MIXTURE DESIGNS APPLIED TO THE STUDY OF THE LIQUID-CHROMATOGRAPHIC SEPARATION OF L-PHENYLALANINE, L-TYROSINE AND L-TRYPTOPHAN

Mixture designs were used to improve the separation of L-phenylalanine, L-tyrosine and L-tryptophan on a C18 bonded stationary phase. The pH of the mobile phase and retention time of each amino acid were measured for 35 mobile phase formulations containing various concentrations of H3PO4, NaH2PO4, NaClO4, sodium 1-octane-sulfonate and methanol. A non-linear six-parameter model was used to describe the measured pH as a function of the eluent composition with a standard error of estimate of 0.10 pH unit. A different non-linear ten-parameter model was used to describe the measured retention time as a function of eluent composition for each of the three amino acids with a standard error of estimate of 0.08 min. This last model was then used to predict the composition for which the separation would be optimum. The optimum separation was confirmed experimentally.