Iron Spectral Interference in the Determination of Zinc by Atomic Absorption Spectrometry William R. Kelly and Carleton
B. Moore
Center for Meterorite Studies and the Department of Chemistry. Arizona State University. Tempe. Ariz. 85287
Several authors (1, 2 ) have cautioned against the determination of zinc by atomic absorption in an iron matrix because of an interfering effect. The exact nature of this alluded-to interference has not been investigated. In the early days of atomic absorption spectrometry, it was generally believed that this technique was free of spectral interferences. Recently, atomic interferences (3-5) as well as molecular (6) and light scattering interferences ( 7 ) have been shown to exist in atomic absorption. During the atomic absorption analyses of iron meteorites in our laboratory it was found that zinc determinations were erroneously high. Iron meteorites are essentially a Ni-Fe alloy ranging in Ni content from 5 to 20% Xi. Zinc determinations in a Ni matrix are free of interference effects. It was concluded that iron was producing the erroneously high results and a successful attempt was made to characterize this interference.
EXPERIMENTAL A p p a r a t u s . A Perkin-Elmer 403 atomic absorption spectrophotometer equipped with a Perkin-Elmer Deuterium Arc Background Corrector was used for all measurements. A single element zinc hollow-cathode lamp ( P - E Intensitron L a m p ) was used for all zinc determinations. Reagents. Hydrochloric acid used for dissolution a n d extraction of samples was prepared by bubbling Matheson Electronic Grade HC1 gas (purity 99.99% min) into doubly distilled water. Acid made in this manner attained a normality of 13.5 and was stored in Teflon bottles. Iron and nickel solutions were prepared from Johnson Matthey Specpure sponges. Zinc standards were prepared from the Specpure ZnO. Procedure. Solutions of varying iron concentration were prepared by dissolving weighed amounts of iron sponge in 15 ml of 13.5N HCI in Teflon beakers. The samples were heated t o incipient dryness and diluted t o a 50-ml volume with doubly distilled water and stored in polypropylene bottles. Nickel solutions were prepared in a n analogous manner. Since the purpose of this study was to demonstrate t h a t iron solutions, completely free of zinc, absorbed a t t h e zinc resonance line (2138.56 A), it was necessary to accurately determine the zinc content of the Specpure iron used for study. If the zinc concentration of t h e iron solutions used was below t h e detection limit of the instrument, the iron solutions could be aspirated directly without purification. Any interference could then be attributed to the iron only. Iron was quantitatively extracted from 50-ml solutions containing 2-g quantities of iron using isopropyl ether. T h e iron was initially dissolved in 15 ml of 13.5N HC1 and heated to incipient dryness. Fifty milliliters of 8N HC1 was added a n d the resulting solution oxidized by bubbling Cln gas (Matheson, High Purity) through the solution for 30 min. T h e samples were then extracted three times with 25-ml portions of HCl saturated isopropyl ether. Two reagent blanks were treated the same way as the samples. This technique has long been demonstrated to be effective in re-
(1) D. C . Burrell, Nor. Geol. Tidsskr., 45, 21 (1965). (2) K . J. R. Rosman and P. M . Jeffery, Chem. Geol.. 8, 25 (1972). (3) D. C . Manning and F. Fernandez, A t . Absorption Newslett.. 7, 24 (1968). (4) V . A. Fassel. J. 0. Rasmuson, and T. G . Cowley, Spectrochim. Acta. Part 5 , 23,579 (1968). (5) J. E . Allan, Spectrochim. Acta, P a r t B , 24, 13 (1969). (6) D. T. Coker, J. M . Ottaway, and N. K . Pradham, Nature (London). 233,69 (1971). (7) S. R. Koirtyohann and E. E. Pickett, Ana/. Chem.. 38, 1087 (1966).
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Table I. Instrument Settings
Lamp
Zinc, single element
Resonance line
2138.56 A 4 15 m A
Slit
Lamp current Scale expansion
, 5x 16 I./rnin 6 I./min
Air flow
Acetylene flow
Table Il.aApparent Zinc Concentrations of Zinc Free Iron Solutions Analyzed at the Resonance Line of Zinc Apparent zinc in Apparent zinc in iron sample, pg/g O h Fe in soln soin, pg!g iron 0.20 0.40 0.60
0.80 1 .oo 1.60
2.00
0.031
0.059 0.087 0.12 0.14 0.21 0.26
16 15 14 14 14 13 13
Values based on aqueous zinc standards without deuterium background correction.
moving ferric iron from a host of elements including zinc (8).T h e resulting iron free solutions and blanks were then analyzed for zinc. T h e zinc content of the Johnson Matthey Specpure iron was less than 0.1 p g / g . Therefore. a 2% iron solution (1.0 g Fe/50 ml) would contain less t h a n 0.002 p g / g Zn which is below the detection limit of the instrument. We also determined the amount of zinc lost from the aqueous phase during extraction. A series of 2-g iron samples were spiked with varyins amounts of zinc and then carried through the same procedure described above. T h e zinc loss was consistantly less than 5%. This loss was attributed to mechanical loss during the isopropyl ether extraction. since less than 0.01% Zn is extracted into the ether phase bv this procedure ( 8 ) . In Table I are listed the instrument settings used. A PerkinElmer burner with a 10-cm slot was used. The top of t h e burner was set 0.2 cm below the light p a t h of the lamp. With the burner aligned and ignited. the air flow was adjusted t o yield the minimum absoriinnce !iy the flame and. therefore, maximum transmission 0:' !:".e zinc resonance line through the flame. This is a variation of the procedure described by Abbey (9).T h e use of the deuterium background corrector has been described by K a h n (101.
RESULTS AND DISCUSSION In Figure 1, the absorbances of iron solutions on the zinc resonance line are plotted. Curve 1 shows slight curvature whereas curve 2 is linear. Curve l represents the additive effects of background absorption and atomic ab(8) E. H. Swift. "Introductory Quantitative Analysis," Prentice-Hall, New York. N. Y.. 1950. pp428-432. (9) Sydney Abbey, Geological Survey of Canada, Paper 67-37, 1967. (10) H. L . Kahn. A t . Absorption Newslett., 7, 40 (1968).
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, p483. (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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