Gas Chromatography Detectors

Gas chromatography detectors are devices that detect the presence of solute vapors as they are eluted from a gas chromatographic column.

Traces of vapor modify the properties of a gas far more extensively than traces of solute modify the properties of a liquid. As a consequence, the detection of vapors in gases is easier than the detection of traces of solutes in liquids.

It follows, that GC detectors are generally far more sensitive than LC detectors and there are more of them. The early GC detectors, the gas density balance (that responded to the change in density of the gas),

The thermal conductivity detector (TCD) (that responded to the change in specific heat and thermal conductivity of the gas) and the flame thermocouple detectors (that responded to the heat of combustion of a gas) all had sensitivities of about 5 x 10-7 g/ml at a signal to noise ratio of 2. As demands for improved sensitivity became greater to permit high efficiency columns to be operated, more sensitive GC detectors were developed.

One of the first high sensitive detectors to be described was the flame ionization detector (FID) (that responded to the ion current produced in the flame during the combustion of carbon containing solutes). This detector, although giving a relatively small ion current response (ionization efficiency only 0.0015%), had very low noise, and, consequently, a sensitivity of about 5 x 10-12 g/ml for n-heptane at a signal-noise ratio of 2.

A development of this detector, the nitrogen phosphorous detector (NPD) also provided very high sensitivities, but selectively to nitrogen and phosphorous containing compounds. The argon detector (developed at about the same time as the FID) provides sensitivities about an order of magnitude greater than the FID. It has an ionization efficiency of about 0.5 % but far greater noise than the FID. It functions by producing metastable argon atoms that have energies of 11.6 electron volts, sufficient to ionize almost all organic compounds. On collision with an organic molecule, the molecule is ionized and the metastable argon atom reverts to its normal state.

A further development of the argon detector was the electron capture detector (ECD) which exhibits a sensitivity nearly an order of magnitude greater than the argon detector, but is highly specific and will only give a significant response to electron capturing substances (e.g. halogenated compounds).

The group of GC detectors mentioned, represent those of historic interest and those most commonly used in general gas chromatography. There are many other GC detectors that are less common and that are generally used for very special applications.