Determination of 5-methyltetrahydrofolic acid and folic acid in citrus juices using stable isotope dilution-mass spectrometry.

A stable isotope liquid chromatography-mass spectrometry (LC-MS) method was developed for the quantitative determination of 5-methyltetrahydrofolic acid (5-MTHFA) and folic acid in a variety of commercial citrus juices. Folates were extracted from juices, and the polyglutamyl side chain of 5-MTHFA was cleaved to the monoglutamate form using rat plasma conjugase. The folates were purified on a Bond-Elut column and analyzed by LC-MS with electrospray ionization. The analytes were quantified using the (13)C(5) analogues of 5-MTHFA and folic acid as internal standards. The relative standard error of the method was 3.35% based on replicate analyses (n = 4). This method was then applied to the determination of 5-MTHFA and folic acid in a variety of citrus juices obtained from local supermarkets. It was observed that although both "store" brands and "national" brands of fresh (nonfrozen) juices contained similar concentrations of 5-MTHFA, the "store" brands of fresh juices had on average >5-fold the amount of folic acid compared to the "national" brands. In addition, the "total" folate concentrations were generally below values listed on the food label.