A simple on-line absorbance imaging detection system was constructed for detection of capillary isoelectric focusing (CIEF). Because of the use of the CIEF-imaging detection system, separation and detection of protein sample could be completed in 2 minutes. The use of the imaging detection system also allowed for studying the focusing process. When a 4 cm long, 200-pm i. d. square capillary was used as the separation column, the concentration detection limit of the detectoir reached 5 pg/mL. This is the first report of applying absorbance imaging1 detection system to capillary electrophoretic techniques. The absorbance imaging detection system was compared with the refractive index glradient imaging detection system. For the capillary with large i. d., the absorbance imaging system showed higher sensitivity than the refractive index gradient imaging detector. However, the refractive index gradient imaging system was more suitable for narrow capillaries because it applied a laser as the light source which could be easily focused into narrow capillaries. The sensitivity of the refractive index gradient imaging detector was expected to be higher for narrower capillaries than the absorbance imaging detector.
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