Beam deflection refractive index detector

Figure 9. Diagram of a beam deflection (refractive index) detector (24)...

Figure 3.47 Refractive index detector (based on deflection of light beams).

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. 

An ultraviolet-laser based thermo-optical absorbance detector for micrometer capillaries was used by Qi et al. [76] to monitor the separation of a mixture of 13 phenylthiohydantoin-amino acids. A modulated pump laser beam periodically illuminated the capillary at a point near its end. Complex deflection and diffraction effects occur at the capillary-solution interface. Perturbation of the refractive index at this interface changes the intensity of the probe beam that is measured using a small photodiode. 

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. 

Commercial instruments based on two different principles are available. One uses a vertically divided cell. The top view is shown in Figure 2.52a. One of the chambers contains a reference fluid, typically the pure solvent, and the other chamber contains the sample solution. A laser beam passes the divided cell twice before reaching a two-part photodetector. The detector is placed so that the beam hits the two parts equally when the two chambers of the cell have the same refractive index. A difference in the refractive index in the chambers deflects the beam, resulting in unequal intensities on the two parts of the detector. Thus the imbalance of the two intensities gives the refractive index difference An. 

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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