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The Science Behind the Scenes: How Do Thermal Conductivity Detectors Work Without Destroying the Sample?
In gas chromatography analysis, the thermal conductivity detector (TCD) plays an irreplaceable role. This instrument, called the katharometer, can not only accurately measure the concentration of each compound in the sample, but also ensure that the sample is not polluted. Cause damage. Whether in the medical, energy or food industry, the application of TCD has been deeply rooted in people's hearts, making people full of interest in this technology.
When an analyte elutes from the column, the thermal conductivity of the effluent gas decreases, producing a detectable signal.
Working Principle of Thermal Conductivity Detector
TCD consists of a heating wire and a temperature-controlled detector. Under normal circumstances, the heating wire continuously transfers a steady amount of heat to the detector body. When the analyte elutes, if its thermal conductivity is lower than that of the carrier gas (usually helium or hydrogen), the heating wire will heat up due to the change in heat flow, resulting in a change in resistance. This change can be measured using a Wheatstone bridge circuit, which produces a measurable voltage change.
TCD is considered a universal detector that can detect almost all compounds, both organic and inorganic.
Comparison with other detection technologies
The advantages of TCD over flame ionization detector (FID) are its non-specific and non-destructive characteristics. This means that the TCD can be used more broadly when performing preliminary sample analysis, whereas the FID is only effective for flammable compounds. The detection limit of TCD is comparable to that of FID, both of which can reach low concentration levels. However, due to the high flammability of hydrogen, many places prefer to use helium as a carrier gas, which further highlights the safety of TCD.
Notes during operation
There are several important considerations when using TCD. For example, when the heating wire is at high temperature, the gas flow must remain stable to avoid burning. Also, although the heating wire is usually chemically passivated to avoid reaction with oxygen, the passivation layer may be damaged if it comes into contact with halogen compounds, so such compounds should be avoided as much as possible during analysis.
When detecting hydrogen, using helium as the reference gas will cause the peak value of hydrogen to appear as a negative value. This problem can be avoided by using argon or nitrogen as the reference gas, but this will significantly reduce the detection sensitivity of other compounds. .
Applications of Thermal Conductivity Detectors
TCD is widely used in many fields. It is used not only in lung function tests in medical devices, but also in gas chromatography. Although it takes longer to obtain results than mass spectrometry, TCD is still favored in certain situations due to its low cost and good accuracy. In addition, TCD has also shown its value in the following applications:
- Monitor hydrogen purity in hydrogen cooled turbine generators.
- Detect helium leaks in helium storage tanks for MRI superconducting magnets.
- Quantify the amount of carbon dioxide in beer samples.
- Quantifying the heating value of methane in biogas samples in the energy industry.
- Quantification and verification of food packaging atmospheres in the food and beverage industry.
- In the oil and gas industry, quantifying the percentage of hydrocarbons found in a formation when drilling.
As science and technology continue to advance, we can look forward to how future developments in thermal conductivity detectors will change the way we analyze and apply various types of samples. Are you also curious about what kind of breakthroughs and changes future technology will bring?
