Determination of methionine by gas–liquid chromatography: Methods for the elimination of an interfering component present in some fish meals

Abstract

Intact methionine residues in food proteins, including a limited number of fish meals, have been determined using gas chromatography to measure methyl thiocyanate released after CNBr reaction. However, methionine values for certain fish products determined by that method have been found to be low and variable compared with those determined by ion-exchange chromatography after performic acid oxidation. This was traced to the presence of a component of many fish meals that, when reacted with CNBr, gave a product which co-eluted with the internal standard, ethyl thiocyanate. A method was developed to eliminate this interference and allow methionine determinations to be carried out using the same gas chromatographic procedure.