Patent GB-1230749-A (GB)
IPC: G01J3/42
Filed: 1969-05-07
Abstract: 1,230,749. Photo-electric absorption measurements. AMERICAN OPTICAL CORP. May 7, 1969 [May 15, 1968], No. 23231/ 69. Heading G1A. [Also in Division G2] In a spectrophotometer in which the path length in a fluid sample can be automatically changed by a fixed amount, the light from a lamp 54 is directed into the fluid in one of several cuvettes 90 in a rotatable turret 88 via a lens system 62, 64 and a light transmitting rod 72, the radiation being chopped by a disc 58 (driven by a motor 56) and being made monochromatic using either a fixed wavelength filter 66 or a variable wavelength interference wedge filter 74. Alternatively, a prism or grating monochromator or a single line vapour lamp may be used. On emerging from a transparent bottom window 91 of the cuvette, the radiation activates a photo-detector 40 and the signal produced is amplified and fed to a specific absorbance meter 140, Fig. 2, (not shown). The whole unit 22 may be raised and lowered manually, Fig. 8 (not shown) by a lever (134), Fig. 1 (not shown), the lever rotating a shaft 132 (mounted in a block 124) and hence a cam 118 whereupon a cam follower 120 activates a push-rod 102. The cam 118 has three dwell sections such that the end of rod 72 may be clear of the liquid, as shown, or in either of two predetermined positions in the liquid itself. The path length in the liquid can thus be easily changed by a fixed amount and the absorbence of the liquid determined from the output signals in the two cases. To correct for the change in angle subtended by the detector as the rod 72 is moved, it is arranged, in a manner described, that if the rod moves a distance T the detector moves a distance T/n in the same direction, n being the refractive index of the liquid. The electrical circuitry may be arranged to give output readings on a logarithmic scale by either using a linear amplifier and a logarithmic scale meter or by using a logarithmic amplifier and a linear meter, the light beam being chopped to allow the use of A. C. amplifiers, and an arrangement such as in Fig. 11 may be used in which the output when the rod 72 is in the short path length position may be adjusted to a reference zero level whilst the output with the rod in the other position gives the absorbence value directly. For this purpose the gain of two cathode coupled triodepentode valves 162, 164 is controlled by the voltage across a large bias capacitor 184. The voltage may be set to a fixed value of about four volts by closing a switch 232 and, with switch 138 open and the rod 72 in the short path position, the gain of the amplifier coarsely adjusted by switch 172. The rectified and filtered output from valve 192 is then applied to the capacitor 184 by closing the switch 138 whereupon the gain of valves 162 and 164 automatically adjusts to give a balanced circuit and the equal currents through triodes 206, 214 produce zero voltage at the meter terminals 220, 225. The rod 72 may then be moved to the long path length position and switch 138 opened whereupon the absorbance value is displayed at the meter. In a modified optical arrangement, Figs.12A, 12B (not shown), a lens (254) and an apertured plate (250) are arranged between the cuvette and the detector such that the aperture of the plate lies in the focal plane of the lens. The whole arrangement is raised and lowered and the angle subtended by the detector remains constant. In a further modification, Figs. 13, 14A, 14B (not shown), a light beam from a source (274) passes through a cuvette (270) containing some of the liquid under test and a transparent, eccentric rod (272). Rotation of the rod causes the path length in the liquid to change by a fixed amount.