Ion-pair reversed-phase liquid chromatography for determination of polar underivatized amino acids using perfluorinated carboxylic acids as ion pairing agent

An isocratic liquid chromatography method using volatile ion pairing agent and evaporative light scattering detection was developed for the analysis of 10 underivatized amino acids (Asp, Asn, Ser, Gly, Gin, Cys, Glu, Thr, Ala, Pro) which are not sufficiently retained and resolved in the LC system commonly used. Five perfluorinated carboxylic acids (trifluoroacetic, heptafluorobutyric, nonafluoropentanoic, tridecafluoroheptanoic and pentadecafluorooctanoic acid) have been evaluated as ion-pairing agents in various concentrations (0.25-1 mM) using three analytical reversed-phase columns (LiChrospher RP-Select B, Purospher RP-18e and Supelcosil ABZ(+) Plus). The quantities adsorbed on the chromatographic supports and the equilibration times of each system are also studied. Excellent separation with satisfactory baseline resolution was achieved on the chromatographic system made up of a Purospher RP-18e and an aqueous mobile phase containing pentadecafluorooctanoic acid at 0.5 mM. (C) 1999 Elsevier Science B.V. All rights reserved.