Determination of octanol–water partition coefficients using a micro-volume liquid-liquid flow extraction system Original Research Article

An extraction system consuming less than 1 μl of sample and organic solvent (combined volume) has been developed for the determination of 1-octanol–water partition coefficients (log Kow). The system comprises a capillary, a micro-volume stepper piston pump, a peristaltic pump and a detector. The stepper piston pump is programmable, enabling full control and selection of each step in the pumping sequence. Coupled to the pump is a 250 μm i.d. silica capillary, extending 240 mm from its aperture to the detector cell which houses an on-capillary spectrophotometric detector allowing direct monitoring of both the aqueous sample and the organic solvent plugs. Programmed portions of air, aqueous sample, organic solvent and a second portion of air are sequentially introduced into the capillary from respective vials. Typically, 250 nl of both the aqueous sample and organic solvent were introduced in the trials reported here. Since all liquids are aspirated directly into the capillary there is no need to pre-flush the system (as is required in most other continuous flow systems). Thus, the entire portions of the sample and the organic solvent are used in the analysis. The analytical procedure used for determining octanol–water partition coefficients is essentially an automated and miniaturised version of the batch method recommended in the OECD manual. The log Kow values obtained agreed well with literature data and/or manual batch determinations. Typical analysis time was 4 min when applying a mean flow rate of about 4 μl min−1, including a wash cycle between the samples. The system is suitable for screening purposes and could readily be automated using a robot.