Preconcentration in micro-electromembrane extraction across free liquid membranes

Preconcentration potential of micro-electromembrane extraction (μ-EME) across free liquid membrane (FLM) was examined with an anionic and a cationic dye, 4,5-dihydroxy-3-(p-sulfophenylazo)-2,7-naphthalene disulfonic acid, trisodium salt (SPADNS) and phenosafranine, respectively. For the first time, it was shown that the spatial flexibility of FLMs enabled application of tailored extraction units with mutually different shapes and migration cross-sections for FLMs, donor and acceptor solutions. Thus, e.g. conical units enabled easy and reproducible formation of a three-phase extraction system (donor/FLM/acceptor) with sub-μL volumes of acceptor solutions as well as rapid and highly efficient preconcentration of the two dyes. Quantitative measurements of resulting solutions were carried out by UV–vis spectrophotometry and enrichment factors of up to 98 were achieved for μ-EMEs of 20 μM SPADNS (50 μL) preconcentrated into 0.5 μL of pure water across 1-pentanol at −150 V for 18 min. Visual monitoring of the entire extraction process (with USB microscope camera) was possible across transparent extraction units, moreover, important extraction parameters, such as FLM dimensions and donor-to-acceptor solution volume ratio, which determine the mechanical stability of the membrane and maximum enrichment factor, respectively, were readily adjusted. Combination of μ-EME across FLMs with capillary electrophoresis (CE) was further shown suitable for preconcentration and determination of perchlorate in drinking water samples. Good repeatability of the μ-EME-CE method (RSD values better than 9.5%), linear relationship for the analytical signal vs. concentration (r² better than 0.997) and enrichment factors of up to 30 were achieved for μ-EMEs of perchlorate across 1-pentanol and 1-hexanol based FLMs.