Determination of alpha-dialkylamino acids and their enantiomers in geological samples by high-performance liquid chromatography after derivatization with a chiral adduct of o-phthaldialdehyde.

Derivatization with o-phthaldialdehyde (OPA) and the chiral thiol N-acetyl-L-cysteine (NAC) is a convenient and sensitive technique for the HPLC detection and resolution of protein amino acid enantiomers. The kinetics of the reaction of OPA-NAC with alpha-dialkylamino acids was investigated. The fluorescence yield of alpha-dialkylamino acids was only about 10% of that of protein amino acids when the derivatization was carried out at room temperature for 1-2 min, which is the procedure generally used for protein amino acid analyses. The fluorescence yield of alpha-dialkylamino acids can be enhanced by up to ten-fold when the derivatization reaction time is increased to 15 min at room temperature. The OPA-NAC technique was optimized for the detection and enantiomeric resolution of alpha-dialkylamino acids in geological samples which contain a large excess of protein amino acids. The estimated detection limit for alpha-dialkylamino acids is 1-2 pmol, comparable to that for protein amino acids.