An investigation into the determination of oxalic acid in vegetables by capillary electrophoresis

The presence of natural levels of oxalate in foods may have important dietary implications through interference with calcium availability as well as an effect on renal stone formation. Recently, capillary electrophoresis (CE) has been used to separate a number of commonly occurring organic and inorganic anions, including oxalate. Initial attempts to determine oxalic acid by CE using an electrolyte consisting of 5 mM sodium chromate and 0.05 mM OFM Anion-BT reagent were unsuccessful in the presence of the large chloride concentration from the extract solution due to co-migration of oxalate, molybdate (chosen as the most suitable compound for use as an internal standard) and naturally occurring nitrate. Changing the composition of the electrolyte to 10 mM sodium chromate, 4 mM Anion-BT reagent and adding 10% methanol produced baseline separation of oxalate, nitrate and molybdate in vegetable extracts, despite the presence of a huge chloride peak. The levels of oxalic acid in a variety of vegetables were determined by CE and compared favourably with the levels determined by both the high-performance liquid chromatographic and the enzymatic procedure and levels reported in an Australian food composition table. The CE procedure is more convenient and quicker than the traditional titrimetric and chromatographic methods.