Gas Chromatography Detectors - Ionization Detectors > The Simple or Macro Argon Detector Sensor > Page 58

Unfortunately, the implied higher sensitivity is not always realized. In fact, in many sensors the sensitivity of the macro argon detector is about 10 times less than the FID (the minimum detectable concentration is an order of magnitude higher). This is because the large primary current carries a high noise level compared with the FID (more than two orders of magnitude greater) and thus the signal to noise (which determines the sensitivity) is ten times less.

Although the argon detector is a very sensitive it was not popular, largely because its linearity did not extend over more than two orders of magnitude of concentration (0.98 < r > 1.02) and its response was not predictable. In addition, the early detectors employed a "hot radioactive source" to provide the ionization (90strontium) which was also unacceptable. Less active sources are now available that work perfectly well with the argon detector and are acceptable from the point of view of safety. In addition, modern solid state electronic linearizing circuits might wellgivethe detector a much wider linear dynamic range. Nearly all organic vapors and most inorganic vapors have ionization potentials of less than 11.6 electron volts and thus are detected. The short list of substances that are not detected include H2, N2, O2, CO2, (CN)2, H2O and fluorocarbons. The compounds methane, ethane, acetonitrile and propionitrile have ionization potentials well above 11.6 electron volts, but, in fact, do provide a slight response (between 1 and 10% of that for other compounds).

The sensitivity of the macro argon detector is 4 x 10-11 g/ml. The main technical disadvantage of the argon detector was its large sensor volume which precluded its use with capillary columns. This provoked Lovelock to design the micro argon detector