Solid phase microextraction (SPME) is an extraction technique that uses a polymer-coated fiber as the extraction device. After extraction, the compound of interest can be desorbed from the fiber and subsequently analyzed by GC or HPLC. One of the properties of SPME is that only the freely dissolved fraction of a chemical is available for partitioning to the extraction device. The method can be applied in a way that small amounts are extracted from the sample, which allows negligible depletion extraction. These two properties make SPME devices particularly suitable for measurements of free concentrations. In toxicological studies the free concentration is considered to be a more relevant parameter, concerning toxic effects, than the nominal concentration that is used most frequently. In the current study, the usefulness of this method to measure phospholipid/water partition coefficients and free concentrations in three different in vitro test systems (rat hepatocytes in primary culture, 9000 g and 100,000 g homogenate fractions of rainbow trout liver) was demonstrated. Results show separate relationships between phospholipid/water and n-octanol/water partition coefficients for a set of polar and nonpolar organic chemicals, respectively. These observations suggest that phospholipid/water partition coefficients may be a more suitable parameter in modeling the kinetic behavior of organic chemicals. Additionally, differences between the nominal and the actual free concentration in in vitro systems are more pronounced for more hydrophobic compounds, as was expected based on theoretical considerations. To our knowledge, the approach presented here is the first analytical method to measure toxicologically relevant concentrations in in vitro test systems in a fast and efficient way.

The solid phase microextraction technique uses polymercoated fused-silica fibers to extract organic chemicals from an aqueous or gaseous phase. In the current paper, the partitioning behavior of organic chemicals from water to polyacrylate coating is described in terms of a twocompartment model and first-order kinetics. Experimental results show that, through agitation, it is possible to achieve a sufficiently small aqueous diffusion layer around the fiber to prevent aqueous diffusion from being the limiting factor in the absorption process. This then implies that the equilibration time is completely determined by the polyacrylate phase. In addition, the kinetic rate constants derived from the experimental results, viz. the uptake rate constant and polyacrylate-water partition coefficient, are modeled by multivariate techniques, using physicochemical and quantum chemical descriptors. These models clearly show that, besides hydrophobicity expressed as the octanol-water partition coefficient, the energy of the lowest unoccupied molecular orbital (and related properties) and the most positive charge on any hydrogen atom in the molecule are important descriptors. This indicates that hydrogen bonding plays a significant role in polyacrylate-water partitioning. The models that are presented can be used to predict absorption profiles of organic chemicals to polyacrylate-coated fibers, thereby giving the opportunity to predict the kinetics (including equilibration times) by computations alone.

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