Isolation and desalting with cation-exchange chromatography for compound-specific nitrogen isotope analysis of amino acids: application to biogeochemical samples.

We have established a procedure for removing interfering materials from extracts of geological and biological samples, in order to determine precise compound-specific nitrogen isotopic compositions of amino acids. We employed cation-exchange chromatography of protein and non-protein amino acids prior to derivatization for gas chromatographic separation. The average recovery of a standard amino acid solution was better than 94%, without nitrogen isotope fractionation during the cation-exchange chromatography. We applied the procedure to various environmental samples including 'hard' (calcareous, siliceous, rock and sediment samples) and 'soft' materials (aggregated microbial samples and biological soft tissue samples). We conclude that cation-exchange chromatography is a pre-treatment procedure which should be widely useful for the determination of compound-specific nitrogen isotopic compositions of amino acids.