Polarization modulation Fourier transform infrared spectroscopy with digital signal processing: Comparison of vibrational circular dichroism methods

Digital signal processing (DSP) has been implemented in a step-scan FT-IR spectrometer in a modification that enables processing of high-frequency polarization modulation signals. In this work, direct comparison is made between vibrational circular dichroism (VCD) spectra measured on the same instrument, with the same samples, under the same conditions, using this new DSP method and a conventional rapid-scan technique (employing a lock-in amplifier for demodulation). In this initial test, both techniques generated high-quality VCD for solution phase, rigid chiral molecules such as alpha -pinene and camphor. Noise and reproducibility of known spectral features, as well as enhancing signal measurability and discrimination, were used as criteria for the selection of optimal DSP measurement parameters. Both DSP and rapid-scan VCD methods produced qualitatively reasonable spectra for biologically related molecules such as poly-gamma -benzyl-L-glutamate, poly-L-proline, and duplex RNA homopolymer. In most cases, the DSP method had a slight signal-to-noise advantage based on standard deviations of the noise trace data over the rapid-scan measurement, but the final results did depend on the details of the data collection and the phase correction methods inherent in both methods.