Stability of o-phthalaldehyde-sulfite derivatives of amino acids and their methyl esters: electrochemical and chromatographic properties.

RP-HPLC coupled with 16-channel coulometric electrode array detection was used to monitor the decomposition of five amino acid o-phthalaldehyde (OPA)-sulfite derivatives (Ala, Arg, Glu, Ser, Tyr) and their methyl ester derivatives as well. At fixed OPA and sulfite concentrations inclusion of methanol and EDTA in the derivatization media has increased most effectively the room temperature stability of both derivatives measured at pH 9.2 (amino acids) and pH 8.2 (methyl esters). Decreases in product concentrations by 6% have occurred after more than 15 h for amino acid derivatives and 8 h for methyl ester derivatives. The oxidation potential maxima for OPA-sulfite derivatives of amino acids were found at 600 mV while the same methyl ester derivatives had 60-120 mV higher maxima with the exception of tyrosine. The detector responses were found to be linear in the studied 0.1-10 microM concentration range for both derivative forms and their detection limit was 100-200 fmol injected on the column. The RP-HPLC retention of amino acid methyl ester OPA-sulfite derivatives was very similar to the amino acid OPA-2-mercaptoethanol ones while the more polar amino acid OPA-sulfite derivatives were eluted earlier (k' < 1) under the same chromatographic conditions.