Selective preconcentration of amino acids and peptides using single drop microextraction in-line coupled with capillary electrophoresis

Single drop microextraction (SDME) can be in-line coupled with capillary electrophoresis by attaching a drop to the tip of a capillary. With a 2-layer drop comprised of an aqueous basic acceptor phase covered with a thin organic layer, acidic analytes in an aqueous acidic donor phase can be extracted into the organic layer and then back-extracted into the acceptor phase. However, preconcentration of amino acids and peptides by SDME is difficult since their zwitterionic properties prevent them from being partitioned in the middle organic phase. When amino acids were derivatized with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F), amino acids without a charged side chain were converted to carboxylic acids. In the acidic donor phase, those NBD-amino acids were predominantly neutral and they were successfully concentrated into the basic acceptor phase. In the meantime, amino acids with a charged side chain after NBD-F derivatization were not concentrated via SDME. With this selective SDME, we were able to extract acidic and neutral amino acids obtaining several hundred-fold enrichments within 5 min at 25 °C, while leaving basic amino acids—Arg, Lys, and His—in the acidic donor phase. Furthermore, detection sensitivity was enhanced by employing laser-induced fluorescence detection. We then applied this technique to the selective concentration of peptides.