Fresnel detector

Deviation refractometers are the most commonly used. This version of the DRI measures the deflection in the location of a light beam on the surface of a photodiode by the difference in refractive index between the polymer solution and pure solvent. The Fresnel-type refractometers operate on the principle that the intensity of light reflected from a glass-liquid interface is dependent on the incident angle and the RI difference between the two phases. The deviation and Fresnel detectors typically have cell volumes of 5 to 10 pi, detection limits of about 5 x 10-6 refractive index units (RIU), and a range of 10 7 to 10 3 RIU.156 The deflection-type DRI is relatively insensitive to the buildup of contaminants on the sample cell and is therefore of special utility in laboratories that process large numbers of samples, such as industrial laboratories. 

Refractive Index. Commercial LC refractometers are based on one of two designs, deflection or Fresnel, as shown in Figure 9.28. Both types require reference and sample cells typical of bulk property detectors. 

Figure 9.28. Optical diagrams for refractive index detectors for LC (a) Fresnel type by permission of LDC/Milton Roy (h) deflection type by permission of Millipore Corporation, Waters Chromatography Division.

The Fresnel method of refractive index measurement has also been used in the design of commercially available detectors. The different... 

However, it was the one of the first refractive index detector to work on the Fresnel method. 

A Diagram of the Optical System of a Refractive Index Detector Operating on the Fresnel Method Courtesy of the Perkin Elmer Corporation... 

Figure 19-44. Fresnel reflection type Rl detector. (Courtesy - Varian Corp., Walnut Creek, CA)...

A second method utilizes the Fresnel principle (Figure 19-44), which relates the transmittance of a dielectric interface to the refractive indices of the interface materials. Such an interface may be formed between a glass prism of selected optical properties and the liquid whose refractive index is to be measured. These detectors are difficult to use with gradient elution systems, and temperature control of the solvents is critical. 

Figure 6.25 A Fresnel prism RI detector. (Reproduced by permission of Perkin Elmer Ltd.)...

The linear range is stated to be between 3-10 and 3-10 for the deflection-type instruments, while no figure is available for the Fresnel type. Generally, however, column overloading will occur before the detector becomes non-linear. ... 

The second type of differential refractometer is based on Fresnel s law, which states that the amount of light reflected from a glass-liquid interface is dependent on the angle of incident light and the refractive index difference between the two phases. The two collimated light beams are focused on to the sample and reference cells on the face of the prism. The intensity of the reflected light is a function of the refractive index of the liquid in the cells and is measured by the twin element photodetector. Milton Roy/LDC manufacture a detector of this type. 

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