Katherometer detector

The pressure sensitivity of a detector is extremely important as it is one of the detector parameters that determines both the long term noise and the drift. As it influences long term noise, it will also have a direct impact on detector sensitivity or minimum detectable concentration together with those other characteristics that depend on detector sensitivity. Certain detectors are more sensitive to changes in pressure than others. The katherometer detector, which is used frequently for the detection of permanent gases in GC, can be very pressure sensitive as can the LC refractive index detector. Careful design can minimize the effect of pressure but all bulk property detectors will tend to be pressure sensitive. 

The separation shown in figure 7 of a mixture of aliphatic and aromatic hydrocarbons was monitored using both a katherometer detector that responded to all solutes and the emissivity detector that selectively responded to the aromatics. It is seen that the emissivity... 

As the detector filament is in thermal equilibrium with its surroundings and the device actually responds to the heat lost from the filament, the katherometer detector is extremely flow and pressure sensitive. Consequently, all katherometer detectors must be carefully... 

It is seen that a sensitivity change is made on the katherometer detector after the carbon monoxide had been eluted. The separation is adequate and is efficiently monitored by the katherometer. 

The presence of a solute in the mobile phase will change the thermal properties of the mobile phase and, thus, any device that can monitor the thermal properties of the solvent could potentially perform as a LC detector. The GC katherometer detector acts on this principle and... 

The katherometer detector is used extensively in gas analysis and for this purpose is the most popular detector. It has only moderate sensitivity, which, however, is quite adequate for most gas analysis applications. It has a linear dynamic range that covers about three orders of magnitude and has the distinct advantage of requiring only one flow-controlled gas supply. Unfortunately, it is very sensitive to changes in flow rate and ambient temperature and thus must be well thermostatted. It is compact, rugged, requires very simple ancillary electronic equipment and thus can be relatively inexpensive. Besides its popular use for gas analysis it is also popular as a teaching instrument. 

The katherometer detector is a mgged, very forgiving detector that can be quite seriously abused in operation and still provide accurate quantitative results. This, of course, is a direct result of its relatively low sensitivity. The main problem with the katherometer is instability due to poor control of operating and ambient conditions. The katherometer is very sensitive to temperature changes and changes in columns flow rate (the reasons have already been discussed). Consequently, the flow controller and the sensor oven controls must be well maintained to ensure a precise column flow rate and a precise... 

There are a large number of GC detectors available but the majority of GC separations are monitored by the flame ionization detector (FID), the nitrogen phosphorus detector (NPD), the electron capture detector (BCD) or the katherometer detector (or Hot Wire Detector). Tlie latter is almost exclusively used in gas analysis and rarely used in chiral chromatography and so will only be briefly described here. Furthermore, the FID is used in probably 90% of all chiral analyses. However, before describing the construction and function of each detector the subject of detector specifications needs to be discussed. 

Concentration sensitive detectors provide an output that is directly related to the concentration of solute in the mobile phase passing through it. In GC the katherometer would be an example of this type of detector whereas in LC the UV absorption detector would be typical of a concentration sensitive detector. 

The noise level of detectors that are particularly susceptible to variations in column pressure or flow rate (e.g. the katherometer and the refractive index detector) are often measured under static conditions (i.e. no flow of mobile phase). Such specifications are not really useful, as the analyst can never use the detector without a column flow. It could be argued that the manufacturer of the detector should not be held responsible for the precise control of the mobile phase, beitmay a gas flow controller or a solvent pump. However, all mobile phase delivery systems show some variation in flow rates (and consequently pressure) and it is the responsibility of the detector manufacturer to design devices that are as insensitive to pressure and flow changes as possible. 

The sensitivity of the detector was similar to that of the katherometer i.e. about 1 x 10 g/ml. Unfortunately, the practical lifetime of this detector was also relatively short as it was eclipsed by both the FID and the argon ionization family of detectors introduced by Lovelock. Nevertheless, the development of these early detectors not only helped establish GC as a viable analytical technique but also stimulated the development of other types of vapor sensing devices. The future... 

McWilliams and Dewer [12] described the flame ionization detector which was to be the work horse of all future gas chromatographs. Further developments of the flame thermocouple detector were described by Primavesi etal. [13], the design of the katherometer was simplified by Stuve [14] and Grant [15] described the first thermal emissivity detector. 

The Katherometer and Some of the Less Well Known Detectors... 

The out-of-balance signal caused by the presence of sample vapor in contact with the sensor is amplified and fed to a recorder or computer data acquisition system. For maximum sensitivity hydrogen should be used as the carrier gas, but to reduce fire hazards, helium can be used with very little compromise in sensitivity. The sensitivity of the katherometer is only about 10 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). Although the least glamorous, this detector can be used in most GC analyses that utilize packed columns and where there is no limitation in sample availability. The device is simple, reliable, and rugged and, as already stated, relatively inexpensive. 

The sensitivity of the detector was similar to that of the katherometer, i.e. about 10 g/ml. Its response was partly determined by the distribution coefficient of the solute vapor between the carrier gas and the absorbing layer as well as the chemical nature of the solute vapor. As a consequence, the response varied considerably between different solutes. Within a given homologous series, the response increased with the molecular weight of the solute but this was merely a reflection of the increase in the value of the distribution coefficient with molecular weight. Although an interesting alternative method of detection, this detector has been little used in GC and is not commercially available. 

Gas chromatography is an analytical technique that has reached what might be termed a "steady state" and this is also true for the development of GC detectors. The four detectors FID, BCD, NPD and the katherometer are now well established, are the most popular and are employed in over 95% of all GC applications. Of the four, the FID is the most versatile, sensitive and linear, and should be the first to be considered when facing the challenge of a new, and hitherto unknown sample containing unfamiliar substances. Nevertheless, specific and unusual samples can demand special detecting conditions particularly for certain environmental and forensic analyses. In such cases, one of the many other detectors described may well be found more appropriate. Fortunately, there is a fairly wide range of practical GC detectors from which to choose that are commercially available. 

The TCD or katherometer is a bulk property detector, that is, it responds to some overall physical property, the thermal conductivity, of the carrier gas. 

GC detectors exhibit a wide range of sensitivities. At one extreme the katherometer has a sensitivity of about 1 X 10 g/ml and at the other, the electron capture detector can detect eluents at levels of 2 x lO g/ml. 

Flow sensitivity is measured as the change in detector output for unit change in flow rate through the sensor cell. The response of the FID is virtually unaffected by flow rate changes and, in fact, only responds to the mass of solute passing through it per unit time. In contrast, the katherometer is very sensitive to changes in flow rate and requires to be operated with a reference cell to compensate for any fluctuations in column rate. 

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