The Saga of the Electron-Capture Detector

Feb 01, 2007

Leslie S. Ettre
In addition to the universal detectors used in gas chromatography (GC), selective detectors also have played an important role in the rapid spreading of the utilization of the technique. Probably the most important selective GC detector is the electron-capture detector, with a very high sensitivity to organic compounds containing chlorine and fluorine atoms in their molecules. The electron-capture detector had a vital role in environmental protection and control — its use helped to prove the ubiquitous presence of chlorinated pesticides in nature and halocarbons in our atmosphere, and made us aware of the global extent of pollution. It was the electron-capture detector that made concentration ranges of parts-per-billion (ppb: 1:109) or even parts-per-trillion (ppt: 1:1012) detectable. Today, these terms are used routinely without realizing how formidable such a sensitivity really is: 1 ppb means that a spaceship (or a UFO, depending upon one's inclination) could pick up a particular family of six from the whole living population of the Earth, and 1 ppt means that it could even find one piece of chewing gum in the pocket of one of the children. Lovelock — the inventor of the electron-capture detector — illustrated its superior sensitivity by the following metaphor (1): If one would pour about one liter of a perfluorocarbon liquid onto a blanket in Japan, and left it out to dry in the air by itself, a few weeks later one could detect on the west coast of England the vapor that had evaporated into the air in Japan from that blanket and carried by the jet stream around the world.

The electron-capture detector is an ionization detector and its response is based upon the ability of molecules with certain functional groups to capture electrons generated by the radioactive source. The detector chamber contains two electrodes and a radioactive foil as the radiation source. Using an inert carrier gas with no affinity for electrons, the ions formed by the ionizing radiation can be collected, creating a steady standing current in the detector. When molecules of certain electron-absorbing solutes enter the detector chamber, they will capture electrons, resulting in a decrease of the standing current, giving a negative peak. In practice, the recorded peaks are made positive by reversing the polarity of the recorder.

Figure 1.
Since its invention, the design of the electron-capture detector has undergone a number of changes, but its principles have remained the same, as shown in Figure 1. Also, different radioactive sources have been used: in Lovelock's original design, the foil contained 90Sr, but soon this was changed to tritium occluded in titanium foil. Today, it almost universally contains 63Ni. However, questions regarding the detector's construction are not our subject.

James E. Lovelock (born in 1919) graduated in 1941 as a chemist from Manchester University (Manchester, UK) and then, in 1948, obtained the Ph.D. degree in Medicine from the London School of Hygiene (London, UK). After 1941, he was associated with the British Medical Research Council for almost 20 years. From 1958 to 1959, he was a visiting scientist at Yale University Medical School (New Haven, Connecticut) and then, from 1960 to 1964, he was associated with Baylor College of Medicine in Houston, Texas, and the University of Houston, Houston, Texas, as a professor. Since 1964, he has been a freelance scientist serving as a consultant to various companies and institutions; among others, he also cooperated in NASA's space programs. In the early 1970s, Lovelock proposed his theory of Gaia, the living Earth, functioning as a superorganism in which the physical environment and the lifeforms inhabiting the planet interact, maintaining a more-or-less steady state. His fundamental contributions to our understanding of the impact of environmental pollution were recognized by three major awards: the Heineken Prize for the Environment of the Royal Dutch Academy of Sciences (1991), the VOLVO Prize (1996), and the Blue Planet Prize (1997), the latter generally considered as the environmental equivalent of the Nobel Prizes.