Patent GB-1401306-A (GB)
Title: Method of and apparatus for quantitative spectral analysis
IPC: G01N21/31
Filed: 1972-09-12
Abstract: 1401306 Photo-electric spectral analysis INSTITUT VYSOKIKH TEMPERATUR AKADEMII NAUK SSSR 12 Sept 1972 [13 June 1972 #2] 42239/72 Heading G1A The concentration of a substance in its gaseous phase is measured by passing radiation through a periodically interrupted atomic beam of the substance, isolating a portion of the transmitted spectrum containing an absorption line of the substance, applying it to a photo-detector, and synchronously detecting the resulting signal. As shown, Fig. 1, a light beam 2 from a lamp 1 is focused on to a target comprising an atomic beam 4 and the transmitted light is analyzed by a Fabry-Perot interferometer 15 and a monochromator 16 to isolate a portion of the spectrum. A double slit arrangement and a reflecting prism 34 select the flank of the emission line to provide a feedback signal to stabilize the spectrum analyzer and the peak of the emission line to provide a signal which is integrated and recorded and is also applied to a synchronous detector 42 the output of which is integrated and recorded. The light source 1 comprises a globular lamp filled with an inert gas and containing a small quantity of Cd 114, and is excited by an electrodeless discharge within the envelope by means of an H.F. generator. The atomic beam 4 is produced in an evacuated enclosure 5 by heating a sample of Cd in a chamber 6 by means of a heating coil 7 The beam is collimated by aligned apertures in thermal shields 8 and is periodically interrupted by a disc 10 having spaced apertures 11 rotated by a motor 65. The Fabry-Perot interferometer 15 comprises a pair of half silvered mirrors 19 one of which is supported by means of Piezo-electric rods 20 by means of which its position can be adjusted. The mono-chromator 16 comprises a system of reflectors and a dispersing prism (29) arranged so that the light passes twice through the prism in its passage between the entrance and exit slits. The portion of the spectrum selected can be adjusted by rotation of the prism and one of the reflectors. The resulting portion of the spectrum is focused on to an adjustable slit 32 having a removable strip 33 inserted in its centre to form two slits which select the middle of the flank and the peak of the Cd emission line which are reflected by prism 34 on to photo-multipliers 35 and 36 respectively. The flank signal is amplified and applied to the energizing circuit for the Piezo-electric rods of the Fabry-Perot interferometer to stabilize the spectral analyzer. The peak signal is applied to an amplifier 39 the output of which is applied to an integrating digital voltmeter 40 and thence to a recorder 41. The output of amplifier 39 is also applied to a synchronous detector 42 which is supplied with a reference signal by means of a second apertured disc 61, mounted on the same shaft as disc 10, co-operating with a lamp 63 and a photo-cell 64. The aperture 62 in disc 61 are smaller than those in disc 10 to ensure that the reference pulse does not occur while the atomic beam is partially interrupted. The ouput of the synchronous detector 42 is applied to a second digital integrating voltmeter 66 and the output is recorded. The ratio of the recorded values may be utilized to calculate the absolute concentration of the substance (Cd) in the target beam 4. In a modification, Fig. 5 (not shown), the Fabry-Perot interferometer is mounted inside the vacuum enclosure 5 and arranged so that the atomic beam passes between the two mirrors. The use of spherical mirrors is mentioned. The light source may comprise a turnable laser or may be an excited atomic beam of the substance whose concentration is to be measured. A block diagram of the synchronous detector is given, the reference pulses being converted into antiphase square waves by an arrangement utilizing sawtooth generators level detectors and a frequency-to-voltage converter, (Fig. 4).