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,

nbsp; The Katharometer Detector The first alternative GC detector to be devised was the katharometer introduced by Ray [7] (now known more prosaically as the hot wire detector (HWD)). It consists of two heated filaments, situated in the arms of a Wheatstone bridge, one suspended in the eluent gas from the column and 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

A filament carrying a current is situated in a tubular cavity through which flows the column eluent. Under equilibrium conditions, the heat generated in the filament is equal to the heat lost and consequently the 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

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 analytical detectors for preparative work

nbsp;                                                                          where (A) is the surface area of the plate and (Z) is the thermal conductivity of the plate and its contents, given in appropriate units. Substituting for (dt) from equation (66),                                                            (69) Substituting

process can be considered as almost instantaneous. The filament wire is usually made from tungsten or platinum as both metals have high temperature coefficients of resistance and at the same time are relatively inert. The column and reference filaments are situated in the arms of a Wheatstone Bridge and a suitable current is passed through the filaments to heat them significantly above ambient temperature. To ensure temperature stability, the sensors and their conduits are installed in a high thermal conductivity metal block which is thermostatted by means of a separate oven. The performance of the in-line sensor is almost identical to that of the off-line sensor. For maximum sensitivity hydrogen or helium is used as the carrier gas. The katharometer sensitivity is only about 10-6 g/ml (probably the least sensitive of all GC detectors) and has a linear dynamic range of about 500 (the response index being between 0.98 and 1.02).    Courtesy of Supelco Inc