Liquid-chromatographic direct determination of phenylalanine and tyrosine in serum or plasma, with application to patients with phenylketonuria.

With this method, picomole amounts of phenylalanine and tyrosine can be rapidly separated and directly measured by absorbance at 206 nm. As little as 25 microliters of serum or plasma suffices. Plasma is deproteinized and 20 microliters of the supernate, representing 1.8 microliters of plasma, is applied to a reversed-phase column and eluted isocratically with a solution containing methanol and H3PO4, pH 2.4. The tyrosine and phenylalanine peaks appear at 6 and 10 min, respectively. We used the method to measure phenylalanine in plasma from six children with phenylketonuria.