Quantitative determination of l-Phe in physiological fluids is crucial in the diagnosis and therapy of disorders of phenylalanine catabolism, such as phenylketonuria (PKU), a genetic disorder characterized by a deficiency in the liver of the hepatic enzyme phenylalanine hydroxylase that catalyzes the conversion of phenylalanine into tyrosine, leading to an excessive l-Phe accumulation in the serum (> 120 μM) [18]. Here, PAH is linked to phenylketonuria.