Gas Chromatography Detectors - Ionization Detectors > The Simple or Macro Argon Detector Sensor > Page 57

The cascade effect is controlled by the linearizing resistance. As the current increases due to the presence of organic vapor, the voltage drop across the linearizing resistance is also increased which reduces the voltage applied across the electrodes. For example, if 1300 volts is applied to the detector and when a solute is eluted, the current increases to 10-7 amp, this will cause a 300 volt drop across the linearizing resistance of 3 x 109 (10-7x 3 x 109 = 300) and consequently reduce the voltage across the electrodes to 1000 volts. In this way the natural exponential response of the detector can be made sensibly linear.

In a typical detector, the primary current consists of about 1011 electrons per second. Taking the charge on the electron as 1.6 x 10-19 coulombs this gives a current of 1.6 x 10-8 amp. According to Lovelock [20], if each of these electrons can generate 10,000 metastables on the way to the electrode, the steady state concentration of metastables will be about 1010 per ml (this assumes a life span for the metastables of about 10-5 seconds at NTP). From the kinetic theory of gases it can be calculated that the probability of collision between a metastable atom andan organicmoleculewillbeabout 1.6 : 1. This would lead to a very high ionization efficiency and Lovelock claims that with sensors of more advanced sensors design, ionization efficiencies of 10% could be achieved. Ionization efficiencies of at least 0.5 % are readily obtainable, which, compared with that of the FID, is very large indeed.