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Functional principle of the most important detectors

TCD: Thermal Conductivity Detector

Together with the carrier gas from the separation column the sample components one after the other reach the measuring channel of the detector. A second channel serves as a reference channel and only pure carrier gas flows through it. In both channels there are electrically heated resistance wires.

The thermal conductivity in the measuring channel changes through the individual sample components. The altered heat dissipation at the resistance wires results in a temperature change and so in a resistance change in the wires.

This change is compared with the resistance value of the wires of the reference channel, through which only pure carrier gas flows.

A "Wheatstone Bridge" transforms the resistance change into a current/voltage signal.

This signal is directly proportional to the concentration of the sample components (mg/ml) in the carrier gas of the measuring cell.

FID: Flame Ionization Detector

The individual sample component (the eluent) reaches the detector together with the carrier gas stream from the separation column. Directly at the column end there is the burner nozzle.

Depending on the carrier gas (H2, He, N2) hydrogen and synthetic air will be added before the detector.

In the flame ions and free electrons are formed.

The charged particles cause a measurable current between nozzle (cathode) and the cylinder shaped anode.

There flows an increased current compared with the signal of the pure carrier gas/fuel gas flame. This signal difference informs us about the sample amount, which passes per time unit (mg/s) through the detector.

ECD: Electron Capture Detector

Together with the gas stream from the separation column the individual components of the sample one after the other reach the detector.

Fast beta particles from a radioactive Ni63 source hit the molecules of the carrier or makeup gas and form by impact ionization slow free electrons, which cause a measurable current between the two electrodes.

Electrophilic (electron capturing) sample components reduce the number of slow electrons.

The reduction of the electron flow (compared to a signal without sample components) is proportional to the amount of the electrophilic sample components (mg/s), which passes per time unit through the detector.

Nitrogen Phosphorus Detector (NPD)/Thermionic Ionization Detector (TID)

In the Thermionic Ionization Detector (TID) there is an electrically heated silicate bead, which contains rubidium. With nitrogenous or phosphoric substances it reacts by the formation of ions, which can be proved similar as with the FID. Because we can prove with this detector up to 1 pg of these substances it is also called Nitrogen Phosphorus Detector (NPD).


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