The thermal conductivity detector (TCD) is one commonly used with gases. It is especially favoured for the measurement of permanent gases such as hydrogen, nitrogen or argon.
The TCD utilises differences in the thermal conductivity of gases to speciate between them. As the gases flow over the detector they remove heat at different rates. This cooling of the detector influences its electrical resistance, which can be measured using a Wheatstone bridge electrical circuit. A reference gas, generally helium, is passed over another arm in the Wheatstone bridge.
As long as the detector is operational, a continuous flow of reference and sample gas are required to pass over the heated TCD coils. An interruption in flow will result in overheating of the detector and damage to the instrument. For this reason, it is recommended to use an automatic changeover gas supply panel for the TCD reference gas, or GC-TCD carrier gas.
The response rate of a TCD is very fast and can generally be measured in microseconds. For this reason, the technique is ideal for chemical process control applications, where a rapid feedback loop between measurement and process control intervention is ideal for chemical process optimisation.
Since all gases have different thermal conductivities, the TCD method is referred to as universal, meaning that it can analyse all possible gaseous species. This feature has led to the adoption of the TCD as a favoured method of measurement for so-called "permanent" gases such as argon, nitrogen, hydrogen or carbon dioxide, which do not contain any H-C bonds and are therefore not detectable using an FID.
The combination of the above two properties has made TCD instruments a popular choice for process control applications that involve hydrogen purity analysis, such as in steam methane reformers where syn-gas and hydrogen are produced then purified using pressure swing adsorption systems. Coregas can manufacture a range of calibration gas mixtures for such applications.
The main disadvantage of the TCD is that it has a relatively high detection limit: in the order of 100ppm. So for more sensitive analysis requirements a more specific instrument should be used, for example an FID for hydrocarbons, where the sensitivity would be one order of magnitude better, down to the 10 ppm range.
The combination of gas chromatography for species separation and a TCD detector for quantitative analysis is one of the most common and generic GC setups and is referred to as GC-TCD. Use of a thermal conductivity detector (TCD) can be combined with other detectors because this is a non-destructive technique. So, GC-TCD-FID is a possible instrumentation setup.
When used in combination with gas chromatography, the TCD requires no additional detector gases because helium, as the most common carrier gas for gas chromatography, is used to transport the sample over the TCD element. Helium is also used as the reference gas in the TCD Wheatstone bridge circuit.
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