Adsorption versus Absorption of Polychlorinated Biphenyls onto Solid-Phase Microextraction Coatings

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Abstract
Absorption-based polymeric solid-phase microextraction (SPME) fibers with poly(dimethylsiloxane) (PDMS) coatings were used to determine the partitioning coefficients of polychlorinated biphenyls (PCBs) between the sorptive fiber coatings and water. Previous models showing very good correlations between octanol−water partitioning coefficients (Kow) and absorption-based fiber−water partitioning coefficients (Kdv) for low-molecular-weight analytes failed to predict Kdv values for PCBs. In fact, Kdv values for PCBs were 1−7 orders of magnitude lower than those predicted by Kow and actually showed a strong negative correlation between Kow and Kdv for higher molecular weight analytes (MW >∼200). Kdv values obtained using PDMS fibers with 7- and 100-μm coatings also disagree, demonstrating that Kdv cannot be used to describe the partitioning behavior of PCBs between PDMS and water. However, when PCB partitioning coefficients were calculated on the basis of surface area (Kds), the Kds values obtained using 7- and 100-μm PDMS fibers agreed reasonably well, demonstrating that surface adsorption is the primary mechanism controlling PCB (and likely other higher molecular weight solutes) partitioning from water to SPME sorbents.