GC–MS investigation of distribution of fatty acids in selected Polish brown coals

Brown coals sampled in different Polish brown coal basins were investigated to assess the distribution of fatty acids present in their solvent extracts. Extracts were separated into aliphatic hydrocarbons, aromatic compounds and polar compounds groups by thin layer chromatography. Polar compounds fractions were acidified and fatty acids were methylated. Distribution of fatty acids methyl esters was analysed with gas chromatography–mass spectrometry. Distributions of fatty acids included compounds in the range from n-hexadecanoic acid (C16) to n-tritriacontanoic acid (C33) with the dominating compounds from n-eicosanoic acid (C20) to n-triacontanoic acid (C30). Lighter acids are also present. The most obvious feature is predominance of compounds with even-over-odd-carbon-atom-number in a molecule, which comes from vascular plant fatty acids. In the mine profile, the predominance decreases with depth. Some of brown coals sampled at lower depths do not contain fatty acids with odd carbon atom number in a molecule at all or only in very low concentration. This relationship is promising in assessment of brown coal rank and can be expressed as the proposed even-over-odd-carbon-number predomination ratio (EOP). Distribution range of fatty acids is related to brown coal lithotype, with wider range for detritic brown coals and more narrow for xylithic brown coals.