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Thermal conductivity detector
The TCD is a universal, concentration-dependent detector, as it measures the thermal conductivity of the eluate and converts it into an electrical signal. However, the analyte must have a different thermal conductivity than the carrier gas in order to be determinable. The substances are not destroyed during the detection and the TCD can therefore be connected in series with another detector.
Detection principle: A heating element is blown cold or hot by the gas flow of the eluate.
The heating element has a temperature-dependent resistance (temperature is measured indirectly via a resistance at the heat source); Alternative: thermistors for low temperatures (small detector geometry possible, insensitive to oxidation.)
The carrier gas flows around the heating element and thereby cools it down.
The sample in the carrier gas changes the amount of heat given off by the heating element to the gas flow (result: peaks with positive and negative signs are possible).
When coupling with a Wheatstone bridge circuit, the balance of the bridge is disturbed and a voltage equivalent to the signal is applied.
- Tab. 1
- Carrier gases used in the thermal conductivity detector
|HeliumHe||has a very high thermal conductivity compared to other gases and the samples (and therefore high sensitivity)|
|Hydrogen H.2||like he; Hydrogenation on the heating element possible (destruction of the thermistor by reductive processes)|
|ArgonAr||for the determination of permanent gases|
|Nitrogen N.2||for the analysis of organic compounds|
- Tab. 2
- Thermal conductivity detector
|▪ simple structure (only one type of gas required for operation) ▪ cost-effective; universal detection; substances are not destroyed (series connection possible / analytes can be collected separately in cold traps) ▪ sensitivity of the detector can be influenced by the carrier gas||▪ high carrier gas purity (no H.2O and O2) ▪ well stabilized carrier gas flow, temperature and supply voltage; no mechanical vibrations; reactions of the sample on the heating element; changes between the measuring cell and the reference cell due to oxidative processes (asymmetry of the bridge circuit, increased background signal); ▪ mostly not very sensitive; “bleeding” from the column is detected|
Micro-TCD (flow modulated TCD):
An alternative to eliminate the different behavior of the measuring and reference cell is to lead the measuring gas flow and the reference gas flow alternately (frequency ~ 10 Hz) over the same heating element. This makes it possible to electronically eliminate flow variations immediately and to push the detection limit further (significantly lower detector volume).