Solvent microextraction with simultaneous back-extraction for sample cleanup and preconcentration: Preconcentration into a single microdrop

A preconcentration technique which employs a microliter-size liquid membrane has been developed in order to obtain a large enrichment factor (EF) in a short time. The 30-mu L n-octane liquid membrane, confined inside a small Tenon ring, is layered over 1.60 mL of aqueous source phase (sample), which is buffered at pH 13, The receiving phase is either a 1.0(0)- or a 0.50-mu L aqueous drop buffered at pH 2.1, that is suspended in the 30-mu L membrane phase directly from the tip of a microsyringe needle. When the sample solution is stirred, basic analytes are extracted into the organic membrane phase and back-extracted simultaneously into the microdrop because they are neutral at high pH and protonated at low pH. After extracting for a prescribed time, the microdrop is taken back into the syringe needle and injected directly into an HPLC for quantification. in 15 min, the EFs in the 1.0(0)-mu L receiving drop are about 500 for methamphetamine, mephentermine, and methoxyphenamine, and about 160 for 2-phenylethylamine. Enrichment factors are approximately doubled for the same 15-min extraction time by using a 0.50-mu L receiving drop. The reasons for this significant increase in EF with decreasing drop volume are discussed on the basis of a previously derived kinetic model.