Patent GB-1144805-A (GB)
Title: Improvements in and relating to a method and apparatus for spectrographic analysis
IPC: G01N23/06
Filed: 1966-06-17
Abstract: 1,144,805. Photo-electric, analysis apparatus. OFFICE NATIONAL D'ETUDES ET DE RECHERCHES AEROSPATIALES. June 17, 1966 [July 9, 1965], No.27087/66. Heading G1A. [Also in Division H1] The concentration of an element in a sample is determined by bombarding the sample with electrons and analysing the characteristic X- radiation produced using two detectors receiving the radiation through two filters, at least one of which is gaseous, the absorption spectra of which filters are identical except for one narrow band containing the characteristic wavelength of the element under investigation. Fig.1 illustrates the two-path system, with electron gun 10, diaphragm 11, sample 12, and filter devices 17, 18 all of which are located in a vacuum enclosure, and detectors 30, 31 (proportional counters) and separate detector channels. Each channel comprises a pre-amplifier 40, 41, linear amplifier 42, 43, amplitude selector 44, 45, counter 46, 47 and integrator 48, 49. In setting up the apparatus the filter device 17 is filled with a gaseous element which will transmit the charateristic wavelength of the element to be determined, and the device 18 with another gas, the basic element of which is close to the first in the Periodic Table but which does not transmit the said characteristic wavelength. For example, if the oxygen content of the sample is to be determined, the filters 17, 18 contain oxygen and nitrogen respectively. The pressures of the gases in the devices 17, 18 is adjusted and a certain amount of a heavier gas e.g. in the case of oxygen determination, krypton, can be added to one or both of the filter devices as required, so that a super-positioning of the absorption spectra for the two filter devices is effected, except for the narrow range of wavelengths defined by the two absorption edges of the two basic gases in the filter. (The heavier gas added has no discontinuity in the range of wavelengths that can be sensed by the detectors). Graphs, Figs.3-5 (not shown) indicate how this "balancing" of the two filters is produced. Thus, when counts are taken in the two channels of the spectrometer, the filter 18 absorbs the characteristic wavelength of the element to be determined whereas the other filter 17 absorbs it to a much smaller extent, all other characteristic lines in the radiation emergent from the sample being absorbed equally by the two filters and therefore no errors are introduced into the measurement. The difference between the counts registered at 46 and 47 is an indication of the element concentration in the sample. The pulses from both counters may be fed to a circuit which produces an indication of the difference value directly. Various details of the apparatus used are described. Means for varying the pressure of the gas in the filters 17, 18 includes a pump (27) and manometer (29), Fig.2 (not shown). The apparatus for controlling the electron beam incident on the sample may comprise a condenser (110), a diaphragm (111), scanning device (112), and objective lens and cooler (114), Fig.7 (not shown). The cooler, Fig.8, (not shown) reduces the formation in the vacuum enclosure of carbon deposits on the sample and improves the sensitivity of the spectrometer, Figs. 9, 10 (not shown). The construction of a filter cell and its window are described Figs.11, 12, (not shown). Besides the determination of the oxygen content of a sample, nitrogen may be determined using nitrogen and methane filters, and fluorine, using a gaseous filter of oxygen and a solid one of lithium fluoride. More than one element may be determined at a time, several pairs of filters being used, Fig.13 (not shown). In this case, one filter of a pair, used to determine a particular element, may be solid, the other being gaseous. A graph, Fig.6 (not shown) compares measured results with actual element concentrations.