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Gas Chromatography: Types of Detectors-I

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The thermal conductivity detector, or TCD, measures the thermal conductivity of the surrounding gas using thermistors arranged in a Wheatstone bridge circuit.

A Wheatstone bridge involves balancing four resistances in a bridge configuration to determine an unknown resistance by tuning the voltage to zero.

In TCD, pure carrier gas passes through the reference arm, while the column effluent passes through the other arm of the circuit.

When the column effluent contains the carrier gas alone, the circuit is balanced.

The appearance of sample vapors unbalances the bridge, resulting in a chromatographic signal.

In a flame ionization detector or FID, the column effluent vapors are mixed with hydrogen gas and burned in the air to produce a flame rich in electrons and ions.

When a potential is applied across the flame, a small current is generated, which is amplified to obtain the analytical signal.

In an electron capture detector or ECD, a beta emitter ionizes the mobile phase and produces electrons that generate an electric current between the electrodes.

When solutes with electronegative groups capture electrons, the decrease in the current serves as the signal.

Overview

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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.

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Gas Chromatography

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