Circular Dichroism Spectroscopy: Units

Definition

The CD analogue of the Beer–Lambert Law is

$$ {\displaystyle \begin{array}{ll}\mathrm{CD}& =\Delta A\left(\mathrm{absorbance}\ \mathrm{units}\right)={A}_{\mathrm{\ell}}-{A}_r\\ {}& =\Delta \epsilon c\mathrm{\ell}\end{array}} $$

(1)

Most CD spectropolarimeters measure differential absorbance (at least at differential absorbance signals below 0.1) (Sutherland 2002; Nordén et al. 2010; Rodger and Nordén 1997) via an alternating and direct current measurement. However, largely for historical reasons, most instruments plot a CD spectrum in units of ellipticity, θ , in millidegrees, versus λ, rather than ΔA versus λ. The ellipticity, θ, is obtained from the ratio of the minor and major axes of the ellipse traced out by the electric field vector of the elliptically polarized light when it emerges from the chiral sample onto which linearly polarized light was incident (Rodger and Nordén 1997; Barron 2004; Nordén et al. 2010). The conversion between ΔA and millidegrees is (Barron 2004...