Rock phosphate as a novel solid-phase extraction adsorbent for analysis of oxalic acid in plant xylem saps by RP-HPLC

A simple and effective method for elimination the interference from nitrate ions on oxalic acid in RP-HPLC by solid-phase extraction with rock phosphate was developed. Rock phosphate is a cost-effective and environmentally friendly mineral material; it exhibited good adsorption ability for oxalic acid, other than nitrate ions, malic, or citric acid; therefore, rock phosphate could be used as an excellent solid-phase extractor for solving interference from nitrate ions on oxalic acid in RP-HPLC. The optimum experimental conditions for the separation and determination of organic acids were studied. After the interference was eliminated, oxalic acid was determined by RP-HPLC at the optimum chromatographic conditions with an average recovery ranging between 96.2% and 100.2%; the relative standard deviations (RSD) were less than 3.0%. This method was successfully applied to determine the oxalic acid in xylem sap of pokeweed.