Refractive index detector
A refractive index detector ( English refractive index detector ), and RI detector mentioned, is an in the chemical analysis used measuring device for determining the varying concentration of dissolved substances in the flow of a solvent , especially for chromatographic applications ( HPLC and GPC ). A device for determining the refractive index of a pure substance is called a refractometer .
Refractive index detectors are universal detectors, so they register all substances that have a change in the refractive index of the solution compared to the pure solvent. Their sensitivity therefore depends on the extent to which the refractive indices differ from the pure solvent and the dissolved substance. Because of this generally rather small difference, the detector sensitivity is, for example, rather low in comparison to UV absorption detectors. RI detectors show a linear response behavior over a concentration range which typically comprises about four powers of ten.
A refractive index detector consists of three core elements:
- Measuring block with two compartments for analyte and reference as well as adjustment device and mirror
- Light source
- a photo measuring cell covered by a gap
The liquid to be analyzed flows through the measuring cell; the reference cell contains the same liquid, usually pure solvent , during the calibration and measurement phases. A light beam is guided through both chambers and hits the photocell through the slit opening. In the calibration phase, in which the measuring cell is flushed through by the pure eluent , the gap is positioned so that the photocell is illuminated to the maximum. If there is a liquid with a different refractive index than that of the liquid in the reference cell during the measurement phase, the light beam does not hit the gap opening exactly and the amplitude of the signal from the photocell decreases. This is interpreted by the evaluation unit as a signal which can be converted into the actual concentration of the analyte on the basis of calibrated values.
Since the refractive index of a liquid or solution is highly dependent on temperature, a constant temperature of the eluent is essential for error-free measurements. Modern commercial devices therefore have precisely temperature-controlled measuring cells.
Area of application in chromatography
Refractive index detectors and the like are used. a. in the field of high-performance liquid chromatography , in order to register substances in a liquid (the so-called solvent or eluent) that would not be detectable by other - more sensitive or more selective - measuring methods. Refractive index detectors can be used for substances that do not absorb in the UV range and cannot be detected with conductivity detectors, for example when analyzing sugars . In gel permeation chromatography of polymers, RI detectors are often used due to their simple design, since the refractive index of polymers is constant at different molar masses , so detection is quantitative.
The gradient elution often used in liquid chromatography, in which the solvent composition is changed during the separation of an analysis mixture, is not compatible with the detection of the analytes using an RI detector. Isocratic separation processes (in which the composition of the eluent remains constant) are essential, as changing the composition of the mobile phase would also lead to a change in the refractive index and thus to a detector signal.
- Karl Kaltenböck: Chromatography for Dummies. Wiley-VCH, Weinheim 2010, ISBN 978-3-527-70530-6 , p. 156 ( limited preview in Google book search).
- Sandie Lindsay: Introduction to HPLC. Vieweg, Braunschweig 1996, ISBN 3-528-06759-4 , p. 75 ( limited preview in the Google book search).
- Walter Wittenberger: Chemical laboratory technology. 7th edition. Springer, Vienna / New York 1973, ISBN 3-211-81116-8 , pp. 297-298.