Francois Dr Maystre

The pigtailing approach to optical detection in capillary electrophoresis

A novel approach, called pigtailing, is presented for the construction of optical detectors for capillary electrophoresis. Optical components and procedures from other fields, mainly telecommunications, are incorporated into these devices to give miniaturized detection systems. Suitable light sources for the construction of pigtail absorbance, fluorescence, refractive index and thermo-optical detectors are light-emitting diodes (LEDs) and laser diodes. The best optical components are gradient-index lenses, optical fibers or diffractive optical elements. These components are joined to the capillary with refractive-index-matching materials to avoid refraction and reflections at the optical interfaces and to reduce mechanical vibrations. These joints also facilitate fast thermal equilibrium. The performance of absorption detectors depends mainly on the brightness of the selected LEDs. Two types of refractive-index capillary detectors are described: one features a single-mode polarization-preserving fiber whereas the second uses a customized holographic plate as the main optical element.

Aspects of quantitative determinations with polarimetric detectors in liquid chromatography

Abstract The overall performance of polarimetric detectors for liquid chromatography has substantially improved in the last few years. Results show relative standard deviations of 0.50% and as good as 0.25% can be obtained using present models. Under reversed-phase conditions using achiral columns, pseudo-rotation due to refractive index related effects preclude these detectors from performing equally well when operated under static conditions. This situation becomes critical when analyzing compounds having small specific rotations or mixtures with small enantiomeric excess. Due to such pseudo-rotations, the calibration factors of the polarimetric detectors for two R and S enatiomers are not necessarily identical. Although the response functions are linear, the negative and positive parts do not coincide at zero enantiomeric excess.