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Study Notes: More about the FID and TCD

Flame Ionisation Detector (FID)
In an FID, carrier gas exiting from the end of the column is mixed with hydrogen and burned in air. Ions formed in the flame from combustion of the analytes enter the gap between two electrodes and reduce the gap resistance. This causes an increased current flow across the electrodes whereupon a signal is sent to the chart recorder to print out the chromatogram.

A diagram showing the essential parts of an FID including the flame, gases and electrodes.

The higher the concentration of ions, the greater the current flow. That is, the amount of ions formed is directly related to the concentration of the component.

The role of hydrogen and air is therefore to support the flame. Helium is commonly used as the carrier gas with FID as it provides best sensitivity and resolution.

An FID also requires ‘make up gas’ in order to sweep analytes exiting the column into the detection zone. Nitrogen or helium are normally the gases of choice.

The ratio of hydrogen to air is important for keeping the flame alight and providing optimum response. The ratio should be between 8 and 12% with typical flows being around 30 mL/min and 300 mL/min respectively. The make up gas is typically set at about 30 mL/min.

Two graphs showing the FID response to (i) increased air flow, and (ii) increased hydrogen flow.

Notice that increasing airflow will increase the FID response up to a maximum. Hydrogen has an opitmal flow rate with reduced response on either side.

Flow rates should be monitored for each chromatographic run. A soap bubble flow meter is commonly used for this purpose.

Gases are usually supplied from high purity, high-pressure cylinders piped to the GC.

Thermal Conductivity Detector (TCD)
The TCD is based on the principle that a hot body will lose heat at a rate which is dependent upon the composition of the surrounding gas. This heat loss is used as a measure of the gas composition.

Heat is transferred by conduction when the gas molecules strike a heated filament. The greater the number of gas molecule collisions with the filament, the greater the rate of heat loss, which is measured and converted into an electrical signal that is sent to the chart recorder. The chart recorder prints out a chromatogram representing the components in the sample.

It should also be noted that different gases have different thermal conductivity so the choice of the carrier gas and the sample composition may be considerations in the choice of this detector. The smaller the gas molecule, the higher its conductivity will be.

A diagrammatical representation of the TCD showing the position of the filament, where the gas flows in.

The filament is the heart of the TCD.

Increasing the filament current to suit particular separations will increase the filament temperature but higher filament temperatures shorten the filament life so a balance between detector efficiency and filament life is usually determined.

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