Quantification of elemental sulfur in pulping liquors by thin-layer chromatography

The inorganic sulfur compounds that are used as pulping agents in the production of pulp and paper from wood are converted into a variety of sulfur species during processing. How sulfur is allocated to different streams and products in a pulp mill is relevant for efficient mill operation and to avoid harmful emissions. Pulping liquors are a highly challenging and potentially destructive sample matrix. We describe a thin-layer chromatographic method for the direct quantification of elemental sulfur in pulping liquors. The liquors are spotted (not sprayed!) directly onto the plate without prior purification, extraction, or workup. Sulfur is then eluted with cyclo-hexane and detected by densitometry at 285 nm or fluorescence quenching close to the solvent front. The method was validated for a calibrated range from 60 to 2000 ng sulfur on plate. The limit of detection was determined at 20 ng; measurement uncertainty was about 20%. With this method, 27–54 mg/L elemental sulfur were found in Kraft pulping liquors, which corresponds to 0.24–0.48% of the total sulfur in these samples.