SEC Concentration Detectors and Sample Concentration

A number of detector types for specific additional information will be considered later but the most basic SEC application requires a concentration detector to monitor the polymer as it emerges from the column.

The most common form of concentration detector encountered in SEC is a differential refractive index (DRI) detector. These detectors are a cost-effective option which are nearly universal in application but they have limited sensitivity and require a very good temperature control. Since SEC is normally used to look at bulk components (rather than trace components), the poor sensitivity is not normally a problem but some difficulties may be encountered where the differential refractive index for the polymer/solvent combination is so small that baseline noise becomes significant.

The other universal detector used in SEC is the ELSD (evaporative light scattering detector, also known as an evaporative mass detector). These units involve atomization of the eluent into a steady flow of inert gas where the solvent is removed and the remaining solute particles are detected by the light scattering they induce. It is important to distinguish between these ELSD and the light scattering from solutions considered later. The ELSD is more expensive than DRI detectors and there are potentially high running costs for the large amount of inert gas consumed. However, they are less affected by any variation in solvent composition or temperature and are more universal in the size of the response regardless of chemical composition of the polymer.

Figure 2 is a schematic diagram of a commercially available evaporative light scattering detector.

Other types of HPLC detector (such as ultra-violet, infrared or radiochemical) are also used for SEC but are more restricted in applicability and are used more as selective detectors to pick out specific components. There are latter comments with regard to the SEC of copolymers.

Figure 2 A schematic diagram of a commercially available evaporative light scattering detector. Printed with the permission of Polymer Laboratories Ltd, Church Stretton, Shropshire, UK.
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