Interferometric apparatus for monitoring changes of the refractive index of fluid samples in capillary tubes

An interferometric apparatus for monitoring changes of the refractive index of An interferometri1capillary tubes comprising a source of coherent light, which is arranged with respect to a capillary tube which is part of a capillary liquid chromatographic system or a capillary electrophoretic system, such that the coherent light beam strikes the capillary tube about perpendicular to its longitudinal extension. In the forward direction of the coherent light beam, behind the capillary tube, the apparatus comprises a photoelectric detector for monitoring a resulting interference fringe pattern and the shifts thereof, which is connected to an evaluation electronics. Between the light source and the capillary tube there is arranged a beam splitting optical element for generating a converging probe beam, which travels along a probe beam axis and strikes the capillary tube at its bore, and a converging reference beam, which travels along a reference beam axis, that extends generally parallel to the probe beam axis and is located in close vicinity to the probe beam axis, and which does not strike the capillary bore.