Patent GB-1143364-A (GB)
Title: Device for the simultaneous measurement of optical rotation and circular dichroism
IPC: G01N21/19
Filed: 1966-03-14
Abstract: 1,143,364. Photo-electric measurement of optical activity. BODENSEEWERK PERKINELMER & CO. G.m.b.H. March 14, 1966 [March 13, 1965], No. 11087/66. Heading G1A. Apparatus for the simultaneous measurement of the optical rotation and circular dichroism of an optically active sample utilizes a photo-electric detector. In one embodiment, Fig. 2, light from source 20 passes through a monochromator 22 to a polarizer prism 24. The plane of polarization of light from the prism is oscillated about a central position at a frequency w 1 , by means of a Faraday modulator 26. After passing through the optically active sample 28, the beam is applied to a double-refracting element 30 (a “-wave plate). The light then passes through a further Faraday modulator, this time operating at a frequency # 2 , and through the analyzer prism 34 to photo-detector 36. Motors 42 and 44 are provided to operate in response to the detector output effectively to rotate the polarizer and analyzer, respectively. The rotation of the polarizer is such that the central position of the polarization plane of the light (or major axis in the case of elliptically polarized light) is always parallel to the optical axis of the double refracting element 30. Thus in a position of balance, the polarizer position represents the optical rotation occurring in the sample. The rotation of the analyzer with respect to the double refracting element is such that the ellipticity caused by the double-refracting element compensates for the ellipticity caused by the circular dichroism of the sample. The final angular position of the analyzer thus represents the circular dichroism of the sample. The detector output contains frequencies # 1 , # 2 , plus double and other harmonics of these. When the analyzer is exactly crossed with respect to the central position of incident radiation, only the frequencies 2# 1 , 2# 2 , are produced in the detector output. Filters 38, 40 are used so that only signals at frequencies # 1 , # 2 , respectively, are used to control the motors 42, 44. Thus when the system is balanced (frequencies 2# 1 , 2# 2 ) no further movement is produced by the motors. Another arrangement and alternative individual optical elements are referred to. For example, Fig. 6 (not shown), the double-refracting element 30 may be in front of the sample 28, when the polarizer position indicates circular dichroism and the analyzer position is representative of optical rotation. In balancing either of the arrangements, the analyzer can be adjusted parallel to the incident polarization plane instead of crossed with respect to it. However, in this case an undesirable constant intensity is obtained in the detector. Instead of using Faraday modulators, actual mechanical rotary oscillation of the polarizer and analyzer can be utilized. Instead of direct physical movement of the polarizer and analyzer by the servomotors 42, 44 the central positions of the polarization planes, may be changed by changing the geometry of an optically active wedge and plate construction associated with each of the polarizer and analyzer. Alternatively, these central positions can be adjusted by utilizing Faraday effect devices, the current for these being derived from the detector signal and being measurable to indicate the amount of rotation of the polarization plane. The double refracting element may be an electro-optical crystal, or a Soleil-Babinet compensator. The variation of both optical rotation and circular dichroism with wavelength may be measured by use of the monochromator.