HPLC Refractive-Index (RI) Detector

RI detector measures change in reflex index. A glass cell is divided into two chambers (cells). The effluent from LC column flow through the "sample cell", while other cell called "reference cell" is filled with only mobile phase. When the effluent going through the sample cell does not contain any analyte, the solvent inside both cells are the same (Figure 1A). When a beam is irradiate on the cells, the observed beam will be straight in this case. However, in a case the effluent contains any components other than mobile phase; bending of the incident beam occurs due to the reflex index difference between the two solvents (Figure 1B). By measuring this change, the presence of components can be observed.

RI detector has lower sensitivity compared to UV detector, and that's the main reason why RI is not as commonly used as UV. However there are some advantages over UV detector.

  • It is suitable for detecting all components. For an example, samples which do not have UV absorption, such as sugar, alcohol, or inorganic ions obviously cannot be measured by a UV detector. In contrast, change in reflective index occurs for all analyte, thus a RI detector can be used to measure all analyte.
  • It is applicable for the use with solvent that has UV absorbance. A UV detector cannot be used with solvent which has UV absorbance. Sometimes the organic solvent used for GPC analysis absorbs UV, and thus UV detector cannot be used.
  • It provides a direct relationship between the intensity and analyte concentration. The amount of UV absorbed depends on each analyte, thus the intensity of UV detector peak does not provide information on the analyte concentration. While intensity observed by a RI detector is comparable to the concentration of analyte. Because of those advantages, RI is often used for the detection of sugars and for SEC analysis.