Used as a stable isotope-labeled internal standard in mass spectrometry and liquid chromatography-mass spectrometry (LC-MS) assays for accurate quantification of L-tryptophan in biological samples such as plasma, serum, and urine. Its deuterated structure allows clear differentiation from endogenous L-tryptophan, minimizing interference from matrix effects and improving measurement precision. Commonly applied in pharmacokinetic studies, metabolic research, and biomarker validation where reliable detection of tryptophan levels is critical. Also utilized in tracer studies to monitor tryptophan metabolism pathways, including serotonin and kynurenine production, without altering the biological behavior of the molecule.