Determination of polydimethylsiloxane (PDMS)-seawater distribution coefficients for polychlorinated biphenyls and clorinated pesticides by solid-phase microextraction

ABSTRACT Despite the growing popularity of solid-phase microextraction (SPME) among the community of analytical chemists, applications of SPME in the measurement of very hydrophobic organic compounds (VHOCs) have remained limited. This is due, in part, to the difficulty of calibrating SPME devices for VHOCs. Here, we present an analytical procedure used to determine the distribution coefficients (Kf values) for a large suite of polychlorinated biphenyl (PCB) congeners and chlorinated pesticides between a polydimethylsiloxane (PDMS) phase (100 μm thickness) and seawater. Losses of analytes to sample containers and stirring bars were accounted for in the determination of Kf. The correlation between log Kf and log Kow, the octanol-water partition coefficient, was positively linear for PCB congeners with log Kow up to ~6.5, but became negatively linear for PCB congeners with log Kow >6.5. When grouped based on the number of chlorines (homolog), log Kf increased linearly with increasing log Kow for homologs 3-5 and decreased for homologs 6-10. These findings were inconsistent with existing data acquired using thinner PDMS coatings (7 and 15 μm), which exhibited a positively linear relationship between log Kf and log Kow for all PCB congeners. We postulate that the larger PCB congeners cannot readily sorb into the bulk PDMS phase, as would be required to maintain a consistent sorptive capacity for the thick 100 μm fiber coating. This effectively lowers the sorption capacity of PDMS-coated SPME devices for high molecular weight PCB congeners. This hypothesis contributes additional insight toward understanding the mechanism of SPME processes with PDMS phases. INTRODUCTION Solid-phase microextraction is an organics extraction method based on quantitation of analytes sorbed on a polymeric phase, often coated on a glass fiber, in contact with water or the headspace above the aqueous phase. Since the introduction of SPME as a quantitative analytical technique by Arthur and Pawliszyn (1990) more than a decade ago, a large amount of data has been obtained concerning the fundamental mechanisms governing the SPME processes and potential applications of SPME in a variety of research areas (Pawliszyn 1997, 1999, Heringa and Hermens 2003, Mayer et al. 2003). The success of SPME can be attributed to its simplicity of use, combination of extraction and concentration in a single step, relatively short sample processing times, minimal solvent use, and cost effectiveness. Successful development and implementation of a robust SPME-based method are strongly dependent upon a thorough understanding of the factors dictating the distribution of analytes between the SPME sorbent phase and sample matrix. Many SPMEbased methods have been developed for specific sample matrices only; therefore, their usefulness is limited. A more universal approach is to employ the coefficient of distribution (Kf) of an analyte between the sorbent phase and water for quantitation, with additional considerations of matrix effects if necessary. Consequently, accurate determination of Kf values becomes a critical step toward the development of a robust SPME-based method. While Kf values for organic compounds with low hydrophobicity are relatively easy to determine with precision, Determination of polydimethylsiloxane (PDMS)-seawater distribution coefficients for polychlorinated biphenyls and chlorinated pesticides by solid-phase microextraction