Gas Chromatography Detectors - Ionization Detectors > The Thermal Argon Detector > Page 62


The column eluent (argon) passes through a stainless steel tube, which acts as the anode, into a cylindrical glass tube 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 reported. The authors claimed sensitivities at least as good as those of the FID and a linear range of three orders of magnitude or more (0.98 < r < 1.02). Employing the propensity for glass to conduct at elevated temperatures they also designed a sensor completely free of metal and demonstrated it worked as effectively as the sensor shown in figure 31. A diagram of the metal-free detector sensor is shown in figure 32.


Figure 32 The Sensor of the Metal–Free Argon Detector