THE ELECTROANALYSIS OF MANNITOL, XYLOSE AND LACTULOSE AT COPPER ELECTRODES - VOLTAMMETRIC STUDIES AND BIOANALYSIS IN HUMAN URINE BY MEANS OF HPLC WITH ELECTROCHEMICAL DETECTION

The electrochemical behavior of mannitol, xylose and lactulose has bee n investigated at a copper working electrode. A sensitive, accurate an d precise method employing HPLC with electrochemical detection in the d.c. amperometric model has been developed and validated for the deter mination of mannitol and lactulose in human urine. The ratio of these probe carbohydrates is altered in conditions that cause damage to the intestinal mucosal barrier. Systematic studies employing cyclic voltam metry indicate that the electrode reaction involves an electrocatalyti c oxidation of each carbohydrate in a process yielding a single irreve rsible anodic wave that is dependent on the ionic strength of the sodi um hydroxide supporting electrolyte solution. High performance liquid chromatography with electrochemical detection was performed using a th in-layer cell housing a custom manufactured copper working electrode. The optimized HPLC method can detect 72, 57 and 319 pg of mannitol, xy lose and lactulose injected on column, respectively. The corresponding linear calibration ranges are 359 pg-2.24 mu g, 57.4 pg-896 ng and 41 9 pg-262 ng, respectively. Solid-phase extraction of human urine on po lar sorbents, and direct injection after simple 1 + 99 dilution in 0.0 25 M NaOH were compared for bioanalysis. Direct injection was selected for further method development as the technique proved robust and sim ple. The optimized method was validated for the determination of manni tol and lactulose in human urine over the concentration ranges predict ed when assessing intestinal permeability (0.25-2.5 mg ml(-1) mannitol and 0.05-1.0 mg ml(-1) lactulose). Over these ranges intra-and inter- assay bias is < +/- 6.5%, and imprecision (coefficient of variation) i s < 9% for each carbohydrate. The validated method provides a useful a lternative to HPLC with pulsed-amperometric detection at gold electrod es. (C) 1998 Elsevier Science B.V. All rights reserved.