Optimisation of pressurised liquid extraction for elimination of sulphur interferences during determination of organotin compounds in sulphur-rich sediments by gas chromatography with flame photometric detection

A simple method for species-selective analysis of organotin compounds (OTCs) (butyl and phenyl) in sediments was developed. The sample preparation procedure was specifically optimised for sulphur-rich sediments to eliminate interferences from elemental sulphur and organosulphur compounds. Tin species were extracted from sediment samples using pressurised liquid extraction technique (PLE), ethylated – with simultaneous extraction to isooctane – in aqueous phase with sodium tetraethylborate (NaBEt4) and separated/detected by gas chromatography with flame photometric detection (GC–FPD). PLE operational variables (extraction temperature and pressure, solvent composition and number of static extraction steps) and extract handling routine were fine-tuned to minimise the amount of extracted interferents while keeping OTCs recovery at an acceptable level.Best results were obtained after extraction of sediment samples with methanol/water (75% v/v methanol) solution of acetic acid/sodium acetate with tropolone addition (0.6 g l−1). Derivatisation of low temperature, high-pressure (50 °C, 13.8 MPa) extracts gives isooctane extracts which are clean enough to be directly analysed by GC–FPD without any further cleanup. Interferences from elemental sulphur were completely eliminated while concentrations of other interferents were reduced to the level not impairing quantitation of OTCs under the study. No negative effects in terms of chromatographic column deterioration were observed after repeated injections of such extracts.Two certified reference materials, BCR646 and PACS-2, were analysed to assess performance of the method. Recoveries of all OTCs under the study, except MBT, were in the range of 91–114%. MBT extraction efficiency was low (34–47%) therefore the method is unsuitable for precise determinations of this compound.