The Chemical Analysis of Argonne Premium Coal Samples

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

Determination of 29 Elements in 8 Argonne Premium Coal Samples by Instrumental Neutron Activation Analysis

By Curtis A. Palmer

ABSTRACT

Twenty-nine elements have been determined in triplicate splits of the eight Argonne Premium Coal samples by instrumental neutron activation analysis. Data for control samples NIST (National Institute of Standards and Technology) 1633 (fly ash) and NIST 1632b (bituminous coal) are also reported. The factors that could lead to errors in analysis of these samples, such as spectral overlaps, low sensitivity, and multiple sources of interfering nuclear reactions, are discussed.

INTRODUCTION

The U.S. Geological Survey (for example, Zubovic and others, 1979, 1980; Oman and others, 1981; Currens and others, 1986, 1987) and other laboratories (for example, Gluskoter and others, 1977) have used instrumental neutron activation analysis (INAA) for the determination of major, minor, and trace elements in thousands of coal samples. The application of INAA for the analysis of coal has been described in several papers (for example, Block and Dams, 1973; Ondov and others, 1975; Rowe and Steinnes, 1977a,b; Swaine, 1985; Palmer and Baedecker, 1989). The analysis of coal by INAA is especially useful because determinations are made on the whole coal in contrast to other techniques in which the ash is used as the sample matrix. Therefore, INAA can be used to measure elements that might be volatilized during ashing, such as bromine. All elements are determined on the same sample split so that element ratios used in understanding geochemical environments are not affected by inhomogeneities in a coal sample. In addition, INAA has very low detection limits for many elements, can be easily automated, and provides precise data for many major, minor, and trace elements.

ACKNOWLEDGMENTS

The author would like to thank Jeff Grossman and Phil Baedecker for help with the derivation of the fission product correction formulas, Mike Pickering for assistance in counting the samples, and Jean Kane, Phil Baedecker, and Frank Walthall for suggestions to the manuscript.

EXPERIMENTAL

Three splits of approximately 500 mg of each of the eight Argonne Premium Coal samples were weighed and heat sealed in 1.5-cm 3 polyethylene vials. These samples were irradiated for 8 hours in the TRIGA research reactor facility of the U.S. Geological Survey in Denver, Colo., at a neutron flux of 3 x 10¹² neutrons/cm² sec. After a delay of 3 days to eliminate or reduce short-lived activity, the samples were shipped by overnight delivery to laboratories in Reston, Va., for gamma-ray counting.

The samples were counted at three different times on high-resolution coaxial germanium and germanium (lithium) detectors for gamma-ray spectroscopy. The first count was started approximately 4 days after irradiation. A second count was started at 17 days after irradiation after allowing the short-lived activities (especially ²⁴Na, half-life=15 hours) to decay, and then a third count was begun approximately 2 months after irradiation to obtain higher precision on the measurement of the long-lived radionuclides. The gamma-ray detectors were coupled to multichannel pulse-height analyzers, which are capable of dividing the spectrum into 4,096 energy increments or channels. An automatic sample changer similar to that described by Massoni and others (1973) was used to change the samples. All spectra were processed by using the computer program SPECTRA (Baedecker and Grossman, 1989, 1994).

SAMPLES AND STANDARDS

The eight Argonne National Laboratory Premium Coal samples used in this study have been described previously (Vorres, 1990). The convention for sample identification is the same as described by Palmer in the Introduction of this volume. Three multiple-element standards, NIST (National Institute of Standards and Technology) 1632a, NIST 1633a, and Eastman-Kodak TEG-50-B, and two control samples, NIST 1633 (fly ash; different from 1633a) and NIST 1632b (bituminous coal), were included with each irradiation. The element concentration values for the NIST standards used for analysis have been reported previously (Palmer and Baedecker, 1989) and are largely based on the results of Ondov and others (1975).

A comparison of the results of this study with literature values for the control samples is given in table 1. The analytical errors reported for the control NIST 1632b in this study are based on counting statistics at the one-sigma level. NIST certified and information values are shown for NIST 1632b. Our determinations of concentrations in control 1632b agree with all certified values within the stated errors and generally agree, within 10 percent, with the NIST information values that have no reported errors.

