Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > The Interferometer Detector > Page 29

Light from an appropriate source strikes a half silvered mirror and is divided into two paths. Part of the beam is reflected by a plane mirror back along the same path and onto a photocell. The other part of the beam passes through the sensor cell to a plane mirror, where it is reflected back again through the sensor cell to the half silvered mirror that reflects it onto the photocell. Interference takes place with the other half of the light beam on the surface of the photocell. The trace resulting from the elution of 8 ml of dioxane through the cell is shown in figure 18.

Figure 18 Chromatogram from the Bakken and Stenberg Interferometer Detector

Each peak shown in figure 18 represents the passage of a fringe across the surface of the photocell. The four interference peaks represents a single chromatographic peak. The number of fringes will be directly proportional to the total change in refractive index, which, in turn, will be proportional to the total amount of solute present. In this form the detector has limited use, but has been developed into a commercially viable instrument called the Optilab DSP by Wyatt Technology Inc. A diagram of the optical system of the Optilab interference detector is shown in figure 19.

Light from the source is linearly polarized at -45o to the horizontal plane. Horizontal and vertical polarized light beams are produced and after passing through the Wollaston prism, one beam passes through the sample cell and the other beam through the reference cell. The sample cell beam is horizontally polarized and the reference cell beam is vertically polarized. After passing through the cells, the beams are focused onto a second Wollaston prism and then through a quarter-wave plate which has its fast axis set -45o to the horizontal plane. A beam that is linearly polarized in the fast axis plane after passing through the plate will lead another linearly polarized but orthogonal beam by a quarter of a wavelength. The phase difference results in a circularly polarized beam.