Sorption of very hydrophobic organic compounds onto poly(dimethylsiloxane)and dissolved humic organic matter. 1. Adsorption or partitioning of VHOC on PDMS-coated solid-phase microextraction fibers - A never-ending story?

Solid-phase microextraction (SPME) using nonpolar fiber coatings is a very useful method for determining concentrations (more precisely, activities) o f environmentally relevant very hydrophobic organic compounds (VHOC: alkane s, PCBs, and PAHs). The issue of adsorption (surface effect) versus absorpt ion (partitioning) is of huge importance for the application of SPME to det ermine VHOC in environmental samples. Competition effects, which are associ ated with adsorption processes, would result in concentration-dependent and mixture-dependent responses. The confusion in the literature about the pro cesses responsible far analyte extraction by the poly(dimethylsiloxane) (PD MS) fiber coatings turned out to be mainly attributed to experimental error s when applying conventional static SPME approaches. Determining fiber coat ing distribution coefficients (K-f) using dynamic systems is more accurate in comparison with static systems because analyte losses in the system (due to the fiber uptake, sorption on the walls, etc.) can be compensated for, thus ensuring constant concentration of the dissolved analyte(s) during the experiment. Fiber distribution coefficients of VHOC on PDMS coatings are s trongly correlated with the analyte hydrophobicity, expressed by the octano l-water partitioning coefficient(K-ow). This indicates that partitioning be tween the sample and the coating is the prevailing process. Therefore, equi librium SPME extractions in multicomponent systems allow the determination of concentrations of any of the VHOC, provided that the extraction is carri ed out in a depletion-free system and that appropriate partition coefficien ts of the analytes, which can be estimated on the basis of their K-ow data, are available.