RESULTS AND DISCUSSIONOptimization

RESULTS AND DISCUSSIONOptimization of CE separation conditionsEffect of buffer type, pH and concentrationIn order to improve the resolution and sensitivity for the ten amino acids, borate, phosphate, and borate-phosphate mixture were investigated as running buffer in the CE separation. It was found that the separation effect of borate-phosphate mixture buffer was the best.Changing pH has an impact on the separation efficiency and selectivity. Therefore, the optimization of pH of the borate-phosphate mixture buffer could further improve the separation efficiency of analytes. Herein, the influence of buffer pH on the separation of OPA￾amino acids was evaluated from 6.0 to 10.0. The results of different buffer pH conditions were illustrated in Fig. 1. As shown, when pH of the running buffer was 9.5 without β-CD, most amino acids could achieve baseline separation, except for L-glutamine and L-asparaginate. Therefore, the pH of the running buffer was set to 9.5, considering the separation efficiency of the further experiment.In CE, the concentration of buffer can significantly affect the separation of analytes. When the pH of the buffer solution was 9.5, the effect of the concentration of the buffer on the separation was examined in the range of 25-80 mmol L-1. In all cases, an applied voltage of 15 kV was used. According to the experiments results, the migration order of each amino acid slightly changed, but the resolution did not improve as the concentration of the buffer increased. Moreover, the migration time became longer. The experimental results showed that the concentration of borate-phosphate mixture buffer of 50 mmol L-1 was used as the optimum condition