Determination of total folate in plant material by chemical conversion into para-aminobenzoic acid followed by high performance liquid chromatography combined with on-line postcolumn derivatization and fluorescence detection.

A procedure involving chemical conversion of all forms of folate present in plant material into para-aminobenzoic acid (PABA) and a liquid chromatographic-fluorimetric determination with on-line postcolumn derivatization is reported. All folates are cleaved with liberation of PABA by hydrogen peroxide followed by acid hydrolysis using concentrated hydrochloric acid (37%) at 110 degrees C for 6 h. The reaction yield for individual folates conversion to PABA ranged from 44.4 to 97.3%. PABA could be determined sensitively by on-line postcolumn derivatization with fluorescamine, the detection limit for PABA being 3.02 nM. On the basis of this principle, a method for the determination of total folate in plant material, including a purification step on an affinity column, is presented, which offers a sufficient sensitivity and selectivity for routine analysis of total folate in natural samples. The total folate contents of tomatoes, carrots, white cabbage, and spinach were determined, and the results were quite comparable to the data reported. The recovery of PABA and the comparison of total folate analysis in spinach on different occasions (over 6 months) are also reported. The method is reliable, universal for all folates, including polyglutamate and monoglutamate forms, and eliminates the need for a deconjugation step and multiple conversion reactions.