Quantum Cascade laser based VCD characterised by a high temporal resolution

Abstract

Vibrational Circular Dichroism (VCD) is a mid-IR spectroscopy technique that offers information about the chirality and structure of the chosen analyte, without the need for additional reactants or time-consuming pre-processing steps. As such, VCD is an efficient tool to evaluate chiral molecules or biological superstructures like DNA and proteins in a non-destructive way. It is routinely used to determine the absolute configuration of molecules in a pharmaceutical context. However, VCD signals are 4 to 6 orders of magnitudes weaker than classical absorbance, necessitating long measurement times in the order of several hours to achieve reasonable signal to noise ratios. Recently, the high spectral power density of broadly tunable quantum cascade lasers (QCL) has been shown to allow mid-IR absorption spectroscopy at higher sensitivity and short measurement times than even high end FTIR instruments, especially when working with highly absorbing solvents.This and the fact that they emit highly polarised light makes QCLs attractive sources for VCD, too. We constructed a QCL based setup, capable of recording VCD spectra with competitive noise levels in a matter of minutes. Showcasing the improvements offered by the application of QCLs, an enantiomeric excess study in CHCl4 was performed. It was possible to resolve the comparatively weak VCD signals (~2-3*10-5 ΔAU) of R and S form of our analyte 1,1′-Bi-2-naphthol within 5 minutes of measurement time. The signal intensity and wavenumber location of these signals are comparable to those of proteins in solution, serving as promising starting points for fast VCD studies of biological molecules.