SOLID-PHASE MICROEXTRACTION AS A TOOL TO DETERMINE MEMBRANE WATER PARTITION-COEFFICIENTS AND BIOAVAILABLE CONCENTRATIONS IN IN-VITRO SYSTEMS/

Solid phase microextraction (SPME) is an extraction technique that use s a polymer-coated fiber as the extraction device. After extraction, t he compound of interest can be desorbed from the fiber and subsequentl y analyzed by GC or HPLC. One of the properties of SPME is that only t he freely dissolved fraction of a chemical is available for partitioni ng to the extraction device. The method can be applied in a way that s mall amounts are extracted from the sample, which allows negligible de pletion extraction. These two properties make SPME devices particularl y suitable for measurements of free concentrations. In toxicological s tudies the free concentration is considered to be a more relevant para meter, concerning toxic effects, than the nominal concentration that i s used most frequently. In the current study, the usefulness of this m ethod to measure phospholipid/water partition coefficients and free co ncentrations in three different in vitro test systems (rat hepatocytes in primary culture, 9000g and 100000g homogenate fractions of rainbow trout liver) was demonstrated. Results show separate relationships be tween phospholipid/water and n-octanol/water partition coefficients fo r a set of polar and nonpolar organic chemicals, respectively. These o bservations suggest that phospholipid/water partition coefficients may be a more suitable parameter in modeling the kinetic behavior of orga nic chemicals. Additionally, differences between the nominal and the a ctual free concentration in in vitro systems are more pronounced for m ore hydrophobic compounds, as was expected based on theoretical consid erations. To our knowledge, the approach presented here is the first a nalytical method to measure toxicologically relevant concentrations in in vitro test systems in a fast and efficient way.