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

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 (K(ow)) and absorption-based fiber-water partitioning coefficients (K(dv)) for low-molecular-weight analytes failed to predict K(dv) values for PCBs. In fact, K(dv) values for PCBs were 1-7 orders of magnitude lower than those predicted by K(ow) and actually showed a strong negative correlation between K(ow) and K(dv) for higher molecular weight analytes (MW >∼200). K(dv) values obtained using PDMS fibers with 7- and 100-μm coatings also disagree, demonstrating that K(dv) 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 (K(ds)), the K(ds) 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.

SPME of 52 pesticides and polychlorinated biphenyls: Extraction efficiencies of the SPME coatings poly(dimethylsiloxane), polyacrylate, poly(dimethylsiloxane)-divinylbenzene, Carboxen-poly(dimethylsiloxane), and Carbowax-divinylbenzene

“…Nevertheless high molecular weight congeners like cb-180 with high octanol-water partition coefficients have small K fw . These results agree with those obtained by Yu Yang and co-workers [33] where K fw values were calculated for PDMS coating for different congeners from cb-8 to cb-209. Similar results were observed with a decrease in K fw for the high molecular congeners despite of the high log K ow .…”

Novel stir bar sorptive extraction methods for environmental and biomedical analysis

“…For a typical 100 m PDMS fiber, which is the most widely used fiber, the volume of the extraction phase is approximately 0.5 l. Consequently, the extraction efficiency for solutes that are partially water soluble is quite low [36]. For very polar compounds, however, competition can occur between the aqueous phase, the SPME fiber, the glass wall of the extraction vessel, and the surface of the polytetrafluoroethylene stir bar used to stir samples [37,38].…”

Study into the Equilibrium Mechanism between Water and Poly(dimethylsiloxane) for Very Apolar Solutes: Adsorption or Sorption?

“…PCBs (and also PAHs) are among the compounds with the lowest water solubility and are known to easily adsorb on glass or other surfaces (e.g., the stir bars used in SPME). We suspected this to be occurring in the experiments and data presented by Yang et al With increasing apolarity, compounds will adsorb more strongly on active surfaces and will increasingly be lost, which should explain the observed effect. It was already mentioned by Yang et al that the stir bars could not be reused since that causes contamination problems (by the adsorbed PCBs).…”

“…Although this correlation seemed to hold for a wide range of solutes, for high-molecular-weight and very apolar solutes such as polyaromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), the correlation between K O/W and K PDMS/W was no longer valid. For PCBs it was found that, with decreasing polarity (increasing K O/W of a factor of 4 × 10 4 ), the measured K PDMS/W decreased by a factor of 40, which would mean a misprediction in K PDMS/W of a factor 2 × 10 6 . The authors explained their data by assuming surface adsorption on the PDMS fibers instead of bulk partitioning (sorption) and emphasized by comparison of data from two PDMS fibers (7 and 100 μm) the validity of this model.…”