The Micro Argon Detector A diagram of the micro argon detector sensor is shown in figure 29. This sensor is designed to have a very small "effective" sensing volume to facilitate its use with capillary columns where the flow rate may be as low as 0.1 ml/min or less. In the micro argon detector sensor, the anode is withdrawn into a small cavity about 2.5 mm in diameter. This ensures that the electrons can only reach the anode along a restricted path and the electric field around the electrode resides within a few diameters of the anode tip. The anode is tubular in form and the capillary column can slide up inside the anode until it is within a millimeter or so of the electric field. Metastable argon atoms are formed as a cloud of around the anode tip and any solute

in figure 30. It is seen that the sensor volume has no effect on the column performance and the detector is now very suitable for use with capillary columns. The modifications carried out to reduce the effective sensor volume did not improve its linearity nor increase its  linear range. However, the noise level was reduced by about two orders of magnitude and thus the  sensitivity was commensurably increased by the same amount making it 10 times more sensitive than the FID. The Thermal Argon Detector Beres et al. (24) showed that the argon detector could be made to function without a radioactive source or other electron producing device providing the argon and sensor system was operated at temperatures above 150oC. Glass becomes conducting at temperatures of 150oC and above, and so glass could be employed as one of the electrodes. A diagram of a sensor is shown in figure 31. Figure 31 The Thermal Argon Detector Sensor

Lovelock introduced the argon ionization detector (20-22), an ionization detector that functioned on an entirely different principle. The argon type detectors utilize noble gases to produce metastable argon atoms which have sufficient energy to ionize most organic compounds.   Noble gases, have their outer octet of electrons complete and, thus, collisions between argon atoms and electrons are perfectly elastic. Consequently, if a high potential is set up between two electrodes in argon, and ionization is initiated (for example by a suitable radioactive source) electrons will be accelerated towards the anode and will not be impeded by energy absorbed from collisions with argon atoms. If the potential of the anode is high enough, the electrons will develop sufficient kinetic energy that on collision with an argon atom, energy can be absorbed, and a metastable atom can be produced.   A  metastable atom carries no charge but adsorbs its energy from

by collision between a metastable atom and an organic molecule, the electron, simultaneously produced, is immediately accelerated toward the anode. This results in a further increaseinmetastableatoms and a consequent increase in the ionization of the organic molecules. The resulting cascade effect, unless controlled, results in an exponential increase in ion current. The control of this cascade production of ions involves a negative feed back control on the ion production The Simple or Macro Argon Detector Sensor       Figure 28 The Macro Argon Detector

held at 150oC or above. The tube is insulated from the glass tube by a PTFE sleeve. The argon exits from the sensor by a length of PTFE tube. A metal band round the glass acts as an electrical connection to the amplifier, the other input of the amplifier being connected to the –ve side of the power supply. The +ve side of the power supply is connected to the metal tubular anode. Electrons, thermally emitted from the glass surface, are accelerated under the high potential and on collision with argon atoms produce metastable atoms in the usual manner which collect round the anode. Organic vapors are sensed in the same way as the normal argon detector, i.e., by collision between the organic molecules and the metastable argon atoms. The electrons and organic ions produced are collected and the resulting current is monitored by a high impedance amplifier. The performance of the detector using potentials ranging from about600Vto 1500 V and sensor temperatures of 150oC, 200oC and 250oC were

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