Refractive index detector applications

The refractive index detector, in general, is a choice of last resort and is used for those applications where, for one reason or another, all other detectors are inappropriate or impractical. However, the detector has one particular area of application for which it is unique and that is in the separation and analysis of polymers. In general, for those polymers that contain more than six monomer units, the refractive index is directly proportional to the concentration of the polymer and is practically independent of the molecular weight. Thus, a quantitative analysis of a polymer mixture can 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 ... 

The normalization method is the easiest and most straightforward to use but, unfortunately, it is also the least likely to be appropriate for most LC analyses. To be applicable, the detector must have the same response to all the components of the sample. An exceptional example, where the normalization procedure is frequently used, is in the analysis of polymers by exclusion chromatography using the refractive index detector. The refractive index of a specific polymer is a constant for all polymers of that type having more than 6 monomer units. Under these conditions normalization is the obvious quantitative method to use. 

The 1/16" x 0.02" i.d. transfer line also functioned as a sample dilution device in other applications, a stainless steel column packed with glass beads has been found to be useful for dilution. This simple dynamic dilution technique has been used extensively in flow injection analysis.3 A refractive index detector is typically used to measure the sample transfer time. As shown in Figure 4, approximately 5 minutes is required to transfer the sample plug to the Rheodyne valve. As the apex of the sample band passes though the Rheodyne valve, the valve is activated and 1 pi injected onto the liquid chromatographic column. The sample transfer time was checked periodically over 1 year of operation and found to be stable. 

Burggraf, N., Krattiger, B., de Rooij, N.F., Manz, A., de Mello, A.J., Holographic refractive index detector for application in microchip-based separation systems. Analyst 1998, 123(7), 1443-1447. 

Four detectors have found widespread application. These are the ultraviolet-visible detector, the fluorescoice detector, the refiactive index d ector, and the electrochemical detector. Only the refractive index detector can be considered as a imiversal detector as virtually all compounds cause a (diange in refiactive index whoi solved in [Pg.202]

The four most commonly used LC detectors are the UV detector, the fluorescence detector, the electrical conductivity detector and the refractive index detector. Despite there being a wide range of other detectors to choose from, these detectors appear to cover the needs of 95% of all LC applications. This is because the major use of LC as an analytical technique occurs in research service laboratories and industrial control laboratories where analytical methods have been deliberately developed to utilize the more straight forward and well established detectors that are easy and economic to operate. LC detectors are more compact than their GC counterparts and need much less ancillary support. Most operate solely on the mobile phase and need no other fluid supplies for their effective use. All LC detectors are 3-5 orders of magnitude less sensitive than their GC counterparts and thus sensor contamination is not so severe, and generally less maintenance is required. 

The choice of detector can be quite critical. Uv detectors are very sensitive but are of little use if molecules without chromophores are being separated. A refractive index detector is universally applicable but has the drawback that gradient elution is virtually impossible. 

Another detector, which has found considerable application, is based on the changes in the refractive index of the solvent that is caused by analyte molecules. In contrast to most of the other detectors listed in Table 32-1, the refractive index detector is general rather than selective and responds to the presence of all solutes. The disadvantage of this detector is its somewhat limited sensitivity. Several electrochemical detectors that are based on potentiometric, conductometric, and voltammetric measurements have also been introduced. An example of an amperometric detector is shown in Figure 32-9. 

The refractive index detector is applicable to all compounds, although it is not as sensitive as the uv detector. An RI detector can detect differences of about 10 RI units, which means that about 5x10 g/mL must pass through the detector for a favorable response. As a general rule, the sensitivity in mg of sample is almost equal to the reciprocal of the differences in refractive index between the solvent and the sample. Figures 19-43 and 19-44, show two basic methods of refractive index measurement. 

Refractive index detectors are bulk property detectors with a near universal response, albeit limited by poor sample detectability for some applications [101 -106]. Differential detection is employed to minimize background noise with the result that the detector response is in some way related to the difference in refractive indices of the mobile phase and the mobile phase containing the sample. Consequently, an analyte will be detected only if its refractive index is different from that of the mobile phase. Peaks in both a positive and negative direction may be observed in the same chromatogram depending on whether the analyte has a higher or lower refractive index than that of... 

The evaporative light-scattering detector (ELSD) is a near universal detector suitable for the determination of (mainly) neutral compounds that are less volatile than the mobile phase used for the separation [151,152]. Primary uses include the detection of compounds with a weak response to the UV detector, especially carbohydrates, lipids, surfactants, polymers and petroleum products. Its greater sensitivity and ease of use in gradient elution separations makes it preferable to the refractive index detector for these applications. The ELSD is compatible with most volatile solvents used for normal and... 

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