Simultaneous monitoring of inorganic cations, amines and amino acids in human sweat by capillary electrophoresis

The determination of cationic constituents of sweat is widely recognized as a difficult analytical task due to its complex composition and minute sample volumes available for the individual analysis. Capillary electrophoresis (CE) has been evaluated as a simple routine method to measure sweat metal cations, biogenic amines, and amino acids using a sampling procedure previously developed in one of collaborative teams. The carrier electrolyte, which consisted of 10 mM 4-methylbenzylamine, 6.5 mM α-hydroxyisobutyric acid, and 2 mM 18-crown-6 at pH 4.25, allowed the separation of five cations (NH₄⁺, K⁺, Ca²⁺, Na⁺, Mg²⁺) and four amino acids (ornithine, histidine, lysine, arginine) to be completed in about 13 min with a positive polarity of the applied voltage (30 kV). By increasing the sample volume (due to employing hydrodynamic instead of hydrostatic injection mode), it was also possible to detect indirect UV signals of Zn²⁺, diethanolamine, and trithanolamine. Sweat samples were collected from the fingers and forearms of three healthy male volunteers and analyzed by CE. A good repeatability and reproducibility of peak area responses based on five intraday and three inter-day assays (average %R.S.D. less than 3.5 and 2.5, respectively) were obtained. The limits of detection were in the range of 3.2-5.8 μM for alkali and alkaline-earth cations (hydrostatic injection) and 0.27-0.79 μM for other target analytes (hydrodynamic injection). The analytical results for particular analytes were found to vary, depending on the sampling spot and individual, but in general correspond well to clinical concentration ranges.