Study into the equilibrium mechanism between water and poly(dimethylsiloxane) for very apolar solutes: Adsorption or sorption?

Recently several publications appeared correlating octanol-water partitioni ng coefficients (K-O/W) with solid-phase microextraction (SPME) extraction coefficients on poly(dimethylsiloxane) (PDMS) fibers. This correlation seem s very good for medium-polar to polar compounds but cannot explain the obse rvations for apolar compounds. It is shown that for polychlorinated bipheny ls (PCBs) the published data are erroneous, because of system adsorption ef fects. PCB concentrations up to 10 times higher were measured on the stir b ar compared to the SPME fiber. Using a short packed PDMS trap, it is shown that the true PDMS-water equilibrium constant is indeed proportional to lit erature K-O/W data.