FA was purchased from Sigma and its stock solution (0.05 M) was prepared with diluted NaOH aqueous solution. The vitamin tablets were purchased from Wyeth and urine sample was from a volunteer. OMC was prepared according to our previous report. 0.04 M B-R buffer solutions with different pH values were used. All other chemicals were at least of analytical grade. Water used was doubly distilled by a quartz apparatus.