5-Methyltetrahydrofolic acid (5-MTHF) is an active folate metabolite. It plays an important role in the synthesis of methionine from homocysteine by donating methyl group.5-Methyltetrahydrofolic acid-(glutamic acid-13C5) is an isotope of vitamin B9 wherein all the five glutamate carbons are replaced by13C6carbon.5-Methyltetrahydrofolic acid-(glutamic acid-13C5) can be used as a stable isotope internal standard for accurate data interpretation in citrus juice samples by LC-MS.