The Interference pattern from side-illuminated capillary tubes has been exploited to develop a sensitive universal refractive Index (RI) detector suitable for nanollter 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 AT = 2.0 X 10"* °C is achieved In a Peltier 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-* RIU and baseline drifts of 2 X 10-* RIU h1 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 underlvatlzed carbohydrates using a 50-jim tube.

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A novel refractive index detector suitable for capillary separations is presented and demonstrated in the capillary electrophoretic separation of metal cations in 10-itm-i.d. tubes. The principle of the detector is interferometric; it features a laser diode and, as the main optical element, a holographic optical element which performs several optical functions allowing the use of capillaries with inner diameters as small as 5 /*m. In contrast to the "off-axis" method, the use of holographic plates permits probing of the capillary through its center, where the optical path is larger and diffraction effects are smaller. With an i.d. = 10 /tm capillary the detector resolves 2 juRIU and covers a linear dynamic range of three to four decades. Joule heat effects preclude the detector to operate at its maximum sensitivity which is delivered in the absence of electrical fields. For this reason, the use of smaller inner diameter capillaries and buffers having low conductivities is recommended. Under field-amplified sample injection conditions, the detection limits for the studied cations are in the low nanomolar range. The performance of this detector is compared with indirect UV-absorption detection, which also helps to elucidate the mechanisms leading to refractive index changes in ionic solutions.Considering that holographic optical elements (HOEs) could greatly simplify the construction of optical instruments without compromising performance, their use is gaining acceptance in analytical instrumentation such as spectrographs.1-2 The present investigation represents the first application of HOEs to on-column refractive index (RI) detection for the analysis of transparent substances using capillary electrophoresis (CE).Optically dense compounds are most commonly detected by absorbance methods, and when the substances of interest * Author to whom correspondence should be addressed.(1) Tedesco,

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