High Performance Liquid Chromatography coupled with Refractive Index Detector

High Performance Liquid Chromatography coupled with Refractive Index Detector is a so-called universal analysis technique because of its ability to detect almost all of the compounds dissolved in the eluent.

The detection principle consists in measuring the variation in the refractive index of the effluent from the column passing through the flow cell. This consists of two compartments separated by a glass plate mounted at an angle such that a deflection of the incident beam occurs if the two solutions differ in refractive index. The first half of the flow cell (the sample cell) will contain the column effluent and the other half the mobile phase which is the reference. The detector measures the refractive index of the two components simultaneously.

When the mobile phase passes through the sample cell, the measured refractive index of the two parts remains identical (no deviation of the incident beam). On the other hand, when an analyte passes through the flow cell, the two refractive indices measured are different and a signal can be observed, that is to say a peak in the chromatogram. In fact, the detector measures the deflection of an incident beam caused by the change in the refractive index of the sample compartment compared to that of the reference.

The sensitivity will be higher if the RI difference between the sample and the reference is large. The RI detector has a lower sensitivity than the UV detector; hence it’s less frequent usefulness. On the other hand, it has the advantage of detecting a large range of samples where a change in reflection index can be measured like sugars, alcohols or inorganic ions which cannot be measured in UV.

A disadvantage of this refractive index detector is that it is very sensitive to the evolution of the eluent or to its composition, so it should not be used in phase gradient mode.