Refractive index deflection 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. 

Figure 25-21 Deflection-type refractive index detector. 

The Waters Assoc. R401 differential refractometer is based on an optical-deflection design rather than on light reflection. This permits the use of a single cell throughout the refractive-index range of 1.00-1.75. The system also has a wider dynamic range of linearity for quantitation than the Fresnel-type refractometers. 

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.

Refractive index detectors. There are three types of commercially available RI detectors, namely, deflection, Fresnal and interference. Each measures the change in the refractive index of the base eluant due to the presence of analytes and hence in principle should provide the basis for a universal detector. Difficulties in RI detection arise due to the sensitivity of the solvent RI to fluctuations in temperature, pressure, the presence of dissolved gases and eluant composition. The RI of a solvent changes by 4 X 10 " per °C and by 4 x 10 per atmosphere thus baseline noise is 1 X 10 with a mixed solvent giving corresponding sensitivities of 5 pg ml . ... 

Fig. 1 illustrates the operating principles of the two types. On refraction of a Tight beam at an interface, both the direction and the intensity of the refracted beam are a function of the refractive indices of the media. In the deflection type, the direction of the beam is measured by means of a differential photodetector. Compensation for the refractive index of the reference is obtained by placing in series two prisms, one containing the sample and the other the reference liquid. 

By using the deflection principle of refractometry, it is possible to use one sample cell throughout the entire refractive index range from 1.00 to 1.75. An optical block and heat exchanger are provided to bring the liquid temperature to the temperature of the cell at all flow rates normally encountered. An example of a chromatogram obtained from this type of detector is shown in Figure 2. 

The differential refractometer is perhaps the second most widely used HPLC detector because of its universal nature. Potentially, any substance with a refractive index (RI) different from the mobile phase is detectable. The deflection type RI detector consists of a light source passing two beams of monochromatic light through a double prism that constitutes the sample cell and reference cell. If the mobile phase composition changes, the altered refractive index causes the beam to be deflected from its initial position on the photomultiplier detector. The electrical signal produced is proportional to the light position. 

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