Mh. Ma et Ff. Cantwell, SOLVENT MICROEXTRACTION WITH SIMULTANEOUS BACK-EXTRACTION FOR SAMPLE CLEANUP AND PRECONCENTRATION - QUANTITATIVE EXTRACTION, Analytical chemistry (Washington), 70(18), 1998, pp. 3912-3919
A solvent microextraction technique has been developed to perform simu
ltaneous 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 s
ample phase (al) contained in a 1- or 2-mL microreaction vial and is s
tabilized against mechanical disruption by a small Teflon ring, even w
hen the al phase is stirred at a speed of 2000 rpm. A 0.1- or 0.2-mL a
queous receiving phase (a2) is layered over the o phase. After ex-trac
tion for a prescribed time, an aliquot of the a2 phase is injected dir
ectly into an HPLC for quantification. The technique is efficient and
selective for ionizable compounds. In 30 min, the model compounds, mep
hentermine and 2-phenylethylamine, in the al phase buffered at pH 13 a
re 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 be
cause of its simplicity, reproducibility, selectivity, overall sample
preparation time, and quantitative extraction of compounds with relati
vely small distribution coefficients with submilliliter volumes of aqu
eous back-extractant.