Refractive index detectors reflection-type

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. 

Automatic instruments are usually reflection-type instruments that measure the deflection of a beam of light as it passes from one medium, into the sample and then is reflected back to a detector. The angle at which the light beam exits the medium is related to the refractive index of the sample. Automated instruments are calibrated with standard substances of precisely known refractive index prior to use. 

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. 

Inevitably the choice of solvent must reflect the type of detector used in the analysis. If a refractometer is used, then clearly there must be sufficient difference in refractive index between solvent and polymer solution for good response at the low concentrations required for good molecular separation. Alternatively, an appropriate window in the spectra of the solvent for an IR or UV detector is required. The exact choice of the solvent depends on the polymer and the required sensitivity of the detectors. 

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