11.15 : Gas Chromatography: Types of Detectors-I

There are different types of detectors used in gas chromatography, each with its own specific properties that make it suitable for detecting certain types of analytes. The most commonly used detectors in GC are thermal conductivity detector (TCD), flame ionization detector (FID), and electron capture detector (ECD).

TCD is the earliest and most widely used detector that operates by measuring the changes in the thermal conductivity of the carrier gas. When a sample compound enters the detector, it changes the thermal conductivity of the carrier gas, and this change is detected. TCD offers a linear response over a wide range of concentrations and is nondestructive, allowing solute isolation. However, its detection limit is relatively poor compared to other detectors.

FID operates by the combustion of organic compounds in a hydrogen-air flame. The analytes in a sample elute from the GC column and enter the flame, where they are combusted and produce ions. The current generated by these ions is proportional to the concentration of the analyte, and it is recorded as an electrical signal. The FID is highly sensitive, and it responds to a wide range of organic compounds, especially hydrocarbons. However, FID destroys the sample during analysis.

ECD is a selective detector that utilizes a radioactive beta source to ionize the mobile phase, producing an electric current between electrodes. When a solute with high electron capture potential elutes from the column, the electric current decreases, serving as the signal. ECD is highly selective towards solutes with electronegative functional groups and electron-capturing analytes like chlorinated compounds, and it is relatively insensitive to other compounds. It has an excellent detection limit but a limited linear range of only about two orders of magnitude.

The choice of the detector depends on the specific requirements of the analysis.