Solvent microextraction with simultaneous back-extraction for sample cleanup and preconcentration: Quantitative extraction

A solvent microextraction technique has been developed to perform simultaneous forward- and back-extraction across a microliter-size organic liquid membrane. The organic liquid membrane phase (o), consisting of 40 or 80 mu L of n-octane, is layered over 0.5 or 1.0 mL of aqueous sample phase (al) contained in a 1- or 2-mL microreaction vial and is stabilized against mechanical disruption by a small Teflon ring, even when the al phase is stirred at a speed of 2000 rpm. A 0.1- or 0.2-mL aqueous receiving phase (a2) is layered over the o phase. After ex-traction for a prescribed time, an aliquot of the a2 phase is injected directly into an HPLC for quantification. The technique is efficient and selective for ionizable compounds. In 30 min, the model compounds, mephentermine and 2-phenylethylamine, in the al phase buffered at pH 13 are 100% and 90% extracted, respectively, into the a2 phase buffered at pH 2.1. A kinetic model has been developed, based on the Whitman two-film theory, to describe the extraction process and has been verified experimentally, The technique is promising for routine applications because of its simplicity, reproducibility, selectivity, overall sample preparation time, and quantitative extraction of compounds with relatively small distribution coefficients with submilliliter volumes of aqueous back-extractant.