Editing Circular dichroism (section)

=== Linear dichroism (LD) ===
With the appropriate accessory, it is possible to perform [[linear dichroism]] (LD) measurements on CD spectrophotometers. Linear dichroism is the difference in the absorbance of light polarized parallel and perpendicular to an orientation axis:

'''ΔA = A<sub>para</sub> - A<sub>perp</sub>'''

where '''A''' is the absorbance of the linearly polarized light, and the subscripts indicate parallel and perpendicular, respectively. For a sample to have non-zero LD it must be anisotropic; the anisotropy may be intrinsic, as in liquid crystals for example, or it may be induced, for example, by stretching a polymer film, by the application of an electrical field, or by alignment or deformation in a shear field (e.g., nucleic acids such as DNA).

An LD device uses a [[Couette flow|Couette]] cell to generate a shear field. The sample is contained in the annular gap between two concentric quartz cylinders, the outer of which, the rotor, is rotated about its cylindrical axis, while the inner, the stator, is stationary. This arrangement is named after its originator, [[Maurice Couette]].

Shearing can cause large particles in the liquid to align or deform. For example, rigid, rod-like particles, such as carbon nanotubes and glass fibers, will align, whereas vesicles, micelles, and flexible polymers will deform from their equilibrium conformations, extending in the direction of shear. Linear dichroism is then used to determine the direction of net electron transfer in an absorbed chromophore relative to the shear direction, and thereby to the alignment direction of the absorbing species or the deformation direction of its environment.

For more information on LD and its measurement, consult this book.<ref>{{Citation |last1=Rodger |first1=Alison |title=Circular Dichroism and Linear Dichroism |date=2023 |encyclopedia=Encyclopedia of Analytical Chemistry |pages=1–42 |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/9780470027318.a5402.pub3 |access-date=2025-02-17 |publisher=John Wiley & Sons, Ltd |language=en |doi=10.1002/9780470027318.a5402.pub3 |isbn=978-0-470-02731-8 |last2=Chubb |first2=Joel J.}}</ref>