Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > The Interferometer Detector > Page 30

 

Courtesy of Wyatt Technology

Figure 19 The Optilab Interference Refractometer Detector

Each of the beams focused on the Wollaston prism consists of two such perpendicular beams which, after the quarter wave plate, result in two circularly polarized beams of opposite rotation. These beams will interfere with each other to yield the original linearly polarized beam. A second polarizer is placed at an angle (90 b) to the first, allowing about 35% of the signal to reach the photocell. A filter transmitting light at 546 nm precedes the photocell.

If the sample cell contains a higher concentration of solute than the reference cell the refractive index will be higher and the interfering beams will be out of phase. The refractive index difference (Dn) and the phase difference (Dp) are related by

 

where (L) is the length of the cell,
and (l) is the wavelength of the light.