CONTINUOUS-FLOW SPECTROPHOTOMETRIC DETERMINATION OF AMINO-ACIDS WITH 1,2-NAPHTHOQUINONE-4-SULFONATE REAGENT

A completely continuous-flow method for amino acid determination is pr oposed. The spectrophotometric detection is based on the reaction betw een amino acids and 1,2-naphthoquinone-4-sulphonic acid (NQS) in basic sodium carbonate-sodium hydroxide medium (pH 10). The reaction was de veloped on-line because NQS solutions are unstable in this medium. Phe nylalanine (Phe) was chosen as a model amino acid to develop the metho d. The influence on the absorbance of different variables (pH, buffer and reagent concentrations, temperature, reaction coil length and flow -rate) were studied. For Phe at 470 nm, the calibration graph was line ar up to 1.5 x 10(-4) M, the reproducibility was 2.5% and the limit of detection was 2 x 10(-6) M. The sample throughput was 40 h-1. Measure ments of reaction product spectra were carried out in the range 290-59 0 nm. Spectral differences for nineteen amino acids derivatives were s tudied by using principal component analysis and cluster analysis. On this bases a spectral classification can be made.