The Chemical Analysis of Argonne Premium Coal Samples

Edited by Curtis A. Palmer
U.S. Geological Survey Bulletin 2144

Determination of 18 Elements in 5 Whole Argonne Premium Coal Samples by Quantitative Direct-Current Arc Atomic Emission Spectrography

By Janet D. Fletcher and Carol J. Skeen

ABSTRACT

Quantitative multiple-element analysis of whole Argonne Premium Coal samples by direct-current arc atomic emission spectrography is possible with the use of a lithium carbonate buffer. Two spectrographic methods are described for the determination of 18 trace elements in 100-mg samples of coal. Overall concentrations for calibration standards range from a low of 2 µg/g to a high of 3 weight percent. For concentrations well above the lower determination limit, the typical accuracy is within ±20 percent, and the general precision of the method is ±10 percent.

INTRODUCTION

Most atomic-spectroscopic methods are designed for the analysis of ash from pulverized coals that have been oxidized at 500°C or 750°C (ASTM, 1984). The direct-current (dc) arc functions well for ash that is mixed with graphite powder, and many elements are effectively preconcentrated by the ashing process, thus providing improved detectability (Dorrzapf, 1973), but certain elements associated with organic phases, such as porphyrins, organometallics, or acid salts, may be volatilized and lost during the ashing process. The elements Ag, B, Ga, Ge, Mo, Ni, and Ti, which potentially can be determined by dc arc spectroscopy, are at least partially associated with organic phases in coals (Ruch and others, 1974; Gluskoter and others, 1977; Finkelman, 1980). Direct multiple-element analysis of whole coals circumvents the long intervals required for ashing, the losses due to volatilization, and the further exposure of samples to possible contamination. This paper describes the two directcurrent arc atomic emission spectrographic (DCAES) methods that have produced accurate determinations of 28 elements in the pulverized whole coal (Fletcher and Golightly, 1985). In this study, only 18 elements were determined in 5 Argonne Premium Coals. These methods, which have been applied principally to the analysis of coal microlithotypes, offer the basis for efficient, low-cost, multiple-element analysis of whole coals.

EXPERIMENTAL

Approach

A principal difficulty encountered in attempts to arc small quantities of pulverized coal directly is the rapid evolution of gases that occurs immediately following initiation of the arc and on the subsequent burning of the organic phases that remain in a cup-shaped electrode (anode). The rapidly evolved gases usually blow material from the anode cup, thus creating uncontrolled losses of the previously weighed sample, and the erratic flaming of the organic phase can produce unwanted spectral bands from carbonbased free radicals. These events constitute irreproducible processes that control the transport of material from the hot anode cup into the arc discharge. Such severe problems related to the arcing process have been solved by mixing powdered coal with a lithium carbonate buffer. This controls sample transport and excitation conditions in the arc column and greatly diminishes the possibility for flaming of the hot coal dissociation products. With these important aspects of arcing well controlled for coal samples, the methodology for dc arc spectrographic analysis becomes quite conventional.

Method

Preparation of the Samples

Splits of 100 mg of each whole Argonne Premium Coal sample, 100 mg of lithium carbonate, and 50 mg of pure graphite powder were thoroughly mixed and ground with an agate mortar and pestle to obtain a final homogeneous mixture. For samples that had especially high concentrations of analyte elements, a higher weight ratio of lithium carbonate to sample was necessary, but the ratio was no greater than 10:1. Twenty-five milligrams of the final homogeneous mixture was transferred into the appropriate graphite electrode and firmly tamped (Dorrzapf, 1973). These filled electrodes were dried in an oven at 110° for 4 hours immediately before arcing. The drying step was necessary because it removes water and other readily volatilized components that could cause the loss of sample material from the anode just after initiation of the arc discharge.

Preparation of Standards

Calibration standards consisted of homogeneous mixtures of oxides and carbonates of the analyte elements in a lithium carbonate matrix. Dilutions of commercially available standards, which contain 43 elements in lithium carbonate (Spex Industries, Metuchen, New Jersey), provided calibration standards for the concentration range from 1 to 1,000 µg/g for each element of interest. Individual standards were diluted on a weight-weight basis with high-purity lithium carbonate (<10 µg/g total impurities). Reference standards were prepared from the National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS), coal reference materials NIST 1632, NIST 1632a, and NIST 1635 (NBS, 1974, 1978a,b), which were diluted with lithium carbonate in the same fashion as the samples. Drying and handling of NIST standards followed the procedure used for samples.

Arcing of Samples and Standards

All samples and standards were arced in an argon-oxygen, or argon, laminar stream that is concentric to the anode and is introduced through an alumina nozzle arrangement known as a Helz jet (Helz, 1964). Both the arcing conditions and the atmosphere were chosen to give complete volatilization of analyte elements from the anode cup into the arc column and to effectively excite those atomic energy levels giving the spectral lines listed in table 1, without causing high spectral background. For the volatile elements (group II, table 1), the objective was to vaporize and to excite these elements over a relatively long interval while distilling insignificant amounts of matrix elements into the arc column. The present method was one adapted from that of Annell (1967) for volatile elements in silicate and carbonate rocks. For elements in chemical forms that exhibit low volatility (group I, table 1), total vaporization of each sample into the arc column was necessary for an accurate determination.

