Fluorimetric Method for the Determination of Histidine in Random Human Urine Based on Zone Fluidics

In the present study, the determination of histidine (HIS) by an on-line flow method based on the concept of zone fluidics is reported. HIS reacts fast with o-phthalaldehyde at a mildly basic medium (pH 7.5) and in the absence of additional nucleophilic compounds to yield a highly fluorescent derivative (λ(ex)/λ(em) = 360/440 nm). The flow procedure was optimized and validated, paying special attention to its selectivity and sensitivity. The LOD was 31 nmol·L(−1), while the within-day and day-to-day precisions were better than 1.0% and 5.0%, respectively (n = 6). Random urine samples from adult volunteers (n = 7) were successfully analyzed without matrix effect (<1%). Endogenous HIS content ranged between 116 and 1527 μmol·L(−1) with percentage recoveries in the range of 87.6%–95.4%.