Used as a stable isotope-labeled internal standard in mass spectrometry and metabolic research to accurately quantify L-phenylalanine and its metabolites in biological samples. Enables precise tracking of phenylalanine metabolism in studies related to phenylketonuria (PKU), protein synthesis, and amino acid transport. Its isotopic enrichment (13C9,15N) allows clear distinction from endogenous phenylalanine, improving measurement accuracy in clinical diagnostics, nutritional studies, and pharmaceutical development. Also employed in tracer studies to investigate metabolic flux in cells and tissues without introducing radioactivity.