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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

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

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

As the sample is contained in a frozen argon matrix, the samples can be held for a very long time on the drum without loss or change. Consequently, the spectrum of any particular sample can be taken repeatedly, any number of times, to improve the signal-to-noise ratio and, thus, the overall sensitivity. The interferometer's modulated IR beam is focused on the narrow frozen argon 'stripe' that contains the sample. Employing this device, the IR sensitivity is commensurate with that of the mass spectrometer. Courtesy of Mattson Instruments Inc. Courtesy of Mattson Instruments Inc. Figure 43. Absorption Peaks for Liquid, Solid and Matrix Isolated Samples The improved sensitivity is partly due to the detector element being approximately the same size as the sample 'stripe' and partly due to the sample being in an inert argon matrix, which causes the absorption

a small amount of argon (ca 0.5 %), although the helium can be replaced by nitrogen if more convenient, The carrier gas, on leaving the column, is arranged to impinge onto a rotating gold plated drum, situated in a evacuated box maintained at about 12 ûK by a liquid helium thermostat. The drum, while rotating, also moves very slowly in an axial direction and, thus, the samples are deposited as a thin helical trail on the outer walls of the drum. The sample on the drum surface is frozen with the argon and thus trapped in a cage of solid argon. The argon insulates the sample from the gold surface and so there can be no catalytic decomposition, or molecular rearrangement. The major component of the carrier gas (i.e., helium or nitrogen) is continuously removed by a low-pressure turbo-molecular pump. The pump contains no oil and, thus, the possibility of any long-term sample contamination is eliminated