The refractive index (RI) detector is the only universal detector in HPLC.

The detection principle involves measuring of the change in refractive index of the column effluent passing through the flow-cell.  The greater the RI difference between sample and mobile phase, the larger the imbalance will become. Thus, the sensitivity will be higher for the higher difference in RI  between sample and mobile phase. 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. This factor is severely limiting RI detector application in the analyses requiring the gradient elution, where mobile phase composition is changed during the analysis to effect the separation.

Two basic types of RI detectors are on the market today. Both require the use of a two-path cell where the sample-containing side is constantly compared with the non-sample-containing reference side.

Deflection detectors

The optical schematic of the deflection detector is shown in below. 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). When no sample is present in the cell, the light passing through both sides is focused on the photodetector (usually photoresistor). As sample elutes through one side, the changing angle of refraction moves the beam. This results in a change in the photon current falling on the detector which unbalances it. The extent of unbalance (which can be related to the sample concentration) is recorded on a strip chart recorder.

Optical schematic of a deflection refractive index detector.

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. 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.