Multicomponent determinations using addition-generated reagent profiles and partial least squares regression

The use of addition-generated reagent profiles is proposed for multicomponent determinations when the reactions between the reagent and analytes are slow. The oxidation of cysteine, methionine and homocysteine with dichromate in acidic medium has been used as the chemical system for experimental verification of some theoretical suppositions. The methodology tries to make the most of the differences in the kinetic behaviour. The continuous addition of dichromate (the reagent) to the sample including the amino acids provides the experimental data when the visible signal from dichromate is followed along time with a diode array spectrophotometer. The whole signal-time profile for the reagent is then used to characterize the sample, and partial least squares regression (PLSR) has been applied as calibration algorithm. Concentrations of amino acids between 10−4 and 10−3 M have typically been used. In the present case, mean errors increase from individual determinations (between 2 and 5%) to binary (between 4 and 8%) and ternary mixtures (between 6 and 13%). This tendency can be expected to be of general application. Some validation with pharmaceutical products has been accomplished. A critical discussion on the practical interest of this application is finally included.