Determination of ninhydrin positive substances in sea-water and hemolymph

Abstract

A novel method for the determination of ninhydrin positive substances (NPS) is described, which involves the ninhydrin reaction with α-amino acids and the subsequent oxidation of the complex to liberate ammonia, which is measured using an inexpensive flow injection–gas diffusion technique. Calibration was linear up to a concentration of 60 µmol l^–1 alanine, with a limit of detection of 0.3 µmol l^–1 alanine (relative standard deviation, s_r < 2%, n= 6). Because all the NPS are converted to ammonia by this method, interference from the sea-water matrix on the ninhydrin complexes is eradicated.