A method for analysing amino acids and reagent for use with the same

A method for analysing one or more nitrogen-containing compounds, in which the one or more nitrogen-containing compounds are contacted with hydrindantin at an elevated temperature in a contact zone is provided, the method comprising contacting ninhydrin with one or more reducing agents in a heating zone at a first elevated temperature to produce a hydrindantin-containing mixture; and introducing the hydrindantin-containing mixture into the contact zone and contacting the hydrindantin-containing mixture with the nitrogen-containing compounds at a second elevated temperature. The method is particularly suitable for the analysis by visualisation of amino acids.