Electron capture detector

from Wikipedia, the free encyclopedia
ECD coupled with a gas chromatograph with excitation by 63 Ni.

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

Technology history

Lovelocks electron capture detector

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

Working principle

The detector consists of an ionization chamber with a cathode and an anode . This device also has an inlet and an outlet for the gas flow.

A radioactive source in the form of a thin metal foil, which is coated with the radioactive nickel isotope 63 Ni, serves as the β emitter . The electron source is the cathode at the same time. The β-decay leads to the emission of primary electrons which collide with the N 2 molecules of the carrier gas. Positively charged N 2 molecular ions and free secondary electrons are created. When a voltage is applied, an electric field is created through which the free secondary electrons move to the anode. The current of a few nanoamps (nA) that is generated in this way is known as the ionization base current.

If a sample substance with a high electron affinity is carried along in the carrier gas , then some of the free electrons are captured by this substance, which reduces the ionization background current. This reduction represents the detector signal. In practice this means that the ECD reacts to substances that have an affinity for electrons (e.g. halogenated compounds as well as most persistent environmental toxins ).

Since this classic operating mode has the disadvantage of a small linear range, modern devices are more complex. A voltage pulse is applied at a variable frequency. During the moment (0.5 to 1 µs) in which the voltage is applied, the electrons that have not reacted with the substances in the carrier gas flow are collected by the anode. The pulse time is selected so short that the heavy ions created by the absorption of electrons cannot reach the anode. The frequency is not kept constant, but aims to generate a constant current. If a large number of electrophilic molecules are supplied to the device via the gas flow, the frequency increases to compensate for this, since fewer electrons reach the anode and the current intensity decreases. The detector signal no longer serves to reduce the ionization base current, but rather the frequency with which the voltage is applied in order to keep the current strength constant. The pulse frequency is proportional to the concentration of the electron-capturing molecules.

By varying the pulse-free time, the number of free electrons is largely constant. This means that enough electrons are available for ionization even with high analyte concentrations. The number of electrons adapts to the analyte concentration, which significantly expands the linear range.

Detector sensitivity

In terms of detection limits, the ECD surpasses a flame ionization detector (FID) by several orders of magnitude. Detector sensitivities to be expected for different classes of organic compounds (see also) are:

Chemical group Relative sensitivity (to FID)
Hydrocarbons 1
Ethers, esters 10
Aliphatic alcohols, ketones, amines, 100
Mono-Cl- u. Mono-F compounds, Mono-Br-, Di-Cl- u. Di-F connections 1000
Aldehydes and tri-Cl compounds 10 4
Mono-I, di-Br and nitro compounds 10 5
Di-I, Tri-Br, Poly-Cl and Poly-F compounds 10 6

The table only provides approximate values. The sensitivity varies widely within each group of compounds depending on the compound structure.

Individual evidence

  1. ^ JE Lovelock: A Sensitive Detector for Gas Chromatography . In: Journal of Chromatography A . 1, No. 1, 1958, pp. 35-46. doi : 10.1016 / S0021-9673 (00) 93398-3 .
  2. ^ JE Lovelock: The Electron Capture Detector . In: Journal of Chromatography A . 99, No. 1, 1974, pp. 3-12. doi : 10.1016 / S0021-9673 (00) 90840-9 .
  3. M. Krejči and M. Dressler: Selective Detectors in Gas Chromatography . In: Chromatographic Reviews . 13, No. 1, 1970, pp. 1-59. doi : 10.1016 / 0009-5907 (70) 80005-9 .
  4. ^ ED Pellizzari: Electron Capture Detection in Gas Chromatography . In: Journal of Chromatography A . 98, No. 2, 1974, pp. 323-361. doi : 10.1016 / S0021-9673 (00) 92077-6 .