Used as a stable isotope-labeled internal standard in mass spectrometry and quantitative bioanalysis to accurately measure L-Phenylalanine levels in biological samples. Its deuterated structure allows clear differentiation from the non-labeled compound, improving precision in pharmacokinetic and metabolic studies. Commonly applied in research on phenylketonuria (PKU) and protein metabolism. Also utilized in tracer studies to track metabolic pathways without altering biological activity.