Simple double-beam absorption detection systems for capillary electrophoresis based on diode lasers and light-emitting diodes

Simple double-beam absorption detection systems for capillary electrophoresis (CE) based on diode lasers and light-emitting diodes (LEDs) are developed. Using digital normalization, the intensity fluctuations are greatly reduced and a relative standard deviation of the background noise of 10(-5) to 10(-6) can be achieved. The detection performance is thus improved over those of commercial CE systems. For the diode-laser-based system, the gain in detectability results from both a reduction in intensity fluctuations and a better optical coupling of the laser beam with the small capillary tube, maximizing the effective optical path length. The improvement in the LED-based system is largely due to the excellent stability of the LED augmented by double-beam cancellation of the baseline fluctuations. Indirect absorption detection for cationic and anionic species are also demonstrated, extending the applicability of the system.