Highly sensitive indirect photometric detection of cations by capillary electrophoresis with the cationic dye chrysoidine

Abstract

The cationic dye, chrysoidine, has been used for the first time as a probe for the indirect photometric detection of cations. The dye has been used as a probe at concentrations of 5 m M, which is roughly an order of magnitude higher than for other cationic dyes used previously for the same purpose, in order to minimise electromigrational dispersion. Baseline instability was minimised by a combination of coating the capillary with poly(ethyleneimine), addition of a neutral polymer to the electrolyte, and the application of a small amount (20 mbar) of hydrodynamic pressure during the separation. Separation of a mixture containing alkali metals, alkaline earths, transition metals and lanthanides was achieved by the addition of 2-hydroxyisobutyric and lactic acid as complexing agents. Excellent peak shapes were observed over a wide range of analyte mobilities due to the moderate mobility of the probe. The high absorptivity (26 733 l mol −1 cm −1) provided by chrysoidine in comparison with typically used, less absorbing probes, was reflected in limits of detection which were typically less than 0.5 μ M. These are amongst the lowest reported using hydrodynamic injection without the use of large volume stacking methods. The use of 2-hydroxyisobutyric and lactic acids as complexing agents at pH values close to their p K a values provided suitable buffering which was highlighted by very good reproducibility of migration time, corrected peak area and peak height.