Liquid Chromatography - The Refractive Index Detector 3

The refractive index detector is often a 'choice of last resort' and is selected for those applications where, for one reason or another, all other detectors are inappropriate or impractical. However, the detector has one particular and unique area of application and that is in the separation and analysis of polymers. For those polymers that contain more than ten monomer units, the refractive index is directly proportional to the concentration of the polymer and is practically independent of the molecular weight. A quantitative analysis of a polymer mixture can, therefore, be obtained by the simple normalization of the peak areas in the chromatogram (there being no need for the use of individual response factors). Some typical specifications for the refractive index detector are as follows:-

Typical Specifications for a Refractive Index Detector

 

Sensitivity (benzene)

1x 10-6 g/ml

Linear Dynamic Range

1 x 10-6 to 1 x 10-4 g/ml

Response Index

0.97 - 1.03

A typical application of the RI detector is for carbohydrate analysis. Carbohydrates do not adsorb in the UV, do not ionize and although fluorescent derivatives can be made, the procedure is tedious and time consuming. An example of such an application is shown in figure 25 by the separation of the products of b-cyclodextrin hydrolysis.

Courtesy of TOYO SODA Manufacturing Co. LTD

 

Figure 25. The Separation of Hydrolyzed b-cyclodextrin