TODAY ITS
REPORT FOR ANALYTICAL CHEMISTS
Measurement of Circular Dichroism Classical Methods
The circular dichroism of an op tically active substance may be measured by one of two equivalent methods. The first involves a di rect determination of the difference between the absorption coefficients of the substance in right and left circularly polarized light; the sec ond method measures the ellipticity, denned as the arctangent of the ratio of minor to major axes, intro duced to a linearly polarized light beam traversing the optically active substance. The difference between the absorption coefficients, {kL-kR), is related to the ellipticity per unit length, Θ, according to Equation 7. In a classical paper on optical activity published in 1896, Cotton (S) describes two apparatus which he designed and used to measure the circular dichroism of optically active tartrates. Cotton's first ap paratus is represented in Figure 15. Light from source Pi is collimated by lens L x and linearly polarized by Nicol Νχ. Part of the beam from Ni traverses the Fresnel rhomb F x oriented to give a 45° angle between the plane of polariz ation and the plane of incidence at the total internal reflection surfaces. The circularly polarized beam from Fj passes through the sample S of pathlength c into a second Fresnel
rhomb F 2 which changes the polar ization state of the beam back to linear but orthogonally oriented with respect to the initial polariza tion. The portion of the beam which is attenuated due to sample absorption is now compared with the portion which did not pass through the sample. The compari son is carried out by viewing both portions through a second Nicol N 2 and orienting N2 such that the two portions are of equal intensity. If the two beams are represented by their electric vectors a and b, it may be noted from Figure 15 that there are two angular positions of N 2 for which the intensities of the transmitted beams will be equal. The absorption coefficient of the sample, for the circularly polarized light used, may be deduced from Beer's law and the measured angle, 2a, between the two positions of N 2 . Beer's law expresses the intensity of the attenuated beam in terms of the incident intensity and the ab sorption coefficient of the sample; for right circularly polarized light:
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TB = Toe-1'"(25) The ratio of the two electric vec tors α and b is equal to the tangent of the half-angle between the two positions of N 2 ; for right circularly polarized light tan aB = (a/b)B. (26) Since the electric vector is propor tional to the square root of the in-
Cotton's opporatus for measuring absorption coefficients and left circularly polarized light, (1896).
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tan ν
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Name Address City
Figure 15.
Measurement of circular dichroism
State Circle No. 86 on Readers' Service Card
V O L . 3 8 , N O . 7 , JUNE
1960
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49
A
