Spheri-5, PerkinElmer, Waltham, Massachusetts, USA). Isocratic chromatographic separation was carried out using a mobile phase of Milli-Q water with acetic acid (0.1 mL/100 mL) and methanol in a relative proportion of 95:5 (mL:mL). The eluent flow-rate was 0.7 mL/min, and the column temperature was 30 C. Ascorbic acid was identified by comparing the retention time of the sample peak with that of the ascorbic acid standard at 254 nm. Quantification was carried out using external standardization