, in terms of the physical properties of the plate and the volume flow of mobile phase that passes through it must be derived. Consider the (n)th theoretical plate in a GC column, as depicted in Figure 21. The properties of the plate are defined as follows, vg is the volume of gas in the plate, vl is the volume of liquid (stationary phase) in the plate, vS is the volume of support in the plate, Sl is the specific heat of the stationary phase, SS is the specific heat of the support, rl is the density of the stationary phase, rS is the density of the support, Xl(n) is the concentration of solute in the stationary phase in plate (n), Xg(n) is the concentration of solute in the mobile phase (gas) in plate (n), q is the excess temperature of the plate above its surroundings

property detector measures some bulk physical property of the eluent (such as dielectric constant or refractive index) and the solvent property detector, measures some physical or chemical property that is unique to the solute (such as heat of combustion or fluorescence). Detectors can also be classified as concentration sensitive devices such as the katharometer or mass sensitive devices such as the flame ionization detector (FID). Another method of classification is to define detectors as specific or non-specific. An example of a specific detector would be the nitrogen phosphorous detector (NPD), which as its name implies detects only those substances that contain nitrogen or phosphorous. A non-specific detector would be the katharometer detector which senses all vapors that have specific heats or thermal conductivities different from those of the carrier gas. In general (though not always), non specific detectors have lower sensitivities than the specific detectors, the reasons

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 the filament, the detector is extremely

to be specific and sense specific types of sample (e.g., halogenated substances by the electron capture detector). Conversely, those detectors with a catholic response, although highly sensitive compared to LC detectors (e.g. the flame ionization detector) are significantly less sensitive than the specific detectors. The detectors with a catholic response are the most popular and the majority of GC separations are monitored by the flame ionization detector (FID). The most commonly used specific detectors are the nitrogen phosphorus detector (NPD) and the electron capture detector (ECD) The katharometer detector, although having relatively poor sensitivity is widely used in gas analysis. The Flame Ionization Detector The FID, invented by Harley and Pretorious (7), and separately by McWilliams and Dewer (8), evolved from the Heat of Combustion Detector developed by Scott (9). The FID detector employs hydrogen as the combustion gas which is mixed with the

equilibrium conditions, the heat generated in the filament is equal to the heat lost by conduction and convection and consequently the filament assumes a constant temperature. At the equilibrium temperature, the resistance of the filament and thus the potential across it is also constant.     Figure 29. The Katherometer Detector ("In-Line Cell")   The heat lost from the filament will depend on the thermal conductivity of the gas and its specific heat and both these parameters will change in the presence of a foreign gas or solute vapor. The presence of a different gas entering the detector causes the equilibrium temperature to change, producing a change in potential across the filament. This potential change is amplified and fed to a suitable recorder. A diagram of the katherometer is shown in figure 29

are placed equidistant up stream and down stream of the heater. A proprietary set of baffles situated in the main conduit creates a pressure drop that causes a fixed proportion of the flow to be diverted through the sensor tube. At zero flow rate both sensors are at the same temperature. At a finite flow rate, the down stream sensor is heated, producing a differential temperature across the sensors. The temperature of the gas will be proportional to the product of mass flowing and its specific heat and so the differential temperature that will be proportional to the mass flow rate. The differential voltage from the two sensors is compared to a set voltage and the difference used to generate a signal that actuates a valve controlling the flow. Thus, a closed loop control system is formed that maintains the mass flow rate set by the reference voltage. The device can be made extremely compact, is highly reliable and affords accurate control of the carrier gas flow rate