Improved method for elimination of the interference from nitrate ions on oxalic acid in RP-HPLC by solid-phase extraction with nanosized LaPO4

An improved method using nanosized LaPO₄ as solid-phase extractor for avoiding the interference from nitrate ions on oxalic acid in RP-HPLC was developed. Nanosized LaPO₄ possessed lower solubility and higher adsorption ability for oxalic acid than previous adsorbent. Oxalic acid could be adsorbed selectively by nanosized LaPO₄ rather than nitrate, formic acid, malic acid, lactic acid, or citric acid, so the interference from nitrate ions was eliminated, and then the oxalic acid was determined by RP-HPLC at the optimum chromatographic conditions after the solid-phase extraction procedure. The accuracy of the method was confirmed with an average recovery ranging between 97.7% and 107.3%, the relative standard deviations (RSD) were less than 3.0%. This method improved the previous work and was successfully applied to determine the oxalic acid in xylem saps of tomato.