DL-Phenyl-d5-alanine is intended for use as an internal standard for the quantification of phenylalanine by GC- or LC-MS. DL-Phenylalanine is a racemic mixture of the essential amino acid L-phenylalanine and its enantiomer D-phenylalanine . It has been used as a precursor in the synthesis of sulfonamide phenylalanine derivatives with antibacterial, antifungal, and anticancer activity.