Topics - Radioactivity
The spontaneous generation of gamma rays (high frequency X-rays), beta particles (electrons), protons (hydrogen nuclei), alpha particles (helium nuclei), etc., by a substance is termed radioactivity. Radioactivity can persist for a range of time periods depending on the nature of the radioactive substance. For example Radium has a half life of 1500 years whereas Radon (an inert gas and radioactive product of radium) has a half life of only 3.8 days. The lifetime of some radioactive materials may be measured in milli-seconds or even micro-seconds. Radioactive materials must be treated with caution as the emitted particles or radiation can cause cell mutation and produce malignancy. The first radioactive substance to be used in chromatography was Strontium 90 which was used by Boer in the late 1950s in the first ionization detector called the cross-section detector. Hydrogen was used as the carrier gas and an eluted solute was ionized by the high energy beta particles, producing an ion current which was collected by electrodes held at appropriate potentials. Strontium 90 was also used by Lovelock in his first Argon Detector. Strontium 90, however, was a ‘hot source’ which was commercially unacceptable. It was first replaced by Radium, which, due to its emission of alpha particles that had very high ionization cross-sectional areas, only a few micro-curies were needed to produce adequate ionization. However, radium was deemed to be dangerous due to its biological effects. Tritium held in silver foil was then tried, but the source was not stable at high temperatures, so eventually Nickel 63 was tried and found to have appropriate activity, half life, and thermal stability and is generally used today in most detectors requiring radioactive sources.