The Electron Capture Detector ECD

The development of the ECD by Lovelock in the late 1950s led to its application in environmental studies and the discovery of the wide distribution of chlorinated pesticide residues such as DDT. This was followed by the discovery of the emission of methyl iodide from open ocean waters which again had great significance in the study of the Earth's ecosystem.

The ECD has evolved over many years into an instrument very different in detail from the original even though the principle of detection remains the same. The ionization cross-section detector used a small radioactive source to produce electrons in a cell with two electrodes with about 100 V between them. This caused a small current to flow in the cell as a result of ionization of carrier gas molecules by collision. Compounds eluting from the GC column were ionized to different amounts depending on the cross-section of ionization for each compound. The net result was a variation in the current flowing in the cell which could be recorded electronically. Lovelock modified this detector, firstly by using argon as the carrier gas and secondly by using a much higher voltage ( + 1000 V) across the electrodes. Under these conditions, multiple ionization takes place via excited argon atoms so that there is a small standing current but a very much larger signal current than given by the cross-section detector. After the argon detector had been on the market for some time, anomalous results were reported especially for halogenated compounds. On investigation, Lovelock concluded that compounds with a strong electron affinity such as polyhalogenated compounds were capturing electrons and thereby reducing the signal from the detector. He, therefore, maximized the electron capturing effect by using argon with 5-10% methane as carrier gas and about 50 V across the electrodes. Such a cell has a phenomenally high sensitivity for compounds such as CCl4 and is capable of detecting femtogram quantities (10~15 g). Indeed the ECD has a higher sensitivity under favourable conditions than any other detector with the possible exception of radioactivity detectors.

Molecules that capture electrons form charged entities with a much lower mobility than the free electrons responsible for the standing current in the cell. Because of their lower mobility they lose their charge readily and hence cause a reduction in the standing current. The ECD is the only detector that measures a decrease in a large standing current; all other detectors measure an increase in a small standing current. Since the standing current can only be reduced to zero, the ECD can be easily overloaded and has, even in its modern versions, a relatively limited linear range of about 104. Modern versions of the ECD pulse the voltage on the electrodes rather than using a steady DC potential. This allows equilibrium to be achieved between the electron flux and the gas mixture in the cell and is somewhat analogous to a distillation column under total reflux from which a small grab sample is removed from time to time. By changing the frequency of the pulses the current in the cell can be kept constant and the variation in frequency required to maintain a constant current is monitored rather than changes in the current itself. This is a very simple arrangement electronically that is readily compatible with integrating circuits. The most recent modification to the ECD dispenses with the radioactive source and uses electrons produced by a helium discharge in an extension to the cell proper.

As indicated in the introduction to this section, the very high sensitivity of the ECD to polyhalogenated compounds made it extremely important for the analysis of trace pesticides in crops since many pesticides were compounds of this type. In addition to a very high sensitivity for some classes of compounds it has a negligible response for others such as paraffin hydrocarbons. Table 2 shows that the response to different classes of compounds varies over a range of at least 106.

This response is not amenable to accurate prediction from the chemical structure of the compounds and even apparently small differences in structure can result in large differences in electron affinity. The ECD is not suitable for detection of an unknown mixture since large peaks may be from small amounts of compounds with high electron affinity or large amounts of compounds with a low electron affinity. In spite of its rather anomalous response and limited linear range, the ECD is an extremely important detector under favourable circumstances.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook

Post a comment