An automated technique for the simultaneous determination of cations in nanoliter volumes

Background. The study of ion transport along the renal tubule ill vivo or i n vitro requires a technique capable of analyzing ion concentrations in sam ple volumes of only a few nanoliters. This article describes a method for t he analysis of cations at physiological concentrations in samples of tubula r fluid taken from single renal tubules in vivoe. Method. A background electrolyte composed of 2-[N-Morpholino] ethane-sulfon ic acid (MES) (50 mmol/liter) and L-histidine (50 mmol/liter; pH approximat e to 6.2), with the additives 18-crown-6 (1 mmol/liter) and methanol (30%) was used for the cation separation combined with conductivity detection. Results. Capillary zone electrophoresis was used to separate NH4, K, Na, Ca , Li, Mg, and Ba in six minutes. Simultaneous quantitative analysis was per formed for sodium and potassium, providing detection limits of 0.2 pmol for sodium and 30 fmol for potassium. The calibration plots were linear over t hree orders of magnitude, including the range of interest to clinical analy sis. Data on the reproducibility and repeatability of peak areas and of the repeatability of migration times are reported. Conclusion. The results for sodium and potassium are in close agreement wit h those obtained by atomic absorption spectrometry, indicating that this is a suitable technique for the routine measurement of these cations in tubul e fluid samples.