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.
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Radioactivity
identified and the relative activity of each peak
compared and to do this successfully, a proportional radioactive detector was
required. James and Piper described a radioactivity detector 1961-3 [27,28]
suitable for this purpose is still in use today, although the detector has been
fabricated in various different forms by a number of different manufacturers. A
diagram of the radioactivity detector based on the device of James and Piper is
shown in figure 46.
There are two basic forms of theradioactivity detector, one that measures 13C only and the other that measures both 13C and 3H. In both
systems the carrier gas used must be helium or argon and the column eluent is
fed through a furnace packed with copper oxide to oxidize all the solutes to
carbon dioxide and water.
Figure
46 The Radioactivity Detector
GC-Detectors Radioactivity
Author: RPW Scott
Book:Liquid Chromatography Detectors
Section:HPLC-Detectors Transport Modified-Moving-Wire
improvement to appears to have been realized.
Slais and Krejei (42) replaced the normal FID with the NPD detector and used it
to detect chlorine compounds. They mixed the combustion gases with hydrogen and
passed the mixture directly into the NPD. At a column flow rate of 0.37
ml/min., the sensitivity of the detector was stated to be about 3 x 10-7 g/sec,
which is equivalent, in concentration units, to about 1.6 x 10-6
g/ml.
The moving
wire detector has also been modified by Dugger (43) for radioactivity detection (e.g., detection of 14carbon labeled compounds). To detect 14carbon
compounds, the solute on the wire was oxidized to carbon dioxide and the
radioactive gas passed to a Geiger-Muller tube. To detect tritium, the
tritiated water produced on combustion was passed over heated iron to reduce it
to hydrogen and tritium, which was then also passed to a Geiger-Muller tube
HPLC-Detectors Transport Modified-Moving-Wire
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Ionization-Detectors Helium
The thermal argon detector is not
commercially available. However, its high sensitivity, freedom from
radioactivity and electron producing ancillaries make it a very simple detector
to fabricate and operate. The fact that the sensor can be constructed from very
inert material and thus accommodate very corrosive gases could also be an
advantage for certain applications.
The Helium Detector
The helium detector works on exactly the same
principle as the argon detector, but metastable helium atoms are produced by
the accelerated electrons instead of metastable argon atoms. Metastable
GC-Detectors Ionization-Detectors Helium
