Determination of oxalic acid in herbal medicines by semi-micro hydrophilic interaction liquid chromatography coupled with electrochemical detection

In this study, the determination of oxalic acid in herbal medicines was performed by using a hydrophilic interaction liquid chromatography coupled with electrochemical detection (HILIC–ECD) method. A semi-micro column packed with amide–silica particles and an acetonitrile–30 mM phosphate buffer (pH 6.8) mixture (65:35, v/v) were used as the stationary and mobile phases, respectively, in the HILIC–ECD. A potential of + 1.1 V vs. Ag/AgCl was applied to a glassy carbon working electrode. The ratio of the peak height of oxalic acid to that of the internal standard (synephrine) was proportional to the concentration of 0.45 μg L −1 to 1.8 mg L −1 with a correlation coefficient of 0.999. The detection limit (signal-to-noise ratio, S/N = 3) of oxalic acid was 0.17 μg L −1 . By the HILIC–ECD, the oxalic acid content in crude drugs and Kampo medicine extract granules (Zingiberis Rhizoma Processum, Pinelliae Tuber, Sho-seiryu-to, Hange-shashin-to, etc.) were determined with less than 2.9% relative standard deviation (RSD, n = 6), and their recoveries were more than 88.7% with less than 3.3% RSD ( n = 6). In conclusion, we demonstrated that the HILIC–ECD performed measurements that were quite selective, accurate, and precise for the determination of oxalic acid in herbal medicines. Graphical abstract