Patent GB-1482611-A (GB)
Title: Selective detection of a component in an atmosphere
IPC: G01N33/00
Filed: 1974-04-08
Abstract: 1482611 Gas detectors: gas analysis A JENKINS and J E LOVELOCK 7 April 1975 [8 April 1974] 15556/74 Headings G1B and G1N In order to selectively distinguish a constituent of interest from the remaining constituents in a gas sample applied to a gas detector the flow-path of the gas sample to the detector includes a device for alternately interrupting the flow of the constituent of interest, but not the remaining components, and allowing all constituents to flow to the detector. Organic nitro explosives can be detected in the presence of halogenated hydrocarbons and oxygen by passing the sample through a membrane 3 and then together with an argon carrier gas to an electron capture detector 5 via a switching device 8 which under the control of oscillator 9 alternately destroys or diverts any organic nitro explosive from the sample. The detector 5 output after filtering by a band pass filter is synchronously detected in bridge 10 and displayed on meter 7. The synchronous bridge 10 comprises two signal paths, Fig. 6 (not shown) one of which includes an invertor, which are connected alternately by a switch, synchronized with oscillator 9, to an R.C. circuit. The output of the detector 5 is in the form of a smooth curve due to the remaining components on which is superimposed an alternating signal due to the organic nitro explosive constituent the flow of which is switched on and off, Fig. 7 (not shown). After filtering, only those signals close to the switching frequency of the device 8 are allowed through i.e. the signals due to hydrocarbons &c. are suppressed. The output from the R.C. circuit is in the form of a D.C. signal proportional to the amount of explosive present. Normal random noise will be eliminated by the alternate switching between the two paths. Switching Device 8 The device 8 can be in the form of a thin wall metallic tube through which the gas sample flows and which is ohmically heated by an electric current passing therethrough under the control of oscillator 9. During the periods when the tube is heated the nitro compounds are broken down into components which are not detectable by detector 5 whereas remaining detectable components are unaffected and pass to the detector. Between the heating periods the tube is rapidly cooled by a bath so that all components of the sample are passed to the detector. Alternative methods of heating are by a filament or R.F. heating of a ferromagnetic coil. Instead of heating, the nitro explosives may be decomposed by periodic application of ultraviolet radiation e.g. from a pulsed laser. A further form of switching device utilizes the strong electron capture properties of certain compounds and may be in the form of an ionization chamber in which the electrodes are pulsed or an ionization chamber in which thermal electrons are produced in pulses by irradiation of a photo-sensitive layer by U.V. light. Two separate parallel paths may be used instead of a single flow-path, one being maintained at a low temperature and the other at a lower temperature, Figs. 4 and 5 (not shown). An electromagnetically operated valve controlled by the oscillator is provided to control the flow through one or other of the paths. The switching device can employ chemical reactions to convert the component of interest to a component which is not detected e.g. by oxidation. Alternatively a constituent which is not normally detected may be converted into a form in which it is e.g. chlorination of olefines