Commonly used GC detectors

1. Flame Ionization Detector (FID)

The detector is very sensitive towards organic molecules (10-12 g/s, linear range: 106 –107), but relative insensitive to a few small molecules e.g. N2, NOx, H2S, CO, CO2, H2O. If proper amounts of hydrogen/air are mixed, the combustion does not afford any ions. If other components are introduced that contain carbon atoms cations are produced in the effluent stream. The more carbon atoms are in the molecule, the more fragments are formed and the more sensitive the detector is for this compound (-- > response factor). However, due to the fact that the sample is burnt (pyrolysis), this technique is not suitable for preparative GC. In addition, several gases are usually required to operate a FID: hydrogen, oxygen (compressed air), and carrier gas.
2. Thermal Conductivity Detector (TCD)

This detector is less sensitive than the FID (10-5-10-6g/s, linear range: 103-104), but is well suited for preparative applications, because the sample is not destroyed. It is based on the comparison of two gas streams, one containing only the carrier gas, the other one the carrier gas and the compound. Naturally, a carrier gas with a high thermal conductivity e.g. helium or hydrogen is used in order to maximize the temperature difference (and therefore the difference in resistance) between two thin tungsten wires. The large surface-to-mass ratio permits a fast equilibration to a steady state. The temperature difference between the reference cell and the sample cell filaments is monitored by a Wheatstone bridge circuit.

3. Electron Capture Detector (ECD)

The detector consists of a cavity that contains two electrodes and a radiation source that emits b-radiation (e.g. 63Ni, 3H). The collision between electrons and the carrier gas (methane plus an inert gas) produces a plasma containing electrons and positive ions. If a compound is present that contains electronegative atoms, those electrons are “captured” and negative ions are formed, and the rate of electron collection decreases. The detector is extremely selective for compounds with atoms of high electron affinity (10-14 g/s), but has a relatively small linear range (~102-103). This detector is frequently used in the analysis of chlorinated compounds e.g. pesticides, polychlorinated biphenyls, which show are very high sensitivity

4. Mass Spectrometer (GC/MS)

Many GC instruments are coupled with a mass spectrometer, which is a very good combination. The GC separates the compounds from each other, while the mass spectrometer helps to identify them based on their fragmentation pattern.