DETERMINATION OF AMINO-ACIDS BY ION-PAIR LIQUID-CHROMATOGRAPHY WITH POSTCOLUMN DERIVATIZATION USING 1,2-NAPHTHOQUINONE-4-SULFONATE

A new chromatographic method for the determination of amino acids is p roposed. The method is based on the separation of amino acids by means of ion-pair liquid chromatography and post-column derivatization usin g 1,2-naphthoquinone-4-sulfonate. The analytical column was a Spheriso rb ODS 2. Amino acids were separated by an elution gradient with four linear steps based on increasing the concentration of 2-propanol. Two eluents were used to create the gradient profile: eluent A was an aque ous solution of 20 mM H3PO4 + 20 mM H2PO4- + 15 mM dodecyl sulfate and eluent B was a mixture of aqueous (25 mM H3PO4 + 25 mM H2PO4- + 18.5 mM dodecyl sulfate)-2-propanol (1:1, v/v). The injection volume was 10 0 mu l and the total flow-rate for the mobile phase was 0.8 ml/min. Th e chromatographic outlet was coupled on-line to the two-channel deriva tization system which delivered reagent and buffer solutions. The reac tion took place at 65 degrees C in a reaction coil of 4 m x 1.1 mm I.D . The spectrophotometric detection was performed at 305 nm. The separa tion of common amino acids was done in 90 min, although an additional period of 15 min was required to stabilize the column. The repeatabili ty of the method for lysine is 2.1% and the reproducibility is 2.6%. T he detection limit for lysine is 0.09 nmol. The linear range for lysin e is up to 32 nmol with a correlation coefficient of 0.999. The method was applied to the determination of amino acids in animal feed and po wdered milks. The results of the method are in good agreement with tho se obtained with the standard amino acid autoanalyzer method.