sorption at 2138 A. Curve 2 shows atomic absorption only, assuming that the deuterium background corrector has subtracted all background absorption. Aqueous zinc standards gave identical readings with and without background correction. The response was linear in both cases to 0.5 pg/g Zn. A 0.3 pg/g aqueous zinc standard gave a response of 98 a t 5 x scale expansion. Table I1 gives the apparent zinc contents of solutions and the apparent zinc content of the zinc free iron samples. Background absorption is due to light scattering and/or molecular absorption near the resonance line. I t is difficult to distinguish the relative importance of these effects and no attempt was made to do so. The case for atomic absorption of iron on the zinc resonance line is strengthened by the fact iron has a n absorption line a t 2138.59 A ( 1 1 ) which is only 0.03 A away from the zinc line. Therefore, it is possible that there is overlap of the wings of the zinc emission line being emitted by the hollow-cathode tube and the iron absorption line within the flame. Fassel et al. ( 4 ) have summarized these effects. They state that the wavelength interval covered by total hollow-cathode emission and flame absorption line pairs may be in excess of 0.1 A. Frank e t al. (12) have demonstrated that the iron hollowcathode lamp has multielement utility. Indeed, they were able to determine Re with the iron line a t 3459.92 A which is 0.55 A from the Re resonance line a t 3460.47 A. Several points deserve emphasis. Two types of nonzinc absorption are observed a t 2138 A: a molecular and/or light scattering component and a spectral line interference. When the molecular and/or light scattering absorption is corrected for by the deuterium background corrector, there remains a true spectral interference due to the Fe line mentioned earlier. The method of standard additions cannot be used to correct for either of the interferences mentioned (12). Presently, there is no direct, simple way of correcting for this latter interference so long as iron remains in the sample solution.
(11) A. N. Zaidel', V. K. Prokof'ev, S. M . Raiskii, V . A. Slavnyi, and E. Ya. Shreider, "Tables of Spectral Lines," Plenum Press, New York, N. Y.. 1970, p 4 8 3 . (12) C. W . Frank, W . G. Schrenk, and C. E. Meloan, Anal. Chem., 38, 1005 (1966).
Figure 1. Absorption of iron solutions using zinc hollow-cathode tube without background correction (curve 1 ) and with background correction (curve 2) Scale expansion 5X
CONCLUSIONS The determination of low levels of zinc in ferrous materials may lead to erroneously high results unless steps are taken to remove iron from the samples prior to analysis. Solutions which contain between 0.2 to 2.0% iron will be erroneously high by 16 and 13 pg/g Zn, respectively, as shown by Table 11. It is suggested that existing analyses of zinc on ferrous materials should be approached with caution. ACKNOWLEDGMENT We thank M. L. Parsons, Department of Chemistry, Arizona State University, for helpful suggestions. Received for review November 24, 1972. Accepted January 26, 1973. This research was sup-ported by XASA NGL 03001-001 and NSF GA 32297X. One of us (W. R. K.) held a NASA Traineeship during the period this research was conducted.
Determination of Trace Elements in Coal and Coal Ash by Spark Source Mass Spectrometry Richard J. Guidoboni Ledgemont Laboratory, Kennecott Copper Corporation, Lexington, Mass. 021 73
The determination of trace inorganic elements' in coal is one of the most difficult problems facing the analytical chemist. In many cases, there are no methods or standard materials available for these determinations a t the part per million and part per billion levels. Because of the enormous tonnages of coal burned yearly, many elements even a t trace levels assume great importance because of their possible effect on the environment. The spark source mass spectrometric (SSMS) technique is one of the most sensitive instrumental methods for determining inorganic impurities in a variety of materials
( 1 ) . Since the advent of electrical detection, this method has also become much more rapid and reliable ( 2 ) . There has been one previous case reported (3) of trace elements in coal by SSMS. However, these data were based on visual estimates and can only be considered as semiquantitative. This is the first attempt to provide quantitative in(1) A. J. Ahearn, Ed., "Trace Analysis by Mass Spectrometry," Academic Press, New York, N.Y., 1972. (2) C. A. Evans, Jr.. R. J . Guidoboni, and F. D. Leipziger, Appi. Spectrosc., 24, 85 (1970). (3) T. Kessler, A. G. Sharkey, Jr., and R. A. Friedel, Bur. Mines Rep. TRP 42, Pittsburgh, Pa., Sept. 1971. A N A L Y T I C A L C H E M I S T R Y , V O L . 45, N O . 7 , J U N E 1973
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were obtained by grinding and splitting. The samples thus obtained ( < l o 0 mesh) were then weighed into porcelain crucibles and placed in a cold muffle furnace and the temperature was elevated to 300 "C for 0.5 hr. The temperature was then raised to 550 "C for 1 hr and then to 850 "C for an additional hour. The crucibles were then removed from the furnace and the ash dry mixed with the aid of a glass rod. The crucibles were again placed in the furnace a t 850 "C until ashing was complete (usually one additional hour). Separate aliquots of the standards and samples were taken for the AAS and SSMS analysis. The AAS samples were dissolved in the appropriate acids and diluted to volume for analysis. The SSMS samples were reground in a boron carbide mortar and pestle and diluted with two parts of high purity graphite. The samples with graphite were placed in polystyrene vials with
Table I. Operating Parameters
Spark variac Pulse repetition rate, pps Pulse length, psec Source slit Multiplier slit Monitor exposure Multiplier and amplifier gains
35% 100 100 0.002in. 0.002in. 0.3nC'
Variable according to sample elements and concentrations
Electrodes vibrated
Table II. Comparison of Dry Ashing and Wet Ashing Techniques for the Analysis of Coal by AAS Elecu Mn Ni Zn Cda P ba V ASa ment SamD r y Wet Dry Wet Dry Dry Dry ple Dry Wet Dry Wet Wet Dry Wet Wet Wet 1 17 16 67 64 19 15 121 110 2.8 55 41 0.33 0.55 2.8 13 15 2 12 15 179 169 4 6 7 9 0.6 0.6 a 1 6 I O io 0.24 0.40 3 50 50 122 128 a4 85 1420 1450 13.1 12.8 io5 1 1 1 43 43 0.87 1.15 4 13 14 13 12 12 15 10 18 22 0.26 0.50 23 0.6 0.5 13 18 5 9 9 a 7 717 17 0.22 0.40 19
