Optical schematic of a deflection refractive index detector. The optical schematic of the deflection detector is shown in below. The disadvantages are the relatively low sensitivity and a general disability to easily remove and clean or replace the cell when filming or clogging occurs.. Principles . As sample elutes through one side, the changing angle of refraction moves the beam. The refractive index (RI) detector is the only universal detector in HPLC. This results in a change in the photon current falling on the detector which unbalances it.
On the other hand, in complex mixtures, sample components may cover a wide range of refractive index values and some may closely match that of the mobile phase, becoming invisible to the detector. RI detector is a pure differential instrument, and any changes in the eluent composition require the rebalansing of the detector. THE REFRACTIVE INDEX DETECTOR . The detection principle involves measuring of the change in refractive index of the column effluent passing through the flow-cell. This detector based on the deflection principle of refractometry, where the deflection of a light beam is changed when the composition in the sample flow-cell changes in relation to the reference side (as eluting sample moves through the system). Thus, the sensitivity will be higher for the higher difference in RI between sample and mobile phase. The advantages of this type of detector are: (1) universal response; (2) low sensitivity to dirt and air bubbles in the cells; and (3) the ability to cover the entire refractive index range from 1.000 to 1.750 RI with a single, easily balanced cell. Two basic types of RI detectors are on the market today.
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