbon crucible 534352; Leco crucible thimble Crucible assembly axis vs. log monitor exposure produced 537-236; Leco carbon black packing 501-92 a straight line from which the exposure Crucible conditioned with 1-2 grams of similar For F.e, Cu, Ag matrix material before adding sample ratio E J E i could be obtained. This 1-2 grams low-oxygen Pt added to crucible; Pt For W ratio was then converted directly to added with each sample to maintain flux-samde concentration by reference to the ratio > 1 sample taken as standard. Purified helium a t flow rate of 1 liter/minute Carrier gas Due to the large number of tungsten Calibration KHP in Sn capsules samples being processed a t the time of Extraction temperature 2400" C. this work, a computer program by Extraction-collection time 4 minutes Kennicott (9) was used to obtain similar Sample pre aration fractional values which could be con0.05 to 2.5-gram chunks abraded to bright surface For Fe, C?u, Ag with degreased file, rinsed in acetone, and air dried verted to concentration by comparison 0.1 to 1.0 gram powder (
