Characterization of coal-derived humic substances with the aid of low-pressure gel permeation chromatography

Low-pressure gel permeation chromatography (GPC) has proven to be a valuable tool for determining the molecular weights of coal-derived humic substances (HS). A 40×0.9 cm (ID) column packed with Sephadex gel (typically with Sephadex G-100) in combination with alkaline borate-based mobile phase allows an almost complete fractionation of typical HS from young brown coals. After a calibration with a set of globular proteins, a molecular-weight distribution can be determined and average molecular weights can be calculated. The determination of the weight-average molecular weight, Mw, seems to be more reliable than the determination of the number-average molecular weight, Mn. Limitations of the proposed method (and size-exclusion methods in general) were discussed in detail, and some sources of uncertainties were identified.