Circular Dichroism

Circular dichroism is the differential absorption (A = AL – AR) between left and right circularly polarized light in the UV-Visible region. It is a measurement of the optical activity of chiral molecules.


PEM Setting: quarter-wave retardation (λ/4)
Polarization Modulation: right and left circular polarization
Lock-in reference: PEM’s 1st harmonic


After the PEM, the light beam is modulated between right and left circular polarization at the modulator frequency (1f).

When the sample absorbs right and left circular polarization differently, the light intensity on the detector will have a 1f AC signal that is related to A. In principle, two separate spectra, AL and AR, can be taken with left and right circularly polarized light and then subtracted to get A. However, A is normally several orders of magnitude smaller than AL or AR. The stability and dynamic range of a common instrument cannot meet such a requirement. Therefore, PEM modulation makes the CD spectrometer practical.


Structural study and chiral analysis in biochemistry; analytical and physical chemistry; drug and pharmaceutical industry




CD MicroPlate Reader


High-throughput determination of enantiopurity by microplate circular dichroism, Samantha L. Pilicer, Justin M. Dragna, Adam Garland, Christopher J. Welch,, J. Org. Chem. (2020)

Linear & Circular Dichroism Application Note, T.C. Oakberg, Hinds Instruments, Inc. (1991)

Polarisation modulation – the measurement of linear and circular dichroism, A. F. Drake, J. Phys. E: Sci. Instrum. 19, 170 (1986)

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