Balanced detection combined with a Quantum Cascade laser for rapid Vibrational Circular Dichroism

Abstract

Vibrational Circular Dichroism (VCD) combines the broad applicability of infrared spectroscopy with the structure elucidating capabilities of chiroptical spectroscopy.[1] This enables the evaluation of molecular structures in a non-destructive, pre-treatment free manner. However, with signals 4 to 6 orders of magnitude weaker than classical absorbance, measurement times of up to several hours are necessary to obtain satisfactory signal-to-noise-ratios.[1] The recent emergence of broadly tunable quantum cascade lasers (QCL) has the potential to decrease this drawback, similar to the sensitivity enhancement they already provide in absorbance spectroscopy.[2] High spectral power density and highly polarized emission provided by QCLs are well suited for VCD instrumentation. However, QCLs also introduce additional noise in the form of pulse to pulse fluctuations and thermal drifts.[2] For our QCL-VCD instrument, we therefore apply a balanced detection configuration, in which the fluctuations of the laser are compensated, significantly reducing the noise present in the system. These scheme leads to a 4- and 6-fold reduction of noise compared to single detector setups and conventional FT-IR VCD respectively, allowing for short integration times (minutes!) even for weak VCD signals. An enantiomeric excess study of R/S-1,1′-Bi-2-naphthol (BINOL) in CHCl3 serves to showcase this improvement with measurement times below 5 minutes per spectra (see Figure 1).