The oxidative coupling reactions of p-aminophenol with m-and p-aminophenol in the presence of octacyanomolybdate(v) were used as the basis of a differential kinetic method for the simultaneous determination of 0-, m-and p-aminophenol (0-, m-and p-AP) in binary and ternary mixtures. The influence of MoV concentration, acidity, ionic strength and temperature was studied in order to establish optimum conditions. Several oxidants, including Ce4+, CrV1, MnVI1, HzOz and metaperiodate and Fell', Mn'", MoV and WV cyano complexes were tried. The MoV cyano complex was chosen because it has a suitable redox potential and gives rise to kinetically useful runs. The reaction must be implemented under sub-stoichiometric concentrations of oxidant and so the concentration of each aminophenol in the mixture influences the reaction of the others, originating in a system with a high degree of non-linearity. As a result, most methods of data treatment cannot be used and multivariate calibration (PLS) was applied to the data acquired by a diode-array spectrophotometer between 340 and 500 nm. PLS calibration with a number of factors ranging between six and nine allowed satisfactory results for all kinds of mixtures. Aminophenols can be determined over the concentration range 0.1-1.0 mmol 1-1 for o-AP, 0.3-1.0 mmol l-1 for m-AP and 0.25-3.0 mmol l-1 for p-AP with errors of less than 10% in all instances. No systematic errors were detected.

The slow oxidation reactions of cysteine, tyrosine and tryptophan by hexacyanomanganate(IV) were used to investigate partial least squares calibration (PLS) as a method to be applied to the kinetic determination of individual species. All the three reactions are first order in hexacyanomanganate(IV) concentration and the reactions with cysteine and tyrosine are first order in amino acid concentration. The stoichiometry was studied in each case. The determination results were always compared with those obtained by applying the initial rate method to the same kinetic runs. When suitable kinetic curves are obtained and the initial rate is easily determined, both methods give similar results; however, when the initial rate is not easily determined (too fast reaction, slope that changes very quickly, etc.) the results obtained with PLS are better (better precision and wider dynamic range). Moreover, and in contrast to the initial rate method, PLS allowed tryptophan to be determined in the presence of some different feed matrices, provided the calibration set was of the same nature as the samples. This advantage is of general application and makes the analytical method more robust. PLS was tested on uncatalysed reactions but it could equally be applied to catalysed reactions.

Binary and ternary mixtures of cysteine, tyrosine and tryptophan were used to show how kinetic methods together with partial least squares regression (PLSR) can be used to overcome interferences in multicomponent determinations. Two methods were compared: an equilibrium method which uses the characteristic UV spectra of amino acids and a kinetic method which follows the reaction kinetics of the oxidation of the amino acids by hexacyanomanganate(iv). In both cases, multi-wavelength detection with a diode-array spectrophotometer was used. The results obtained from the two methods are critically compared. Lower concentrations of tyrosine and tryptophan can be determined by the equilibrium method because of their high absorptivities. On the other hand, the kinetic method, in conjunction with PLSR, can be considered an example of how selectivity can be improved. This allowed the resolution of mixtures of tyrosine and tryptophan in the presence of several complex spiked matrices (aqueous extracts of feed samples that interfere to different extents) without any previous separation.

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