Lysinoalanine determination in sodium caseinate using the LKB Alpha Plus Amino Acid Analyser

Abstract

Adaptation of the LKB Alpha Plus Amino Acid Analyser to separate and detect lysinoalanine (LAL) in caseinates is described. The separation of LAL was achieved by cation exchange followed by post-column derivatisation with ninhydrin. The effect on the separation of varying the pH and molarity of the Na+ ion in the mobile phase and the column temperatures was investigated, and the information gathered from these tests was used to compile the optimum procedure for use on the Alpha Plus. An isocratic system using 0.2 M Na+ concentration in a citrate buffer with a pH range from 4.9 to 5.5 and column temperatures ranging between 48 and 56 degrees C was found to be the most suitable for the separation of LAL. Many ninhydrin-positive substances which elute close to the LAL peak are likely to cause inaccurate estimations of LAL concentration, and an awareness of the consistent profile of these in a caseinate is essential in the analysis. The method was validated by achieving a recovery of 100% (+/- 3%), limit of detection of 20 ppm and a repeatability relative standard deviation of 7%