Deflection refractometer

The deflection refractometer (Fig. 8.4), which measures the deflection of a beam of monochromatic light by a double prism in which the reference and sample cells are separated by a diagonal glass divide. When both cells contain solvent of the same composition, no deflection of the light beam occurs if, however, the composition of the column mobile phase is changed because of the presence of a solute, then the altered refractive index causes the beam to be deflected. The magnitude of this deflection is dependent on the concentration of the solute in the mobile phase. 

Figure 6.26 Deflection refractometer detector. (Reproduced with permission of Waters.)...

Figure 5.16. Schematic diagram of a refractive index detector employing the principle of refraction (deflection-refractometer).

Deflection refractometer. This method measures the refraction of a parallel beam of light as it crosses the interface of two media. A light beam from a tungsten lamp is passed through the flow cells and is reflected back and focussed onto a photocell. One of the flow cells is the reference cell, the other being the sample. When the solute enters the sample cell the light is deflected. This type of instrument is supplied by Waters (R 400). 

The differential refractometer monitors the deflection of a light beam caused by the difference in refractive index between the contents of the sample cell and those of the reference cell. A beam of light from an... 

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. 

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. 

The most common Refractive Index (RI) detector uses a differential refractometer, which responds to the deflection of a light beam the deflection being caused by the differences in the refractive indices of a cell through which eluant passes and that of a reference cell in which the mobile phase is contained. The response of the detector is proportional to the mass concentration irrespective of the nature of solute being analyzed. 

Differential refractometers operate by utilizing one of the following principles. In the first type, the measurement is based on an optical displacement of the beam (Figure 19-43). A mirror controls the reflection of the light beam. The light passes through the divided cell the first time without being reflected, and this then is followed by a reflected beam. If there is a difference in the solutions, a deflection will occur. The deflection is the sum of the deflection in the two cells. 

Differential refractometers detect the changes in refractive index of eluate when compared with the refractive index of the mobile phase. Four principles are exploited in the construction of differential refractometers deflection of light passing the boundary surface between the substances with different optical density deflexion refractometers), and subsequent light scattering in case of multiple refraction refractometers based on Christiansen effect), the Fresnel law of light reflection reflection refractometers) and, finally, the interference of light interferometers). 

In evaluating Debye s factor H (equation 16), the refractive index increment of the solute must be accurately known. This determination is conveniently carried out with the differential refractometer of P. P. Debye (1946), in which the solution is contained in a hollow glass prism of triangular cross section, immersed in a cell containing the pure solvent. The deflection of a light beam passing through the system is a nearly linear function of the refractive index difference n — n0, which may thus be measured to about 0.000,003. 

The column compartment is reasonably large and can hold up to ten 60-cm X 1-cm analytical columns as well as the differential refractometer—the only standard detector supplied. Other detectors can be fitted (IR, UV, viscometers and LALLS), but these are normally fitted externally and require heated extension tubes with additional plumbing to the equipment. The refractometer has a sensitivity of 1 x 10 RI units full-scale deflection, with an average noise level of 5 x 10 RI units. A quartz halogen bulb light source is used for increased sensitivity. Very good temperature control is required when using a RI detector. 

Most flow-type refractometers use a balanced, split photodiode design to measure the angular deflection of a light ray passing through a split cell, one half of which contains the pure solvent and the other half contains the solvent with polymer. Refractometers usually require special care for stabilization. Their flow cells are often too fragile to withstand much back pressure and so are often placed at the end of a detector train. 

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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