Gas Chromatography Detectors - The General Properties of GC Detectors > Page 30
The General Properties of GC Detectors
The GC detector is designed to respond to very small quantities of vapor contained in a permanent gas. Because the physical and chemical properties of permanent gases differ widely from those of a vapor, a very wide range of detection methods can be employed including the measurement of standard physical properties such as thermal conductivity and light adsorption to more specific properties such as ionization potentials and heats of combustion.
The response of a GC detector can be general or specific but a detector with a catholic response is generally more useful in routine analyses. Aspecificdetector(e.g.,the nitrogen-phosphorus detector (NPD)) can be extremely useful for selectively monitoring compounds such as herbicides and pesticides, when the compounds are not eluted discretely but mixed with a number of other contaminating compounds.
GC detectors should be insensitive to changes in flow rate but, unfortunately, few detectors have this attribute although some, for example the FID, are virtually insensitive to changes in column flow rate. This allows the use of flow programming development if so desired. Flow programming, attempts to achieve the same result as temperature programming which is to accelerate the strongly retained peaks through the column (see Gas Chromatography). Some detectors require no other gas than that used as the carrier gas, other require specific gases to be added to the columns eluent for them to function. In some cases the detector prescribes a certain gas to be used as the carrier gas (e.g., the sensitivity of the katharometer is greater when helium is used as the carrier gas). In addition, if the gas chromatograph is being used for permanent-gas analysis, then helium must be used to differentiate the carrier gas from the other gases being analyzed.
All gas chromatographs are designed to operate over relatively wide ranges of temperature (e.g., -20oC to 400oC). Consequently, to avoid solute condensation in the detector or detector-connecting tubes, the detector should be capable of operating at least 20oC higher than the maximum column temperature. Temperature programming is used in many analyses and to assure temperature stability, the detector is usually thermostatted in a separate oven. Few GC analyses are carried out at sub–ambient temperatures but when they are, care must be taken to avoid condensation, particularly in areas where there are electrical connections to the detector.