Thermal conductivity detector

Thermal conductivity detector

The thermal conductivity detector (TCD) is a bulk property detector and a chemical specific detector commonly used in gas-liquid chromatography. [Grob, Robert L. Ed.; "Modern Practice of Gas Chromatography", John Wiley & Sons, C1977, pg. 228, ] This detector senses changes in the thermal conductivity of the column effluent and compares it to a reference flow of carrier gas. Since most compounds have a thermal conductivity much less than that of the common carrier gases of helium or hydrogen, when an analyte elutes from the column, the effluent thermal conductivity is reduced and produces a detectable signal.

Operation

The TCD consists of an electrically heated filament in a temperature-controlled cell. Under normal conditions there is a stable heat flow from the filament to the detector body. When an analyte elutes and the thermal conductivity of the column effluent is reduced, the filament heats up and changes resistance. This resistance change is often sensed by a Wheatstone bridge circuit which produces a measurable voltage change. The column effluent flows over one of the resistors while the reference flow is over a second resistor in the four-resistor circuit.

A schematic of a classic thermal conductivity detector design utilizing a wheatstone bridge circuit. The reference flow across resistor 4 of the circuit compensates for drift due to flow or temperature fluctuations. Changes in the thermal conductivity of the column effluent flow across resistor 3 will result in a temperature change of the resistor and therefore a resistance change which can be measured as a signal.


Applications

Since all compounds, organic and inorganic, have a thermal conductivity different from helium, all compounds can be detected by this detector. The TCD is often called a universal detector because it responds to all compounds. Also, since the thermal conductivity of organic compounds are similar and very different from helium, a TCD will respond similarly to similar concentrations of analyte. Therefore the TCD can be used without calibration and the concentration of a sample component can be estimated by the ratio of the analyte peak area to all components (peaks) in the sample.

The TCD is a good general purpose detector for initial investigations with an unknown sample. Since the TCD is less sensitive than the flame ionization detector and has a larger dead volume it will not provide as good resolution as the FID. However, in combination with thick film columns and correspondingly larger sample volumes, the overall detection limit can be similar to that of an FID. In conclusion the TCD is not as sensitive as other detectors but it is non-specific and non-destructive.

The TCD is also used in the analysis of permanent gases (argon, oxygen, nitrogen, carbon dioxide) because it responds to all these pure substances unlike the FID which cannot detect compounds which do not contain carbon-hydrogen bonds.

References


Wikimedia Foundation. 2010.

Нужно решить контрольную?

Look at other dictionaries:

  • thermal conductivity detector —    A type of gas chromatographic detector which is sensitive to the change in the ability of the gases emerging from the column to conduct heat. A thermal conductivity (TC) detector is not as sensitive as a flame ionization detector, but it is… …   Forensic science glossary

  • Chromatography detector — A chromatography detector is a device used in gas chromatography (GC) or liquid chromatography (LC) to visualize components of the mixture being eluted off the chromatography column. There are two general types of detectors: destructive and non… …   Wikipedia

  • Flame ionization detector — A flame ionization detector (FID) is a type of gas detector used in gas chromatography. The first flame ionization detector was developed in 1957 by scientists working for the CSIRO in Melbourne, Australia.… …   Wikipedia

  • Gas chromatography — A gas chromatograph with a headspace sampler Acronym GLC, GC Classification chromatography Analytes organic inorganic …   Wikipedia

  • Gas-liquid chromatography — Infobox chemical analysis name = Gas liquid chromatography caption =A gas chromatograph with a headspace sampler acronym = GLC, GC classification =chromatography analytes = organic inorganic must be volatile manufacturers = Agilent (a spin off of …   Wikipedia

  • Dumas method — For the method to determine the molecular weight of an unknown compound, see Dumas method of molecular weight determination. The Dumas method in analytical chemistry is a method for the quantitative determination of nitrogen in chemical… …   Wikipedia

  • Dissolved gas analysis — or DGA is the study of dissolved gases in insulating fluid. [1] Insulating materials within transformers and related equipment break down to liberate gases within the unit. The distribution of these gases can be related to the type of electrical… …   Wikipedia

  • History of chromatography — The history of chromatography spans from the mid 19th century to the 21st. Chromatography, literally color writing , was used mdash;and named mdash; in the first decade of the 20th century, primarily for the separation of plant pigments such as… …   Wikipedia

  • TCD — can be used as: *Chad, ISO 3 letter country code*Teller cash dispenser *Tentative Channel Designation *Thalamocortical Dysrhythmia *Thermal Conductivity Detector, a type of detector used in Gas Chromatography *Transcranial Doppler, a test that… …   Wikipedia

  • gas chromatography (GC) — Type of chromatography with a gas mixture as the mobile phase. In a packed column, the packing or solid support (held in a tube) serves as the stationary phase (vapour phase chromatography, or VPC) or is coated with a liquid stationary phase (gas …   Universalium

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”