ON-COLUMN LASER-BASED REFRACTIVE-INDEX DETECTOR FOR CAPILLARY ELECTROPHORESIS

The interference pattern from side-illuminated capillary tubes has been exploited to develop a sensitive universal refractive index (RI) detector suitable for nanoliter on-column capillary separation techniques. A 2-fold benefit is obtained by surrounding the capillary tube with a RI-matching fluid: the fringe pattern is simplified, and thermal noise is reduced. The key to RI detection in CE, provided that the instrument delivers sufficient sensitivity, has been found to be thermal stability. A thermal stability of DELTA-T = 2.0 x 10(-4)-degrees-C is achieved in a Peitier cooled RI cell having a highly symmetric design aimed to ensure fast thermal response from the thermoelectric system. The linear dynamic range extends to more than 3 orders of magnitude with a typical RMS noise level of 3 x 10(-8) RIU and baseline drifts of 2 x 10(-6) RIU h-1 at 1 Hz. This is about 1 order of magnitude above the calculated shot noise limit for these tube dimensions. These noise levels correspond to an angular deflection of the selected fringe of 100 nrad, which has been obtained by using a large position-sensitive photodiode. The optical geometric arrangement of the photodiode, with respect to the fringe width, has been optimized theoretically. A detailed description of the instrument and its noise sources is presented, and the technique is demonstrated in CE in the analysis of underivatized carbohydrates using a 50-mu-m tube.