Utilization of fluorescein sodium salt in laser-induced indirect fluorimetric detection of ions separated by capillary zone electrophoresis

Abstract

Indirect detection, the principles of which were demonstrated some years ago, has attracted renewed interest in both UV-visible and fluorimetric detection. This paper reports the implementation of laser-induced indirect fluorimetric detection using a commercially available detector and its association with high-performance capillary electrophoresis for the determination of inorganic ions. In order to optimize the response sensitivity, the fluorescent agent being fluorescein sodium salt, the influence of the fluorescein sodium salt concentration, the ionic strength, the electrolyte pH and the gain on the signal-to-noise ratio were studied successively, the injection mode being either electrokinetic or hydrodynamic. In the case of a 75 μm I.D. fused-silica capillary, detection limits were in the ppb range or about 10 ppb for alkali metal cations using injection in the electrokinetic or in the hydrodynamic mode, respectively. As the detection system used had its maximum sensitivity at basic pH, the use of a complexing agent was required in order to perform the determination of alkaline earth metal cations, because they precipitate or are adsorbed at basic pH. Therefore, using ethylenediaminetetraacetic acid sodium salt (EDTA), it is possible to analyse at pH 7.5, in less than 5 min, a mixture containing three alkaline earth metal cations (Ba 2+, Ca 2+ and Mg 2+) and three transition metal cations (Fe 3+, Zn 2+ and Cu 2+) in addition to the previously mentioned alkali metal cations (Li + and K +).