What Is the Unit of the Vertical Axis Δε (M⁻¹ cm⁻¹) in Circular Dichroism Spectroscopy, and What Does M Represent?
In chemistry and physics, circular dichroism (CD) spectroscopy is a technique used to measure the differential absorption of left- and right-circularly polarized light by chiral molecules. The vertical axis of a CD spectrum is typically denoted as Δε, with its unit expressed as M⁻¹ cm⁻¹. In this unit:
M⁻¹ (per molar concentration)
This represents the inverse of molarity (mol/L), as CD measurements are conducted in solutions of known molar concentration. The molar ellipticity (Δε) is directly proportional to the concentration of the analyte.
cm⁻¹ (per centimeter)
This denotes the path length of light in the sample, a standard unit in spectroscopic measurements.
Thus, the unit M⁻¹ cm⁻¹ quantifies the molar absorption difference between left- and right-circularly polarized light per unit path length, providing insights into molecular chirality and electronic transitions in a defined solution environment.
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