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) of environmentally relevant very hydrophobic organic compounds (VHOC: alkanes, PCBs, and PAHs). The issue of adsorption (surface effect) versus absorption (partitioning) is of huge importance for the application of SPME to determine VHOC in environmental samples. Competition effects, which are associated with adsorption processes, would result in concentration-dependent and mixture-dependent responses. The confusion in the literature about the processes responsible far analyte extraction by the poly(dimethylsiloxane) (PDMS) fiber coatings turned out to be mainly attributed to experimental errors when applying conventional static SPME approaches. Determining fiber coating 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 strongly correlated with the analyte hydrophobicity, expressed by the octanol-water partitioning coefficient(K-ow). This indicates that partitioning between the sample and the coating is the prevailing process. Therefore, equilibrium SPME extractions in multicomponent systems allow the determination of concentrations of any of the VHOC, provided that the extraction is carried out in a depletion-free system and that appropriate partition coefficients of the analytes, which can be estimated on the basis of their K-ow data, are available.