Selectivity control in the separation of aromatic amino acid enantiomers with sulphated β-cyclodextrin

Control of selectivity in the enantiomeric separation of three aromatic amino acids (phenylalanine, tyrosine and tryptophan) is demonstrated by electrokinetic capillary chromatography utilising temperature variations coupled with the use of sulphated-β-cyclodextrin (s-β-CD) as a pseudostationary phase. The concentration of s-β-CD and temperature were used as experimental variables to control the observed selectivity. A double-coated capillary was used and proved very robust with reproducibility of migration times being <2.0% R.S.D. between runs and <2.6% on using a new capillary. The system was modelled successfully using an artificial neural network (ANN) comprising one input layer, two hidden layers and one output layer. The model accurately described the observed separations with a correlation coefficient of 0.999 being observed between predicted and observed migration times. Selectivity optimisation was achieved using the normalised resolution product and minimum resolution criteria, with both providing optima at different experimental conditions. The selectivity changes observed also allowed the estimation of electrolyte temperatures within the capillary at high operating currents (>100 μA). Using a 50 μm i.d. capillary and an electrolyte comprising 20 mM phosphate and 15 mM s-β-CD, a temperature of 52 °C was calculated within the capillary at an applied voltage of +30 kV.