The Chemical Analysis of Argonne Premium Coal Samples

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

Determination of 33 Elements in Coal Ash from 8 Argonne Premium Coal Samples by Inductively Coupled Argon Plasma-Mass Spectrometry

By Allen L. Meier

ABSTRACT

Thirty-three elements were determined in the ash of eight Argonne Premium Coal samples by inductively coupled argon plasma-mass spectrometry (ICAP-MS). Two sample digestion procedures were used, a sodium peroxide sinter to dissolve resistant minerals and an acid digestion technique for acid-soluble minerals in the coal ash. Hf, Ta, W, and the rare earth elements La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb were determined by ICAP-MS in the solution from the sodium peroxide sinter. Ga, Ge, As, Rb, Nb, Mo, Ag, Cd, Sn, Sb, Te, Cs, Au, Tl, Pb, Bi, and U were determined by ICAP-MS in the acid solution. These solutions were also used for inductively coupled argon plasma-atomic emission spectrometry (ICAP-AES) determination of other elements to give nearly total elemental coverage except for the volatile elements, halogens, and elements not retained because of combustion in the ashing process. The technique to determine the value for each element was selected to provide the best possible precision and determination limit while minimizing interferences.

INTRODUCTION

Inductively coupled argon plasma-mass spectrometry (ICAP-MS) is one of the newest instrumental analytical techniques to be used for elemental determination in geologic materials. The U.S. Geological Survey (USGS) began use of the technique in 1985 and developed methods using ICAP-MS for the determination of rare earth elements (REE's) and platinum-group elements and for the analysis of coal. ICAP-MS is attractive for these applications because it has multielement measurement capabilities with very low detection limits. Most elements are detected directly in solutions in the range of 1 to 100 pg/mL. These detection limits are often 100 to 1,000 times lower than those routinely achieved by inductively coupled argon plasma-atomic emission spectrometry (ICAP-AES). The response is linear with concentration over about 6 to 8 orders of magnitude, making calibration quite uncomplicated. Another advantage of ICAP-MS is that the mass spectra of elements are relatively simple. Problems with the techniques arise from spectral interferences from molecular species and effects from the sample matrix. These problems are minimized by the use of corrections for spectral overlaps and internal standards for matrix effects. For the analysis of coal, ICAP-MS and ICAP-AES are used as complementary techniques. ICAP-AES is used to determine the elements that normally have higher concentrations in the coal ash. These are primarily the lower mass elements where the ICAP-MS technique has more interferences. The ICAP- AES technique is also used for other elements that are normally found in coal ash above the detection limits for the technique and where the precision by this technique is better than the precision of the ICAP-MS technique. ICAP-MS is used to determine the elements where a lower limit of detection is necessary to determine normal concentrations found in coal and for elements where the ICAP-AES technique suffers from interferences.

EXPERIMENTAL

SINTER METHOD

Hf, Ta, W, and the rare earth elements La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb are made soluble in a 0.1-g coal ash sample by sintering with sodium peroxide, leaching with water, and acidifying with nitric acid in a preparation technique modified from one described by Borsier and Garcia (1983). Details of the procedure are described by Briggs in this volume. The elements were then determined by ICAP-MS at lower reporting limits, in parts per million, of 2.0 La, 3.0 Ce, 0.5 Pr, 2.0 Nd, 0.5 Sm, 0.2 Eu, 1.0 Gd, 0.5 Tb, 0.2 Dy, 0.5 Ho, 0.2 Er, 0.5 Tm, 0.5 Yb, 1.0 Hf, 1.0 Ta, and 1.0 W. Lutetium was added as an internal standard to correct for instrument instability and oxide interferences. Two-point calibration for each element was made by using the average intensity of five blanks taken through the entire procedure, and the intensities were acquired on a solution of a glass reference standard containing a known concentration of each element. The standard solution was run at 15 sample intervals, drift was calculated, and correction was applied between standards. All new determinations reported in this paper result from work done in the USGS laboratory in Denver, Colo.

INTERFERENCES FOR THE SINTER METHOD

Isobaric interferences of some metal oxide ions are quite high for selected REE's. Therefore, conditions that minimize the oxide ions were used. The delivery line from the nebulizer spray chamber to the plasma torch was cooled to 10°C to reduce the amount of water vapor (the main source of oxygen) that enters the plasma. Compromise conditions of power and the sheath gas flow rate were selected to achieve a balance between sensitivity and oxide. A method modified from Lichte and others (1987) was used to minimize and correct for these oxide isobaric overlaps. Oxide interference was subtracted by using the ratio of oxide ions to element ions in single-element standards and the oxide/ion ratio of the internal standard. In a 3-hour period of running samples, the oxide ratios can drift by as much as 100 percent. This is probably due to a gradual closing of the sampler cone, although several factors are involved. The oxide correction of PrO on gadolinium-157 must be very accurate. Even after the oxide abundance is minimized, a 10 percent error in the oxide ratio correction can result in a 20 percent error in the gadolinium result. The metal oxide/metal ion ratios of the REE's all responded similarly to plasma conditions (Lichte and others, 1987). Lutetium, used as an internal standard, was also used to track the drift in the oxide/metal ratio through a sample run. The oxide ratios of overlapping elements were measured in standard solutions and compared to the LuO + /Lu + response. These ratios were used to mathematically subtract the oxide interference and to correct for changes in the other metal oxide ratios due to matrix or drift in the sample run.

