Light-emitting diode-based indirect fluorescence detection for simultaneous determination of anions and cations in capillary electrophoresis

This report presents simultaneous analysis of cations and anions by capillary electrophoresis (CE) in conjunction with indirect fluorescence detection using a blue light-emitting diode (LED), based on the displacement of fluorescein with anionic EDTA–metal complexes and anions. A new focusing system combined with a plastic lens and a 40× objective was developed and used effectively to focus the diverging beam of the LED on the capillary. The optimum compositions for simultaneous analysis of metal ions and anions are the samples prepared in 5 mM borate, pH 9.2, containing 2 mM EDTA and the background electrolytes (BGEs) consisting of 5 mM borate buffer, 5 μM fluorescein, and 1 μM NaCl at pH 9.2. Using this pre-capillary complexation method, the analysis of a sample containing five metal ions and eight anions was accomplished in 8 min, with the relative standard deviation values for the migration times less than 2.0%. The peak heights against the concentrations of the metal ions and anions are linear in 10–1000 and 50–2000 μM, with correlation coefficients better than 0.998, and 0.982, respectively. The limits of detection at a signal-to-noise ratio 3 of up to 14.6 μM for formate and as low as 3.7 μM for Ni2+. The results of the analyses of pond water and a Chinese herbal soup present the advantages of this method, including simplicity, rapidity, reproducibility, and low costs.