Patent GB-1104882-A (GB)
Title: Method and apparatus for detecting traces of substances
Inventor: BARRINGER ANTHONY RENE
IPC: B01F15/00
Filed: 1964-10-16
Abstract: 1,104,882. Photo-electric analysis. BARRINGER RESEARCH Ltd. Oct. 14, 1965 [Oct. 16, 1964], No. 42415/64. Heading G1A. The presence in an unknown sample or atmosphere of a monatomic vapour of an element is detected by passing the light emerging from the sample or region which has been irradiated with light including the wavelength of an absorption line of the element, through first and second resonance radiation cells, the first containing the monatomic vapour of the element, and the second containing vapour having an absorption line adjacent to that of the element, and then comparing the intensities of the resonance reradiation produced from the two cells. The method is designed to obviate the effects of an interfering substance in the test region, which absorbs broadly in the part of the spectrum including the absorption line or lines of the substance under analysis. If the cells contain the most and least common isotope of the element to be detected, the apparatus can be used to measure the isotopic ratio for the element. In one embodiment Fig. 1, a continuous spectrum e.g. black-body, source 10 is used to irradiate a test cell 11 the end walls of which are of quartz, for example. An interference filter 14 which follows the cell 11 passes only a narrow bandwidth of wavelengths centred on the absorption line wavelength of the element of interest. The radiation then passes into resonance radiation cells 15a and 15b which preferably contain vapour of the most common isotope of the element and vapour of the least common isotope, respectively. Resonance re-radiation from the cells is detected at right angles to the optical axis by photo-multipliers 16, 17, respectively, the outputs of which are compared by a differential amplifier 18, to give an indication on meter or recorder 19. Initially, with no sample in the cell 11, the detector gains are balanced so that no indication is given. With a sample being passed through the cell 11 absorption of light by the element of interest (if present) is indicated by a drop in the re-radiation from the resonance cells 15a, 15b. Since different proportions of the isotopes are present in the sample, an unbalance of the photo-multiplier outputs results. The indication is then proportional to the ratio of the amounts of these isotopes in the sample. The light-source may alternatively comprise a metallic vapour lamp containing several isotopes, of known isotopic ratio, or a number of lamp sources of a single isotope each, may be used  successively, and then the isotopic ratio for the sample may be determined. An embodiment Fig. 2 (not shown) is also described in which radiation from the sun which has passed through the atmosphere of a planet is analyzed from a spacecraft to detect the presence of monatomic vapours in that atmosphere. Light from the sun is reflected from the planet (passing twice through its atmosphere), reflected by a mirror (22) to a rotating chopper, having silvered and transparent portions, which is designed to direct to the interference filter and resonance cells alternately light from the planet and light received direct from the sun (via a reflecting sphere (20) and mirror (21)). Reradiation from the resonance cells (26, 27) which contain isotopes of the element of interest, is detected as before by photo-multipliers (28, 29).  The gains of these photo-multipliers are controlled by a ratio control (30) which introduces various corrections. The ratio control reduces the gains of the photo-multipliers when direct sunlight is reaching the cells (26, 27) and reduces the gains when indirect sunlight, from the planet, is passed by the chopper, by comparing the sum of the photo-multiplier outputs with a reference voltage (34). A correction is also necessary to allow for absorption lines in the continuous spectrum from the sun. The ratio control is therefore arranged to equalize the outputs of the photo-multipliers during the periods of direct sunlight and retain this gain ratio during the periods of analysis of the radiation from the planet. The outputs of the photo-multipliers are compared by a differential amplifier (42) whose output only during periods of indirect sunlight is passed by a chopper, synchronized with a chopper in the sun absorption line compensation circuit. The final signal obtained is proportional to the relative absorptions of the two isotopes of cells (26, 27) in the planetary atmosphere.