ACID DIGESTION METHOD

Ga, Ge, As, Rb, Nb, Mo, Ag, Cd, Sn, Sb, Te, Cs, Au, Tl, Pb, Bi, and U were made soluble in a 0.2-g coal ash sample by heating with a mixture of hydrochloric, nitric, perchloric, and hydrofluoric acids (Crock and others, 1983). Details of the procedure are described by Briggs in this volume. The elements were then determined by ICAP-MS at lower reporting limits, in parts per million, of 0.1 Ga, 0.5 Ge, 1.0 As, 0.5 Rb, 2.0 Nb, 0.5 Mo, 0.5 Ag, 0.2 Cd, 1.0 Sn, 0.5 Sb, 0.5 Te, 0.1 Cs, 0.1 Au, 0.5 Tl, 2.0 Pb, 0.1 Bi, and 0.2 U. Lutetium and indium were added as internal standards to correct for instrument instability and oxide interferences. Two-point calibration for each element was made by using the average intensity of five blanks taken through the entire procedure, and the intensities were acquired on a solution of a glass reference standard containing a known concentration of each element. The standard solution was run at 15 sample intervals, drift was calculated, and correction was applied between standards. Oxide interference was subtracted by using the ratio of oxide ions to element ions in single-element standards and the oxide/ion ratio of the internal standard.

INTERFERENCES FOR THE ACID DIGESTION METHOD

Interferences in ICAP-MS come from matrix effects, instrumental drift, and isobaric overlap of some elemental isotopes and molecular ions formed in the plasma, resulting in suppression or enhancement of measured ion intensity. An internal standard was added to minimize matrix effects and instrumental drift. The isotopes measured were selected to minimize isobaric overlap from other elements and molecular species that might be present. Oxide overlaps were subtracted by measuring the ratio of oxide to element for single-element standards in each run and applying this ratio to each sample.

RESULTS AND DISCUSSION

The values obtained for triplicate analyses of the eight Argonne Premium Coal reference samples digested using the sinter method are given in table 1. The values obtained for triplicate analyses of the eight Argonne Premium Coal reference samples prepared by using the acid digestion method are given in table 2.

The wide elemental coverage and the low limits of determination of the ICAP-MS technique make it a worthwhile tool for the analysis of coal ash. The accuracy and precision of the methods are adequate for the determination of trace elements in coal ash. Tables 3 through 6 show values determined by ICAP-MS on solutions obtained by the two dissolution methods of the ash of National Institute of Standards and Technology (NIST) reference standard materials 1632b (coal) and 1633a (coal fly ash). The tables compare values obtained in this study with the reference values. These comparisons show that reasonable accuracy is achieved by these methods. Unfortunately, many elements determined have not been reported for these reference materials, so accuracy cannot be estimated using these materials. Precision is given as standard deviation and relative standard deviation for each element determined in the reference materials. For most elements, precision is better than 10 percent relative standard deviation. Precision for some elements is poorer, especially as detection limits are approached. Silver concentrations determined on solutions obtained by the acid digestion method have the most variation. The lack of precision can be attributed to sampling variation as well as instrumental variation.

REFERENCES

Borsier, M., and Garcia, M., 1983, Analyse automatique d'echantillons geologiques par plasma ICP: Spectrochimica Acta, v. 38B, nos. 1/2, p. 123-127.

Crock, J.G., Lichte, F.E., and Briggs, P.H., 1983, Determination of elements in National Bureau of Standards' geological reference materials SRM 278 obsidian and SRM 688 basalt by inductively coupled argon plasma-atomic emission spectrometry: Geostandards Newsletter, v. 7, no. 2, p. 335-340.

Lichte, F.E., Meier, A.L., and Crock, J.G., 1987, Determination of the rare-earth elements in geological materials by inductively coupled plasma mass spectrometry: Analytical Chemistry, v. 59, no. 8, p. 1150-1157.

National Bureau of Standards, 1979, National Bureau of Standards certificate of analysis, standard reference material 1633a, trace elements in coal fly ash: Washington, D.C., National Bureau of Standards, 2 p.

------1985, National Bureau of Standards certificate of analysis, standard reference material 1632b, trace elements in coal (bituminous): Gaithersburg, Md., National Bureau of Standards, 5 p.

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