Applications of Multivariate Raman Spectroscopy
The vibrational Raman spectrum of water can be used to probe changes in water structure as a function of temperature and due to solute-water interactions. We have demonstrated a diverse set of these applications by combining Raman spectroscopy with self-modeling curve resolution (SMCR). To begin with, the affinity of halogen anions for both benzene and pyridine was quantified by calculating ion partition coefficients based upon vibrational spectroscopic measurements. These partition coefficients were also found to be in good agreement with those obtained from MD simulations performed in collaboration with the Tobias group at University of California, Irvine. Next, we probed the water structure around select biological molecules as a function of temperature, pH and position within the molecule. These proteins, tripeptides and amino acids were challenging due to fluorescence interference and low aqueous solubility. However, promising results related to the sensitivity of phenylalanine to its position within a tripeptide warrant further investigation. Additionally, spectra obtained from both pure and isotopically dilute water were used to selectively probe the influence of intermolecular coupling on the OH (and OD) band shapes and intensities of H2O (and D 2O) in the absence of intramolecular coupling. These results revealed that intermolecular coupling broadens and red-shifts the OH stretch band of HOD in H2O, relative to the isolated OH stretch band of HOD in D2O. This broadening and shifting is also in qualitative agreement with theoretical predictions. Lastly, the temperature-dependent shape and intensity of the OH stretching band of the water spectrum enabled development of an SMCR-based Raman thermometry method. This method was applied to non-invasively measure the temperature of the fluid inside an operating theta tip, a glass capillary used in mass spectrometry experiments. In collaboration with the McLuckey group at Purdue University, we showed the first direct evidence that Joule heating occurs during electroosmosis in a theta tip and that it is positively correlated with electroosmotic voltage.
Ben-Amotz, Purdue University.
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