Used as a stable isotope-labeled internal standard in mass spectrometry and liquid chromatography-tandem mass spectrometry (LC-MS/MS) for the accurate quantification of folic acid in biological samples such as serum, plasma, and tissues. Its deuterated structure allows clear differentiation from endogenous folic acid, improving measurement precision in clinical diagnostics and nutritional studies. Also employed in metabolic research to trace folic acid pathways without altering biochemical behavior.