Solving chemical classification problems using polarized Raman data

Abstract

When solving chemical classification problems, multivariate analysis has proven to be an important mathematical tool. Unpolarized spectroscopic data, IR, NIR, and UV-Visible absorption data and unpolarized vibrational Raman data, are typically analyzed by two-way chemometric methods, e.g. principal component analysis (PCA). When the unpolarized spectra of the different molecules are almost identical, the PCA results in low recognition ratios or even fails. In contrast to absorption processes, Raman scattering can provide polarized data. It is shown, by using mathematical simulations, that the outcome of the PCA can be improved considerably by using the polarized, vibrational Raman data instead of the unpolarized data. The improvement stems from the increased amount of molecular information, which is now available for the PCA of the vibrational data, because the polarization properties of the scattering, expressed through the depolarization ratio (DPR), is very sensitive to small changes in distinct molecular properties and insensitive to sample and experimental variations. For molecules possessing some symmetry, a change of the DPR may be induced by a decrease in symmetry and for highly symmetric molecules non-dispersive Raman modes typically become dispersive. For dispersive modes, a wavenumber-dependant change of the DPR may also result from a small energy shift of an allowed electronic transition. We show that the increased information content inherently present in the polarized data, opens up new possibilities for combining the solution of classification problems with an unveiling of details of the different properties and processes in bio-physic due to various perturbations and changes of the structure of the bio-molecules. It is also demonstrated that the increased access to molecular information enables in vitro detection of molecular changes often encountered when analyzing biological functions, which are reflected in changes in the excited electronic states.

What Is Vibrational Raman Spectroscopy: A Vibrational or an Electronic Spectroscopic Technique or Both?

“…The polarization properties of the resonance Raman signal are more important in resonance than in non-resonance. For example, as discussed in [43], the uniqueness of the polarization-resolved VRRS spectra combined with standard PCA chemometrics enables one to discriminate between closely related biomolecules with almost identical unpolarized VRRS spectra. The key point is that structural molecular change manifests itself through a change of the polarization of the Raman signal (DPR).…”

Recent advances in linear and nonlinear Raman spectroscopy. Part VI

“…carried out information‐theoretic chemometric analyses of Raman data for the study of chemical reactions . Hassing and co‐workers demonstrated a method for solving chemical classification problems by using polarized Raman data . He et al .…”

Determination of the Raman depolarization ratio in optically active samples

“…Along with intensity, frequency, and line shape, the depolarization ratio r is an important parameter when Raman spectroscopy is used to investigate the molecular structure of liquids and solutions. It provides useful information about the symmetry of a vibrational mode, [1][2][3] and therefore can facilitate an improved assignment and interpretation of a spectral feature. The determination of the depolarization ratio is relatively straightforward.…”