Liquid Chromatography Detectors - LC Detectors Based on Refractive Index Measurement > The Fresnel Method > Page 25
The two light beams are arranged to pass through the sample and reference cells respectively. Refracted light from the mobile phase/prism surface passes through the prism assembly and focused onto two photocells. The prism is also arranged to reflects some light to an aperture where the surface of the prism can be observed. The photocell outputs are electronically processed and passed to either a potentiometric recorder or a computer data acquisition system. The refractive index range monitored by the device for a given prism is limited and, consequently, there are usually three different prisms available to cover the RI ranges of 1.35–1.4, 1.31–1.44 and 1.40–1.55 respectively. In figure 15 is shown the separation of a series of polystyrene standards monitored by this type of detector.
Figure 15 The Separation of Some Polystyrene Standards Using a RI Detector Operating on the Fresnel Method
The separation was carried out by size exclusion on a column packed with 5 mm particles and operated at a flow rate 0.8 ml/min.
As a result of limited sensitivity and restricted linear dynamic range, the RI detector is only used for those applications where, for one reason or another. all other detectors are inappropriate or impractical.
This type of detector does, however, have one particular area of application for which its characteristics make it particularly suitable and that is for monitoring the separation of polymers. This is because for those polymers containing more than six monomer units, the refractive index is proportional to polymer concentration and independent of its molecular weight. Consequently, quantitative estimation of each polymer mixture can be obtained by simple normalization of peak areas and no individual response factors are required. RI detectors have sensitivities of about 1 x 10-6 g/ml, a linear dynamic range of about 200 and a response index (r) lying between 0.97 and 1.03.