Free Books and Brochures

the other in a pure reference stream of gas. In the presence of a solute, both the thermal conductivity and the heat capacity of the gas change changing the heat loss and, thus, the temperature of the filament and, consequently, its resistance The bridge is unbalanced and the out-of-balance signal is passed to a suitable monitoring device. This detector is relatively insensitive but responds to all solutes that differ in heat capacity and thermal conductivity from those of the carrier gas. This detector was used extensively in the early days of GC for the analysis of hydrocarbon gases. There was much discourse and dissent with regards to the exact mechanism of detection involved in the katharometer and even today it is considered to respond to a number of different physical properties of the eluent gas with no one property playing a major role.   The Flame Thermocouple Detector The "flame thermocouple detector" was the next detector to be reported which was developed

nbsp; The Katherometer Detector The katherometer detector (sometimes spelt catherometer and often referred to as the thermal conductivity detector or hot wire detector) is relatively insensitive but has survived largely as a result of its catholic response and, in particular, its response to the permanent gases. Consequently, it is often the detector of choice for gas analysis and environmental testing. Its frequent use in these special types of application, somewhat surprisingly, has made it the fourth most commonly used GC detector. A filament carrying a current is situated in the column eluent and, under equilibrium

chromatography detectors can have large sensor volumes and, as the detector is required only to monitor the separation, they need not have a linear response. They do need to tolerate high flow rates and thus, must have low flow impedance. Analytical detectors can be used for preparative purposes but a portion is usually split from the column eluent, diluted with more mobile phase and then passed through the detector. In practice this becomes a rather clumsy procedure. The most commonly used detector in preparative GC is the thermal conductivity detector (hot wire detector). Even this detector, however, is often too sensitive and has too high a flow impedance. Under such circumstances, the procedure mentioned above must be employed. The eluent from the preparative column is split and a small portion diverted through the detector (sometimes with further dilution with carrier gas to reduce sensitivity). In LC, the refractive index detector is probably the most useful of the

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

filament assumes a constant temperature. The heat lost from the filament will depend on both the thermal conductivity of the gas and its specific heat. Both these parameters will change in the presence of a different gas or solute vapor and as a result the temperature of the filament changes, causing a change in potential across the filament. This potential change is amplified and either fed to a suitablerecorder or passed to an appropriate data acquisition system.As the detector filament is in thermal equilibrium with its surroundings and the device actually responds to the heat lost from the filament, the detector is extremely flow and pressure sensitive. Consequently, all katharometer detectors must be carefully thermostatted and must be fitted with reference cells to help compensate for changes in pressure or flow rate.     Figure 13. The Off-Line Katharometer Sensor