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 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.