This method quantifies phenylalanine (Phe) and tyrosine (Tyr) using stable isotope-labeled internal standards (IS): d5-Phe and d4-Tyr. Phe and Tyr are extracted from plasma using methanol:water (50:50) solution containing the IS. The mixture is vortexed and centrifuged to precipitate protein. The supernatant is diluted and then introduced into the tandem mass spectrometer (MS/MS). Chromatography is performed using a C18 (150x4.6mm) column. The concentration of Phe and Tyr are established by comparison of the ion intensity with that of the IS (d5-Phe and Tyr, respectively).(Unpublished Mayo method)
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