XRF is a technique that provides the bulk chemical composition of samples. Most often, samples are powders pressed into pellets, but the fusion pellet process can be used to make glass pellets. Quick semiquantitative determinations can be made for elements with atomic numbers 5 through 92. The XRF can be used in conjunction with chemical fractionation.

Chemical Fractionation

Chemical fractionation is a wet-chemistry technique used to quantitatively determine the modes of occurrence of the inorganic elements in coal, based on the extractability of the elements in solvents. This is an especially good technique for low-rank coals and biomass that can have significant amounts of organically bound elements, which are dispersed within the organic matrix of the material, essentially making them invisible to SEM. The following solvents are used: water – removes soluble inorganics (water-soluble salts like NaCl), ammonium acetate – removes inorganic ion-exchangeable cations associated with organic acid groups, and hydrochloric acid – removes elements held in coordination complexes within the organic structure as well as acid-soluble minerals such as carbonates, sulfates, and oxides, as well as nonextractable elements associated in the coal as silicates, aluminosilicates, sulfides, and insoluble oxides.

Portions of the sample are removed after each solvent extraction, dried, ashed, and analyzed by XRF. Data can be used in combination with computer-controlled scanning electron microscopy mineral data to determine the distribution of all inorganic material in the coal.