Refractive index detectors response characteristics

The ideal HPLC detector should have the same characteristics as those required for GC detectors, i.e. rapid and reproducible response to solutes, a wide range of linear response, high sensitivity and stability of operation. No truly universal HPLC detector has yet been developed but the two most widely applicable types are those based on the absorption of UV or visible radiation by the solute species and those which monitor refractive index differences between solutes dissolved in the mobile phase and the pure mobile phase. Other detectors which are more selective in their response rely on such solute properties as fluorescence, electrical conductivity, diffusion currents (amperometric) and radioactivity. The characteristics of the various types of detector are summarized in Table 4.14. 

The detector converts a change in the column effluent into an electrical signal that is recorded by the data system. Detectors are classified as selective or universal depending on the property measured. Selective (solute property) detectors, such as fluorescence detectors, measure a physical or chemical property that is characteristic of the solute(s) in the mixture only those components which possess that characteristic will be detected. Universal (bulk property) detectors measure a physical property of the eluent. Thus, with refractive index (RI) detectors, for example, all the solutes which possess a refractive index different from that of the eluent will be detected. Selective detectors tend to be more sensitive than universal detectors, and they are much more widely used. Universal detectors are more commonly used in preparative chromatography, where a universal response is desired and sample size is large. 

Cut points specify the beginning and end of the collection of a fraction. Cut points can be based on time, although in practice time is not the most stable parameter, due to possible fluctuations in the flow rate. They can be based on threshold values of the UV-detector signal or on volume of eluent through a mass flowmeter. Cut point locations can also be a combination of a characteristic feature on the chromatogram and volume. Cut point locations for valve switching may also be based on the response of less traditional detection instrumentation such as the on- line measurement of density, conductivity, temperature, spedflc ion detection, near infrared, or refractive index. 

The reliability of polymer characterization by SEC has been significantly advanced by new detection strategies based on series-coupled detectors with complementary response characteristics together with appropriate software for calculating molecular weights from the simultaneous detector inputs [518,522-526]. The three most popular detector combinations are refractive index and viscometry, light scattering and refractive... 

Detectors are based on a selective response for the solute, such as UV-absorbance or fluorescence, or on a bulk property of the mobile phase which is modified by the solute, such as refractive index. Ideally, detectors shoiild have the following characteristics ... 

The most common application of this approach is the tentative identification of compounds that contain characteristic UV or visible absorption spectra. The use of more than one detector, linked in series or parallel after the chromatographic columns, can provide comparative information which reduces the possibility of incorrent assignment of the identity of a component. A simple example is the use of a UV absorbance detector in line with a differential refractive index (RI) detector. The latter will respond to most substances, whereas the former detector is quite selective in its response. 

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