RESULTS AND DISCUSSION

The concentrations and their associated errors based on counting statistics for 29 elements for each of the Premium Coal samples are shown in table 2. Iron is the only major element (concentrations >1 percent) determined, and sodium and potassium are the only minor elements (concentrations <1 percent, >0.1 percent) determined. All other elements determined are trace elements. For many elements, the concentration values ranged over a factor of 5 among the eight Argonne Premium Coal samples.

The errors reported in table 2 are based on counting statistics only. Generally, the precision of the data based on the replicate analyses is within the counting errors for elements where the reported error is greater than 5 percent. For some elements with small counting errors, the analytical precision is poorer because of the other sources of error such as sample homogeneity or positioning during counting.

Errors reported in table 2 were generally less than 10 percent except for nickel, rubidium, and neodymium, in which the concentration was near or below the detection limit for all samples. Errors were also greater than 10 percent for barium in UF PC-1; uranium in PITT PC-4, POCPC-5, and ND PC-8; and ytterbium in UT PC-6 (table 2). Errors reported for potassium are variable even at the same concentration because it has the shortest half-life of the elements determined in this study, and the detection limit varies by nearly an order of magnitude during the 2-day counting cycle for the entire sample set.

In table 2, the concentrations reported for nickel in WYPC-2 and UT PC-6 and for rubidium in UT PC-6 and NDPC-8 are actually below the expected detection limits given in table 3 because the values in table 3 are determined for a 'typical' coal matrix. The detection limits for individual coal samples may change because of variations in the concentrations of the most sensitive elements that dominate the gamma-ray spectrum and because of variations in the intensities of spectral interferences. The percent correction of each spectral interference for all premium coals is given in table 4. Generally, only a small correction is needed for most elements. Some elements, such as nickel, selenium, and samarium in some samples, require changes larger than 10 percent.

In addition to corrections made because of spectral interferences, barium and the light rare earth elements lanthanum, cerium, neodymium, and samarium were corrected for interference because of neutron-induced fission of ²³⁵U; table 5 shows the percent correction for these elements. However, corrections for barium and lanthanum are time dependent and therefore vary during the counting of the samples. The concentration of barium, Bacorr (corrected for the time-dependent fission correction factor), was calculated by using the formula:

Ba_corr = Ba_meas-2.9Ue ^0.0402t

and the concentration of lanthanum, La_corr (corrected for the time-dependent fission correction factor), was calculated by using:

La_corr = La_meas-0.002723Ue ^0.3592t

where t = time after bombardment in days, Ba_meas and La_meas are the uncorrected barium and lanthanum concentrations, and U is the concentration of uranium. The constant 0.002723 in the La equation is calculated by assuming a ²³⁵U cross section of 580 barns, which agrees with the experimental data within ±1 percent. The half-lives for ¹³¹Ba and ²³⁹Np (U) and ¹⁴⁰La were taken from table 3. The half-life of ¹⁴⁰Ba, which decays to the measured ¹⁴⁰La, was assumed to be 12.8 days. The fission correction factors are generally quite small except for barium and cerium in IL PC-3, which are about 16 and 8 percent, respectively, and for barium and neodymium in UT PC-6, which have correction factors as high as 7 and 5 percent, respectively.

REFERENCES

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Palmer, C.A., 1991, The determination of 29 elements in eight Argonne Premium Coal samples by instrumental neutron activation analysis, in Palmer, C.A., and Walthall, F.G., eds., The chemical analysis of Argonne Premium Coal samples: U.S. Geological Survey Open-File Report 91-638, chap. F, p. 50-63.

Palmer, C.A., and Baedecker, P.A., 1989, The determination of 41 elements in whole coal by instrumental neutron activation analysis, in Golightly, D.W., and Simon, F.O., eds., Methods for sampling and inorganic analysis of coal: U.S. Geological Survey Bulletin 1823, p. 27-34.

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Zubovic, P., Oman, C.L., Coleman, S.L., Bragg, L.J., Kerr, P.T., Kozey, K.M., Simon, F.O., Rowe, J.J., Medlin, J.H., and Walker, F.E., 1979, Chemical analysis of 617 coal samples from the Eastern United States: U.S. Geological Survey Open-File Report 79-665, 453 p.

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