Electron capture detector

The electron capture, short ECD ( electron capture detector for English ) is used in combination with gas chromatography, especially in the environmental and trace analysis for the detection of sulfur-containing nitrated and halogenated compounds.

History of Technology

The ECD was developed by James E. Lovelock 1957. Only with the ECD, it was possible to detect chlorinated pollutants such as polychlorinated biphenyls ( PCBs) and chlorinated pesticides such as DDT sensitive by gas chromatography.

Principle of operation

The detector consists of an ionization chamber with a cathode and an anode. In addition, this device has an inlet and an outlet for the gas stream.

A β - emitter is a radioactive source in the form of a thin metal foil coated with the radioactive isotope nickel 63Ni. The electron source is at the same time the cathode dar. The β -decay results in the emission of primary electrons collide with the molecules of the carrier gas N2. It created positively charged N2 molecule ions and free secondary electrons. By applying a voltage, an electric field through which the free secondary electrons move to the anode. The combination forms in this way current of a few nano- amperes ( nA) is called Ionisationsgrundstrom.

If the carrier gas, a sample substance entrained with high electron affinity, then a part of the free electrons of the substance to be captured, making the Ionisationsgrundstrom reduced. This reduction represents the detector signal Represents the means in practice that the ECD responds to substances that have an affinity for electrons ( eg halogenated compounds as most persistent environmental toxins ).

Since this classic mode is the disadvantage of a small linear range, modern appliances work more complex. Here, a voltage pulse is applied at a variable frequency. During the moment ( 0.5 to 1 microseconds ) in which the voltage is applied, the electrons, which did not react with the substances of the carrier gas stream is collected by the anode. The pulse period is selected so short that the damage caused by absorption of electrons heavy ions can not reach the anode. The frequency is not kept constant, but aims to create a more uniform current. So the device will be a large number of electrophilic molecules via the gas stream supplied is increased to compensate for the frequency, because fewer electrons reach the anode, thereby decreasing the current. A signal detector now is not the reduction of the Ionisationsgrundstromes, but the frequency with which the voltage is applied to keep the current constant. The pulse frequency behaves thus proportional to the concentration of elektroneneinfangenden molecules.

By variation of the pulse-free time, the number of free electrons is largely constant. This means that, even at high analyte concentrations, there is a sufficient electrons for ionization. The number of electrons adapts to the analyte concentration, whereby the linear range is extended considerably.

Detector sensitivity

The ECD exceeds detection limits with respect to a flame ionization detector (FID ) to several orders of magnitude. Expected detector sensitivities for different classes of organic compounds ( see also ) are:

The table gives only approximate values ​​. The sensitivity varies depending on the connection structure greatly within each group of compounds.

Swell

  • Detector (chromatography )
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