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.
The concept of a magneto-optic current sensor that employs a fiber-optic Fabry-Perot resonator shaped into a single-turn helix is presented. The helical shape permits construction of a compact device that has the required polarization properties. Compared with a single-pass sensor, a large increase in sensitivity is obtained as a result of the finesse of the resonator. Experimental results confirm the theory and show that for a finesse of F = 6 the sensitivity is four times larger than for a single-pass sensor.
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