Continuous and automated slug flow nanoextraction for rapid partition coefficient measurement

Octanol-water partition coefficients (log K_ow) are widely used in pharmaceutical and environmental chemistry to assess the lipophilicity of compounds. Traditionally log K_ow is determined using a shake-flask method that uses milliliters of sample and solvent and requires hours for preparation, extraction, and analysis. Here, we report an automated system for rapid log K_ow determination for an array of compounds using slug flow nanoextraction (SFNE) enabled by a microfluidic chip. In the method, an autosampler is used to introduce 1 μL of sample into a microfluidic device that segments the injected volume into a series of 4 nL slugs that are each paired to an adjacent octanol slug. Each octanol-water phase pair is compartmentalized by an immiscible fluorous carrier fluid. During flow, rapid extraction occurs at each octanol-water interface. The resulting linear array of slugs flows into an online UV absorbance detector that is used to determine concentrations in the phases, allowing the log K_ow to be measured. The microfluidic device allows toggling between two-phase "aqueous plug" generation (aqueous sample separated by fluorous carrier fluid) and three-phase "phase pair" generation. In this way, online calibration for detection in the aqueous phase can be achieved. The method is applied to determining log K_ow for a panel of seven pharmaceutical compounds, including complete calibration curves, at three different pHs in under 2 h using 5 μL of extraction standard and 2.9 μL of octanol per extraction standard analyzed.