The Chemical Analysis of Argonne Premium Coal Samples
Edited by Curtis A. Palmer
U.S. Geological Survey Bulletin 2144
Determination of 62 Elements in 8 Argonne Premium Coal Ash Samples by Automated Semiquantitative Direct-Current Arc Atomic Emission Spectrography
By Carol J. Skeen, B.J. Libby, and W.B. Crandell
The automated semiquantitative direct-current arc atomic emission spectrographic method was used to determine concentrations of 62 elements in 8 Argonne Premium Coal ash samples. Ashed samples of all eight coals were analyzed in triplicate to verify precision of the method. The precision for most elements was within ±10 percent. The accuracy of this method is assumed to be limited to +50 percent or -33 percent because of the nature of the standard curves for each of the elements. Adjustments to the computer program were implemented to account for unique matrix interferences in these particular coal ash samples.
Emission spectrographic analysis is based on the emission of light by atoms and ions returning to ground state after excitation in a direct-current (dc) arc. The light is passed through the slit of a spectrograph and diffracted by a grating. The slit image is focused on a photographic plate. Distinct wavelength positions for each element indicate the presence of that element, and the darkness of the resulting specific slit image is proportional to the concentration of the specific element present.
The automated dc atomic emission spectrographic analysis of a wide variety of geologic materials is a rapid, economical method for evaluating both the major- and trace-element composition. A total of 62 elements can be determined (Dorrzapf, 1973). This computerized procedure is semiquantitative, because it calculates concentrations by using restored coefficients calculated from previously arced standards.
In preparation of the samples for arcing, 15 mg of the ashed sample (-100 mesh) was mixed with 30 mg of graphite and transferred to a graphite crater electrode. The standard operating procedures for the spectrographic analysis are listed in table 1 (Dorrzapf and others, 1989). The Helz jet was used in preference to a Stallwood jet, because it simplifies the procedure for changing samples (Stallwood, 1954; Shaw and others, 1958; Helz, 1964). An iron bead was arced first and exposed on the plate for the iron calibration reference spectrum. Then each sample was arced at 5 amperes (A) for 20 seconds and then at 15 A for 130 seconds. For the iron and for each sample and standard, a cadmium lamp was exposed in two windows as a reference to be used on the scanning microphotometer.
The photoplate was developed and processed according to standard U.S. Geological Survey (USGS) procedures (Helz and others, 1969). The intensities of the spectra on the plate were recorded by a scanning microphotometer (Helz, 1965, 1973). The data were processed by a Hewlett-Packard 2100 computer, and a report was generated with the 62-element concentration information. This report was evaluated by the analyst, who verified the values reported (Golightly and others, 1977; Dorrzapf and others, 1989).
Programs for data collection and interpretation that were written (Walthall, 1974) for use on a mainframe computer were adapted for use on a minicomputer system (Thomas, 1979). The specific details about the procedure for calibration and the algorithm used for calculations were summarized by Dorrzapf and others (1989).
RESULTS AND DISCUSSION
For eight Argonne Premium Coal samples, an in-depth study was made for specific interference corrections in addition to those that were automatically performed. Because of heterogeneity problems associated with barium, coupled with the suppression of the most sensitive analytical lines for barium due to the matrices of these coals, values from less sensitive lines were used. The values for strontium were improved after correcting for interferences from iron and nickel.
This semiquantitative approach achieves ranges and detectability comparable to those of the visual estimation procedure (Myers and others, 1961). Because the standards used do not closely match the approximate composition of the ash samples to be analyzed, the expected accuracy is limited to ±1 step, which corresponds to roughly +50 percent or -33 percent of the reported value.
In the analyses of these Premium Coals, the National Institute of Standards and Technology (NIST) standard reference materials NIST 1633 and NIST 1633a (National Bureau of Standards, 1975, 1979) were included as control samples for evaluation of both precision and accuracy of this method. Table 2 lists the concentrations provided by NIST certificates, the mean concentrations determined over a 5-month period, and the associated relative standard deviations. Compositions determined for coal ashes are within the limits of precision and accuracy for which the method was designed. The high relative standard deviations for barium and zirconium indicate the heterogeneity documented for these reference materials (Filby and others, 1985). Heterogeneity possibly explains the large range of values for barium and zirconium in the Premium Coals as well as the interferences discussed above.
Table 3 lists data for all 62 elements in these eight Argonne Premium Coals. The 'less than' symbol indicates that the concentration is less than the lower limit for a value that can be determined for that element, and the 'greater than' symbol indicates that the concentration is greater than the highest value that can be reliably determined for that element. An 'H' denotes the occurrence of an unresolved interference. Major elements are reported in percentages, and the trace elements in micrograms per gram.
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