Used as a stable isotope-labeled internal standard in mass spectrometry and liquid chromatography-mass spectrometry (LC-MS) for the accurate quantification of L-phenylalanine in biological samples such as plasma, urine, and tissues. Its deuterium-labeled structure allows clear differentiation from the unlabeled compound during analysis, improving measurement precision and reliability. Commonly applied in metabolic studies, clinical diagnostics, and pharmaceutical research, especially in monitoring phenylketonuria (PKU) and assessing amino acid metabolism. Also utilized in proteomics for protein labeling and turnover studies.