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 s pectrometer in a modification that enables processing of high-frequency pol arization modulation signals. In this work, direct comparison is made betwe en vibrational circular dichroism (VCD) spectra measured on the same instru ment, with the same samples, under the same conditions, using this new DSP method and a conventional rapid-scan technique (employing a lock-in amplifi er for demodulation). In this initial test, both techniques generated high- quality VCD for solution phase, rigid chiral molecules such as alpha -pinen e and camphor. Noise and reproducibility of known spectral features, as wel l as enhancing signal measurability and discrimination, were used as criter ia for the selection of optimal DSP measurement parameters. Both DSP and ra pid-scan VCD methods produced qualitatively reasonable spectra for biologic ally related molecules such as poly-gamma -benzyl-L-glutamate, poly-L-proli ne, 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 d ata over the rapid-scan measurement, but the final results did depend on th e details of the data collection and the phase correction methods inherent in both methods.