Sensitive analysis of amino acids with carrier-mediated single drop microextraction in-line coupled with capillary electrophoresis

In order to analyze amino acids sensitively without derivatization, we have developed carrier-mediated single drop microextraction (SDME). Nonane-1-sulfonic acid was added to an acidic sample donor solution as a carrier to form neutral ion pair complexes with amino acids. The ion pair complexes were extracted to the organic phase, covering a drop of an aqueous basic acceptor phase hanging at the tip of a capillary, and then back-extracted to the basic acceptor phase, where both the amino acids and the carrier have negative charges and the ion pair complexes are broken. The resulting extract of enriched amino acids was injected into the capillary and analyzed by capillary electrophoresis. With 20-min SDME with agitation of the donor phase, enrichment factors of four aromatic amino acids were up to 120-fold, yielding the LOD of 70–500 nM. The linear dynamic ranges for corrected peak areas were 1–100 μM with linear correlation coefficients larger than 0.9959. With internal standardization, the intraday RSDs of migration times and corrected peak areas were 0.01–0.04% and 2.0–3.7%, respectively. The capabilities of sample cleanup including desalting and preconcentration of carrier-mediated SDME were demonstrated with the analysis of human urine after minimal pretreatment of acidification and centrifugation.