[Direct determination of phenylalanine in serum by derivative spectrophotometry (author's transl)].

A simple and precise method is described for the determination of phenylalanine in serum or plasma, using derivative spectrophotometry. After the removal of protein and uric acid, 100 microliters serum or plasma are analysed directly for phenylalanine. The magnitude of the difference between the peak minimum at 254 nm and the peak maximum at 257 nm of the derivative spectrum (2nd derivative of the absorption spectrum) is directly proportional to the phenylalanine content. The precision was 2.1-10.3%, depending on the concentration of phenylalanine. Recovery was between 97.1 and 101.1%. Sensitivity was 5 mg/l serum. The method was compared with a column chromatographic determination of phenylalanine.