Two hundred microlitres of plasma is deproteinised, and chloride is removed with TCA/silver iodate. After centrifugation the phenylalanine in the protein-free extract is oxidised to benzaldehyde with buffered sodium metaperiodate. The benzaldehyde is extracted into iso-octane, and the absorbance of the organic layer is measured at 240 nm. The method gives good agreement with a paper chromatographic method at normal and elevated levels of phenylalanine. All the reagents are stable indefinitely, and a single determination can be completed in 15 minutes.
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