Spectral Similarity: Full-Wavelength Range Calibration of Circular Dichroism Spectroscopy

Circular dichroism (CD) measurements help characterize optically active molecules and higher-order biomolecular structures. The harmonization of interlaboratory CD measurements requires minimizing measurement uncertainties and determinate errors. Most CD measurements utilize a single-wavelength intensity measurement at a spectral peak to calibrate the intensities of the ultraviolet wavelength range. However, such a single-wavelength calibration is inherently less precise than using the CD instrument's spectrum over the entire measured spectral range. A more thorough and informative calibration can be achieved by remapping the spectrum into what we call a spectral similarity plot. This process allows a straightforward, quantitative evaluation of the shape congruence between two spectra over the full spectral range. While preliminary analyses suggest spectral similarity plots can be utilized with a variety of different spectrometry methods, here we illustrate the process applied to circular dichroism. Spectral similarity plots are highly sensitive to deviations due to differences in concentration, pathlength, source and detector properties, circular polarization balance, as well as wavelength nonlinearities and shifts. Deviations in these properties can be quantitated by a linear least squares fit of the remapped data. The remapping enables protocols to correct spectra toward congruence between two spectra. The spectra similarity comparison provides an objective, unambiguous test of the CD instrument quality when, for example, compared to a carefully calibrated system as shown in the examples described in the text.