Polarization Demodulation: A New Approach to the Reduction of Polarization Artifacts from Vibrational Circular Dichroism Spectra

In vibrational circular dichroism (VCD) measurements and other polarization modulation techniques, artifact signals often arise due to polarization sensitivity of the optical components located subsequent to the sample. In this paper, we describe a method for demodulating the light beam after passage through the sample in such a manner that only one linearly polarized state is allowed to reach the detector, thus eliminating these artifacts. The method is a beam reversal technique in which the light subsequent to the sample is redirected through the photoelastic modulator in such a way that the retardation produced on the first traversal through the modulator is removed. The light is returned to its original state of linear polarization prior to detection. The method is applicable to Fourier transform as well as to dispersive spectrometers since demodulation is effected for all wavelengths simultaneously. The results of preliminary demodulation experiments are presented and found to be more than 90% efficient in removing circular dichroism intensity of a birefringent origin.

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Polarization Modulation Fourier Transform Infrared Spectroscopy with Digital Signal Processing: Comparison of Vibrational Circular Dichroism Methods

Applied Spectroscopy ◽

10.1366/0003702011953810 ◽

2001 ◽

Vol 55 (11) ◽

pp. 1435-1447 ◽

Cited By ~ 18

Author(s):

Jovencio Hilario ◽

David Drapcho ◽

Raul Curbelo ◽

Timothy A. Keiderling

Keyword(s):

Signal Processing ◽

Circular Dichroism ◽

Digital Signal Processing ◽

Digital Signal ◽

Vibrational Circular Dichroism ◽

Polarization Modulation ◽

Rapid Scan ◽

Ft Ir ◽

Lock In ◽

Circular Dichroïsm

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 α-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-γ-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.

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The Use of Dual Polarization Modulation in Vibrational Circular Dichroism Spectroscopy

ACS Symposium Series - Chirality: Physical Chemistry ◽

10.1021/bk-2002-0810.ch006 ◽

2002 ◽

pp. 79-88 ◽

Cited By ~ 4

Author(s):

Laurence A. Nafie ◽

Rina K. Dukor

Keyword(s):

Circular Dichroism ◽

Circular Dichroism Spectroscopy ◽

Vibrational Circular Dichroism ◽

Polarization Modulation ◽

Dual Polarization ◽

Circular Dichroïsm

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New Approach to the Theory of Circular Dichroism

Collection of Czechoslovak Chemical Communications ◽

10.1135/cccc19981187 ◽

1998 ◽

Vol 63 (8) ◽

pp. 1187-1201 ◽

Cited By ~ 1

Author(s):

Jaroslav Zamastil ◽

Lubomír Skála ◽

Petr Pančoška ◽

Oldřich Bílek

Keyword(s):

Electromagnetic Wave ◽

Circular Dichroism ◽

Electromagnetic Waves ◽

Maxwell Equations ◽

Optically Active ◽

Semiclassical Approach ◽

New Approach ◽

Isotropic Media ◽

New Formula ◽

Circular Dichroïsm

Using the semiclassical approach for the description of the propagation of the electromagnetic waves in optically active isotropic media we derive a new formula for the circular dichroism parameter. The theory is based on the idea of the time damped electromagnetic wave interacting with the molecules of the sample. In this theory, the Lambert-Beer law need not be taken as an empirical law, however, it follows naturally from the requirement that the electromagnetic wave obeys the Maxwell equations.

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