Direct Current Plasma Emission Spectrometric Determination of Maior, Minor and Trace Elements in Microwave Oven Acid Leachates of Powdered Whole Coal Samples

Major concentrations of Al2O3, Fe2O3, MgO, CaO, Na2O and K2O, minor levels of TiO2, P2O5 and 30 petrologically, geochemically and environmentally significant trace elements have been determined in microwave oven acid leachates of whole powdered coal samples by direct current plasma atomic emission spectrometry (DCP-AES). A single sample preparation procedure is suitable for all the determinations with no additional dilution step for major elements solution. 0.5 g dried samples are treated in low-pressure PFA digestion vessels with HF/HCl/HNO3/HClO4 acids quantitatively extracting the analytes from the bulk material, while leaving the major part of organic matter as a residue. The major constituents of geological samples, in particular the easily ionized elements (EIEs) such as alkali and alkaline earths, may complicate the instrumental determinations in DCP-AES because of differential enhancements of elemental emission intensities and stray light additions. With the concern for these factors, the coal matrix is considered for having very low major oxide totals as compared to many other common geo-environmental and related materials (rocks, sediments, soil, ashes etc..). The sample size employed here, while yielding a relatively concentrated solution to cover a wide range of elemental determinations, provides a sample matrix which diminishes significantly interferences for DCP measurements. The need for closely matching the unknowns and standards is eliminated except for overall acidity and an excess quantity of caesium for EIE buffering. Calibration of the spectrometer is accomplished by simple aqueous single element solutions as high standards in addition to a reagent blank as a low standard. Two points working curves are established to allow for the maximum concentrations of each element expected in the unknowns. The precision of the analysis under routine conditions as well as the reproducibility of the leaching and precision of instrumental measurements have been evaluated. Relative standard deviations (RSD) are of 1-2% for those elements whose concentrations in solid samples are well above the limits of quantification. Method detection limits in the buffered solutions were also evaluated. To evaluate the accuracy of the microwave oven - DCP method a suite of 8 certified coal standards of differing rank, was analised with good agreement with the certified and/or available published data. Results are presented for the uncertified major oxides in the reference materials ARs.