Simultaneous determination of stable isotopically labelled l-histidine and urocanic acid in human plasma by stable isotope dilution mass spectrometry

A capillary gas chromatographic—mass spectrometric method for the simultaneous determination of stable isotopically labelled l-histidine (l-3,3-²H₂,1′,3′-¹⁵N₂]histidine, l-His-M + 4]) and urocanic acid (3-²H,1′,3′-¹⁵N₂]urocanic acid, UA-M + 3]) in human plasma was developed using dl-2,3,3,5′-²H₄,2′-¹³C,1′,3′-¹⁵N₂]histidine (dl-His-M + 7]) and 2,3,5′-²H₃,2′-¹³C,1′,3′-¹⁵N₂]urocanic acid (UA-M + 6]) as internal standards. l-Histidine and urocanic acid were derivatized to ^αN-(trifluoroacetyl)-^imN-(ethoxycarbonyl)-l-histidine n-butyl ester and ^imN-(ethoxycarbonyl)urocanic acid n-butyl ester. Quantification was carried out by selected ion monitoring of the molecular ions of the respective derivatives of l-His-M + 4], dl-His-M + 7], UA-M + 3] and UA-M + 6]. The sensitivity, specificity, precision and accuracy of the method were demonstrated to be satisfactory for measuring plasma concentrations of l-His-M + 4] and UA-M + 3] following administration of trace amounts of l-His-M + 4] to humans.