REPORT FOR ANALYTICAL CHEMISTS where ΔΑ is the difference in ab sorption of the sample for left and right circularly polarized light, 8 is the phase retardation due to the quarter-wave plate and ξ is the error angle in setting the quarterwave plate relative to the polarizer. A novel approach to circular dichroism measurements based on the use of an electro-optic retardation modulator, (Pockel cell), was de scribed by Grosjean and Legrand (14) in 1960 for use in the spectral region 2200 to 6000 A. A diagram of their apparatus is shown in Fig ure 19: a high intensity light source illuminates a single-prism monochromator; the monochro matic beam is collimafced by the exit lens of the monochromator and is linearly polarized by a quartz Rochon polarizer. Both beams from the polarizer, ordinary and extraordinary, pass through an electro-optic modulator consisting of a Z-cut plate of ammonium dihydrogen phosphate sandwiched between two transparent electrodes subjected to an alternating voltage. The plate is oriented so t h a t the electro-optically induced axes make an angle of 45° with the direction of the incident linear polarization. The state of polarization following the plate is modulated, at the same frequency as the driving voltage, alternating between left and right circular polarization. The extraor dinary beam from the polarizer is masked at the entrance to the sam ple space while the ordinary beam traverses a sample cell and is then
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focussed on a phototube. If the sample is circularly dichroic, the output voltage of the phototube will contain an alternating component superimposed on an average com ponent; the alternating voltage is related to the difference in transmis sion of the sample for left and right circularly polarized light, while the average voltage is related to the average transmission of the sample. The ratio of these two voltages is very nearly proportional to the dif ference in absorption of the sample for left and right circularly polar ized light. This ratio is measured in the following manner: the out put voltage of the phototube is sep arated into its two components, Vac and Vdc, by means of a separating amplifier of unit gain. The aver age component, ViC) is inverted in an amplifier of gain —1 and volt ages of -\-Vdc and —Vic are applied across a slidewire. The alternat ing component, Va,, is amplified by a linear AC feedback amplifier whose output is then rectified, fil tered, and compared to the voltage at the sliding contact of the slidewire by means of a servo-amplifier. A servo-motor drives the sliding contact to reduce to zero the differ ence between the two compared voltages. A pen, mechanically coupled to the sliding contact, re cords the circular dichroism of the sample. Grosjean and Legrand es timated this system of measure ment to be sensitive to absorption differences of approximately 10 4 optical density unit. A modified
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ANALYTICAL CHEMISTRY
Figure 19.
Measurement of circular dichroism—Grosjean-Legrand
method
