A chromatographic detector serves to identify and quantify the sample components as they reach it in a sequence after separation in the chromatographic column. Common chromatographic detectors, their properties and their applications are discussed in an earlier article.
Sensitivity versus specificity or selectivity of a detector
It is necessary to understand the difference between sensitivity of detector and its selectivity or specificity. A detector is called sensitive if it responds to a range of compounds that have a common physical or chemical property. On the other hand a selective or specific detector responds to a specific compound bearing a characteristic functional group. A non – selective detector will respond to almost all eluting compounds except the carrier gas itself
Destructive and nondestructive detectors
A detector has a characteristic feature of giving a response when a compound reaches it and gives the signal in proportion to its quantity.Detection is possible through either destructive of nondestructive processes,ie,whether the compound retains its identity or gets destroyed during detection.
Advantages of nondestructive detectors
- Components on passing through the detector do not get decomposed or altered chemically and retain their identity
- It is possible to couple such detectors with other sensitive techniques such as GC-TGA, GC – FT – IR, GC- MS for confirmatory studies on components present and their structures. Destructive detectors cannot be used with such high sensitivity techniques
Concentration versus Mass Flow response
A detector can respond to either changes in concentration of a compound of interest or to the rate of mass transfer of compound in the carrier gas stream.
Non-destructive detectors such as thermal conductivity detector, electron capture detector and photo ionization detector are generally sensitive to concentration changes. Dilution of the stream with make-up gas lowers the detector response.On the other hand destructive detectors such as flame ionization detector, flame photometric detector and nitrogen phosphorus detector are sensitive to mass flow changes of eluents in the carrier gas stream. The response of such detectors also remains unaffected by dilution with make-up gas. In other words any changes in carrier gas flow rate will have negligible effect on detector response.
To summarize detector response should be reproducible for same changes in composition of carrier gas and it should also have a large linear dynamic range to permit response over widely varying concentration of solutes in the samples.