Simultaneous Measurement of Free and Total Concentrations of Hydrophobic Compounds

Hydrophobic contaminants partition between water and sorbent phases such as dissolved organic matter (DOM) in environmental matrices. Consideration of bioavailability increasingly requires the measurement of freely dissolved concentration (C-free) in addition to the total chemical concentration (C-total). However, current practices require the use of separate methods and samples to derive C-free and C-total, and no method affords simultaneous measurement of both concentrations. We employed a method by coupling solid-phase microextraction (SPME) with the use of an isotope-labeled internal standard and gas chromatography-mass spectrometer (GC-MS) for detection. The method was demonstrated using 8 pyrethroids as model compounds and C-13-cis-permethrin as the labeled internal standard. Analysis of 4 spiked sediment porewater and ;4 field-contaminated samples showed that C-total obtained by this method was in close agreement with the spiked concentrations or the total concentrations measured after whole-sample solvent extraction, while C-free was only a small fraction of C-total, suggesting pronounced association of these compounds with DOM. The method displayed good reproducibility with average relative standard deviation <20%, and high sensitivity with method detection limits in the range of 1-5 ng L-1. The method uses a small sample volume (<20 mL, as opposed to 1 L for conventional analysis), and is solventless and fast. The concurrent measurements also allow for estimation of K-DOC, a key parameter that is difficult to derive otherwise. The simultaneous determination of C-free and C-total provides information for better relating chemical concentrations with potential ecotoxicological effects.