DIODE LASER-BASED INDIRECT ABSORBENCY DETECTOR FOR CAPILLARY ELECTROPHORESIS

A near-infrared detector has been developed for use in capillary electrophoresis (CE). The detector has a double beam arrangement with signal-to-reference ratioing and operates with a 670-nm diode laser as the light source. The laser beam can be modulated to allow a.c. signal recovery using lock-in amplification. Near-infrared laser dyes have been investigated as background absorbers, and the singly-charged cationic dye rhodamine 700 found to exhibit suitable characteristics for use in indirect absorbance detection in methanol-water mixtures. Separations of a series of tetraalkylammonium compounds have been performed on both a commercial CE apparatus with indirect UV detection and on a home-made instrument incorporating the diode laser detector. The limit of detection (2 x peak-to-peak noise) for the tetrabutylammonium ion with diode laser-based indirect absorbance is 2 . 10(-5) M. Both positive and negative peaks are found for these positively-charged analytes displacing a positively charged background absorber. Transfer ratios for all peaks are quantitated and the results compared with theoretical treatments of displacement in indirect detection. For analytes with mobility less than the indirect absorbing ion, measurement of peak area leads directly to the quantity of analyte present in the sample solution.