The behavior of o-phthaldialdehyde (OPA)/3-mercaptopropionic acid (MPA) and OPA/Nacef,/I-L-cysteine (NAC) derivatives of amino acids (AAs) have been studied from analytical view point, based on their UVand fluorescence responses, simultaneously. The particular aim was to clarify the background of the believed instabilil',/of AAs, that give more than one OPAderivative, such as glycine, y-amino bu f,/ric acid (GABA), 13-alanine, histidine, ornithine and lysine. To confirm the stoichiometric correlation and to improve reproducibilil",/of interaction the mole ratios of OPA to the SH-group-containing additive, the concentration of reactants, pH of injected analyte, temperature of interaction and reactivil",/of substituted AAs toward the OPA/MPA reagent have been investigated. Varying the mole ratios of [OPAl to [MPA] and [OPAl to [NAC], it is shown that the higher the concentration of SH-group-containing additive the lower the transformation rate of the initially formed OPA-derivative to those forthcoming. Performing all reactions with the same mole ratios, at higher concentrations resulted in increased amounts of the succesively-formed OPA-derivatives. Decreasing the pH of OPA-AAs from 9.3 to 7.2, before injecting them onto the column, lead to considerable loss of response. Increasing the temperature of reactions from 4 ~ to 30 ~ resulted in increased decomposition rate of derivatives. The stabilil',/, and hence reproducibilil",/of reactions can be improved (i) by applying the the same mole ratios of OPA and SH-group-containing additive, (ii) by injecting the analyte of derivatives at the derivatization pH, and, (iii) performing reactions with refrigerated stocksolutions and maintained before injection possible at4 ~
