Role of a Detector
A detector is the final component in the chromatographic system and provides real time information on the separated components. After separation of the components by the chromatographic column they move sequentially to the detector and generate response in proportion to the concentration of solute in the mobile phase stream. Chromatographic detection is based on a set of physical properties such as light absorption, fluorescence, light scattering, refractive index changes, electrical conductivity or differences in mass of separating species. The sensitivity and selectivity of the response is dependent on detector characteristics.
Desirable Detector Features
Detector features play a vital role in the quality of results
- Inertness towards sample and mobile phase
A detector made of material that is not inert to the mobile phase or the sample components is bound to give erroneous results due to loss of analyte or extra peaks resulting from reaction products
- High sensitivity towards solute over mobile phase
The detector should not respond to changes in mobile phase composition except for bulk property detectors such as refractive index detectors. The detector should give a noise free stable base line before the sample is injected.
- Low cell volume
Cell volume should be small enough to prevent overlap of eluting peaks. Commonly available cell
volumes are in the range 2-8μl. Small cell volume mean less dispersion but on the other hand
larger cell volumes provide greater sensitivity. Optimum use of cell volume will give best results.
Limit of detection is the lowest concentration of the substance which can be detected using a detector. It is defined as the concentration which will produce a signal to noise ratio of 3:1. However, authentic quantitation can be obtained at signal to noise levels of at least 10:1.
- Fast Response Time
A sensitive detector is one which provides rapid response to changes in mass of analyte reaching it. Slow response can have adverse effect on peak shape so an ideal detector should produce instantaneous response to mass changes.
- Large Linear Dynamic Range
Dynamic range and linear dynamic range are often synonymously used but are different features. The dynamic range is the solute concentration range over which the detector output will be dependent .The linear dynamic range on the other hand is that range of concentration over which the response is linear with respect to concentration. The choice of a detector is governed by its linear dynamic range over the desired concentration levels as well as sensitivity and selectivity.
A real detector will seldom meet all the desirable features but fortunately chromatographic techniques offer a wide choice depending on principle of separation and detection. You have to decide on your choice depending on optimization of results and cost considerations.