Circular Dichroism

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

SETUP

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

POLARIZATION MODULATION

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, A_L and A_R, 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 A_L or A_R. The stability and dynamic range of a common instrument cannot meet such a requirement. Therefore, PEM modulation makes the CD spectrometer practical.

APPLICATIONS

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

PEMs

I/FS50
II/FS42

INSTRUMENTATION

CD MicroPlate Reader

FURTHER READING

Fundamentals, Advances, and Artifacts in Circularly Polarized Luminescence (CPL) Spectroscopy, Winald R. Kitzmann, John Freudenthal, Antti-Pekka M. Reponen, et.al., Adv. Mater. 35, 44 (2023)

Abstract: Objects are chiral when they cannot be superimposed with their mirror image. Materials can emit chiral light with an excess of right- or left-handed circular polarization. This circularly polarized luminescence (CPL) is key to promising future applications, such as highly efficient displays, holography, sensing, enantiospecific discrimination, synthesis of drugs, quantum computing, and cryptography. Here, a practical guide to CPL spectroscopy is provided. First, the fundamentals of the technique are laid out and a detailed account of recent experimental advances to achieve highly sensitive and accurate measurements is given, including all corrections required to obtain reliable results. Then the most common artifacts and pitfalls are discussed, especially for the study of thin films, for example, based on molecules, polymers, or halide perovskites, as opposed to dilute solutions of emitters. To facilitate the adoption by others, custom operating software is made publicly available, equipping the reader with the tools needed for successful and accurate CPL determination.

High-throughput determination of enantiopurity by microplate circular dichroism, Samantha L. Pilicer, Justin M. Dragna, Adam Garland, Christopher J. Welch, et.al., 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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