Patent GB-1077501-A (GB)
Title: Scanning spectrophotometer
Inventor: DAWSON JOHN BARKER
IPC: G01J3/32
Filed: 1964-02-04
Abstract: 1,077,501. Spectrophotometers. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Jan. 18, 1965 [Feb.4, 1964], No.4751/64. Heading G1A. A spectrophotometer comprises means producing a repeating signal pulse train representing the spectrum of the sample, means producing a repeating reference pulse train whose pulses occur at instants corresponding to predetermined wavelengths of the spectrum, and means to select a pulse of the reference train to gate the corresponding part of the signal train, which part is then measured. A sample is excited at (1) Fig.1 (not shown) to produce a light output which is reflected by plane mirror (3) and concave collimator mirror (4) on to a diffraction grating (5) or a prism, to disperse the beam. The spectral lines produced are reflected by mirror (4) through a narrow slit (6) to photo-multiplier (7). The grating is arranged to oscillate through a small angle about a vertical axis (8) by means of a motor (9) and cam (10) arrangement Fig.2 (not shown) such that in each cycle the grating moves steadily in one direction followed by a relatively rapid flyback which has about a quarter the time duration of the steady movement. The resultant signal output of the photo-multiplier (7) is a pulse train repeated for each sweep of the spectrum across the entrance slit (6). A reference pulse train, synchronized with the signal train is produced by imaging a graticule (14) on to a mirror (16) secured on the back of the grating (5). The lines of the graticule are spaced to correspond with predetermined wavelengths. The system causes the image of the graticule to sweep repeatedly over the entrance slit of a second photo-multiplier (19). The reference pulse train produced is amplified and passes to AND gate 32 (Fig. 3). That part of the train corresponding to the flyback of the grating is prevented from passing through gate 32 by the absence of a gating signal during this period, from amplifier 33, which produces an output only when diode 22 is de-energized. (This de-energization occurs when the diode 22 is not illuminated i.e. when the blackened part of a partly blackened and partly silvered disc 20, rotated by the motor (9) is in the path of light from a source 25). The reference pulses are shaped by shaper 52 and fed to a multi-stage counter 34, for example, a three decade counter. Means for resetting the counter at the end of each forward sweep of the oscillating grating (and mirror) are provided by a second disc 21, with a small reflecting area, producing a reset pulse from diode 23 for each revolution of the disc. The output of the counter is fed to five sets of selector switches 36-40 each of which can comprise three decade switches so that a particular pulse in the reference train can be selected by each set and passed to a corresponding coincidence gating circuit 42-46. The output of photo-multiplier 7 is amplified and also fed to the gates 42-46. When a pulse in the signal train coincides with a selected reference pulse, the magnitude of the resulting gated signal is measured on circuits 47-51, comprising integrating circuits utilising capacitors. In use, one of the five channels is arranged to act as a reference, the selector being set to gate a particular pulse corresponding to a characteristic line of an element known to be present in the sample. The other channels are set to select pulses corresponding to characteristic lines of other elements, the amounts present of which are indicated by the measurements obtained when the reference channel measurement has built up to a predetermined level. The circuitry may be adapted to shift the selected pulses by small amounts to correspond to small wavelength shifts from those corresponding to the graticule markings. Other adaptations of the apparatus include an alternative drive mechanism and gating and resetting arrangements. The apparatus can be adapted for use in absorption flame spectroscopy as well as emission spectra investigation.