Complete details on the spectrographic equipment and the conditions for arcing samples and for making the necessary measurements are given in table 2. Maintaining a 4-mm gap between the tips of the electrodes was essential to the achievement of the accuracy and precision that this approach is capable of producing.

ACCURACY AND PRECISION

The accuracy of analysis by DCAES is dependent on the successful element-by-element calibrations of an instrument with standard materials that closely resemble the materials to be analyzed. For coals, the effective matrix of the 'arced sample' was modified through the use of a lithium carbonate buffer. This modification of the sample matrix made the arced sample resemble the lithium carbonate matrix of the Spex calibration standards. The quantity of lithium carbonate relative to that of the sample was sufficient to control the fusion, vaporization, transport, and excitation processes. The concentration ranges for the elements determined by the dc arc spectrographic methods described in this work are summarized in table 1. Elements exhibiting the largest deviations are aluminum, calcium, manganese, and silicon. Experience in the analyses of other coals, vitrinites, exinites, and inertinites indicates that the deviations for the elements observed here are random, rather than systematic. Measurement errors for the spectrographic method for concentrations well above (>5 times) the determination limits are typically ±20 percent, and the precision of the method is ±10 percent.

RESULTS AND DISCUSSION

Only five of the eight Argonne Premium Coals were available for analysis when this method was developed. Because this method is labor intensive and the accuracy and precision for this method at the detection limits for a majority of the elements are no better than the other methods implemented in the analysis of these coals, the analyses of the other three coals were not carried out.

Only 18 elements were determined in the 5 whole Argonne Premium Coals. The results of these analyses are shown in tables 3 and 4. The determination of silver, boron, and molybdenum required special treatment and preparation time, and so they were eliminated from the routine for analysis. As, Bi, Cd, Hg, Nb, Sn, and Tl were not determined because of the nature of the matrix of these particular coals.

REFERENCES

American Society for Testing and Materials (ASTM), 1971, Methods for emission spectrochemical analysis, general practices, nomenclature, tentative methods, suggested methods (6th ed.): Philadelphia, ASTM, 1094 p.

------1984, ASTM Designation D 3683Ð83, Trace elements in coal and coal ash by the atomic absorption method: 1984 Annual Book of ASTM Standards, v. 05.05, Gaseous fuels; Coal and coke, p. 466-469.

Annell, C.S., 1967, Spectrographic determination of volatile elements in silicates and carbonates of geologic interest using anargon dc arc: U.S. Geological Survey Professional Paper 575-C, p. C132-C136.

Dorrzapf, A.F., Jr., 1973, Spectrochemical computer analysis-Argon-oxygen D-C arc method for silicate rocks: U.S. Geological Survey Journal of Research, v. 1, no. 5, p. 559-562.

Finkelman, R.B., 1980, Modes of occurrence of trace elements in coal: College Park, Md., University of Maryland, Ph.D. dissertation.

Fletcher, J.D., and Golightly, D.W., 1985, The determination of 28 elements in whole coal by direct-current arc spectrography: U.S. Geological Survey Open-File Report 85-204, 14 p.

Gluskoter, H.J., Ruch, R.R., Miller, W.G., Cahill, R.A., Dreher, G.B., and Kuhn, J.K., 1977, Trace elements in coal-Occurrence and distribution: Illinois State Geological Survey Circular 499, 154 p.

Helz, A.W., 1964, A gas jet for D-C arc spectroscopy: U.S. Geological Survey Professional Paper 475-D, p. D176-D178.

Meggers, W.F., Corliss, C.H., and Scribner, B.F., 1975, Tables of spectral line intensities, Part 1-arranged by elements (2d ed.): Washington, D.C., National Bureau of Standards Monograph 145, 387 p.

National Bureau of Standards, 1974, National Bureau of Standards certificate of analysis, standard reference material 1632, trace elements in coal (bituminous): Washington, D.C., National Bureau of Standards, 2 p.

------1978a, National Bureau of Standards certificate of analysis, standard reference material 1632a, trace elements in coal (bituminous): Washington, D.C., National Bureau of Standards, 2 p.

------1978b, National Bureau of Standards certificate of analysis, standard reference material 1635, trace elements in coal(subbituminous): Washington, D.C., National Bureau of Standards, 2 p.

Ruch, R.R., Gluskoter, H.J., and Shimp, N.F., 1974, Occurrence and distribution of potentially volatile trace elements in coal-A final report: Illinois State Geological Survey Environmental Geology Notes, no. 72, 96 p.

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