Separation of Free Amiimo Acids on Reverse Stationary Phases Using an Alkyl Sulfonate Salt as a Mobile Phase Additive

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Abstract
Alkylsulfonate (RSO3 −) salts were evaluated as mobile phase additives for the separation of free amino acids on reverse stationary phases using an acidic mobile phase where the amino acids are cations. The enhanced amino acid retention is the result of two major interactions, one being retention of the RSO3 − salt on the stationary phase and the other an ion exchange selectivity between the amino acid analyte cation and the RSO3 − countercation, or other countercations in the mobile phase. Major mobile phase variables are: type and concentration of RSO3 − salt (the studies focused on C8SO3 − salts), presence of organic modifier, type of countercation present, and mobile phase pH and ionic strength. Alkyl modified silica and polystyrenedivinyl-benzene copolymeric reverse stationary phases were compared. A mobile phase gradient, increasing per cent organic modifier was shown to be best, is necessary for separating complex mixtures of polar and nonpolar or basic amino acids. The procedure is applicable to the identification and/or determination of amino acids in mixtures or in peptides after hydrolysis.