On-line preconcentration of fluorescein derivatives of essential amino acids using sweeping in a nonionic micellar system

A rapid and effective method of nonionic micellar electrokinetic chromatography with laser-induced fluorescence detection and sweeping as an online preconcentration method was developed for the simultaneous determination of free essential amino acids in supplement formulations. The analytes were derivatized using fluorescein isothiocyanate isomer I dissolved in dimethyl sulfoxide. Optimal separation conditions were achieved using an uncoated fused silica capillary (40.0 cm effective length, 50.2 cm total length, 50 mu m internal diameter), 20 mM Na2B4O7, (pH 9.3), 50 mM Brij-35, and 15% (v v(-1)) methanol as running buffer. Complete separation of all analytes was achieved within 15 min. The optimized method exhibited good linearity (r2 > 0.997), acceptable precision (1.3 - 12.4%), and accuracy (88 - 112%) in concentration range 20 - 1000 ng mL(-1). The limit of detection was 6 ng mL(-1). The method successfully determined the quantity of free essential amino acids in a dietary supplement containing spirulina.