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

2001 ◽  
Vol 55 (11) ◽  
pp. 1435-1447 ◽  
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.

Step-Scanning Interferometer with Digital Signal Processing

1993 ◽  
Vol 47 (9) ◽  
pp. 1345-1349 ◽  
Author(s):  
Christopher J. Manning ◽  
Peter R. Griffiths
Keyword(s):  
Signal Processing ◽  
Digital Signal Processing ◽  
Digital Signal Processor ◽  
Modulation Frequency ◽  
Depth Profiling ◽  
Digital Signal ◽  
Scanning Interferometer ◽  
Ft Ir ◽  
Lock In ◽  
Signal Processor

A novel step-scan FT-IR spectrometer incorporating a digital signal processor for demodulation of the detector signal is described. The potential advantages of this method of signal processing are discussed and illustrated. The instrument is based on a commercial cube-corner interferometer which has been modified by replacement of the drive motor with a stepper motor-micrometer and piezoelectric transducer combination. The interferometer retardation is feedback controlled by a 486–50 personal computer, which also controls the digital signal processor and collects spectral data. More than one phase modulation frequency can be imposed simultaneously, allowing for a multiplex advantage in photoacoustic depth profiling. Digital signal processing allows for simultaneous demodulation of multiple frequencies which would normally require several lock-in amplifiers. Data that illustrate the feasibility of these concepts are presented. The suitability of this instrument for double-modulation step-scan FT-IR measurements such as polymer stretching and electrochemically modulated step-scan FT-IR is also discussed.

A Solution to the Artifact Problem in Fourier Transform Vibrational Circular Dichroism

1988 ◽  
Vol 42 (1) ◽  
pp. 32-38 ◽  
Author(s):  
Petr Malon ◽  
Timothy A. Keiderling
Keyword(s):  
Fourier Transform ◽  
Circular Dichroism ◽  
Vibrational Circular Dichroism ◽  
Band Shape ◽  
Ft Ir ◽  
Circular Dichroïsm

Using a newly constructed FT-IR vibrational circular dichroism (VCD) instrument, we have found that elimination of the ellipsoidal collection mirror before the detector and its replacement by a lens leads to a significant improvement in the absorption artifact problem seen previously in FT-IR/VCD. In the mid-IR region, we have been able to measure VCD of a single enantiomer for molecules such as α-pinene, 3-methylcyclohexanone, and dimethyltartrate. More importantly, this reduction in artifact level brings the FT-IR/VCD band shape of some particularly-difficult-to-measure bands, such as carbonyl stretches, into better agreement with those found in dispersive measurements. These results imply that the dispersive results are reliable, though of lower resolution than those obtained with the use of FT-IR/VCD.

New Design for Fourier Transform Infrared Vibrational Circular Dichroism Spectrometers

1994 ◽  
Vol 48 (10) ◽  
pp. 1218-1223 ◽  
Author(s):  
Gang-Chi Chen ◽  
Prasad L. Polavarapu ◽  
Stephen Weibel
Keyword(s):  
Fourier Transform ◽  
Circular Dichroism ◽  
Fourier Transform Infrared ◽  
Vibrational Circular Dichroism ◽  
Racemic Mixture ◽  
Area Detector ◽  
Tight Focusing ◽  
Ft Ir ◽  
High Degree ◽  
Circular Dichroïsm

Fourier transform infrared (FT-IR) spectrometers are commonly designed with small-area detectors and tight focusing mirrors. Vibrational circular dichroism (VCD) measurements made with such FT-IR instruments contain polarization artifacts, and VCD measurements on both enantiomers (or one enantiomer and racemic mixture) are required in order to reduce these artifacts. This restriction limits the VCD measurements to only those samples for which both enantiomers (or one enantiomer and racemic mixture) are available. Recently a modified design was reported in the literature where the mirrors between sample and detector were replaced with a BaF2 lens, and a larger-area detector was substituted for the smaller-area counterpart. These modifications successfully alleviated some of the artifact problems. This design, however, is not suitable for polarizing interferometers, where polarizations exiting the interferometer are to be preserved to a high degree of purity. In addition, it is not clear whether the throughput enhancement advantage realized with a larger-area detector completely offsets the disadvantage from increased noise with detector area. Furthermore, BaF2 lenses reduce the broad range routinely available on an FT-IR instrument. Here we report a new design that replaces all the mirrors at the exit port of the interferometer with two KBr lenses and retains the full spectral range (4000–400 cm−1) of mid-infrared FT-IR spectrometers. VCD measurements obtained with small- (1 × 1 mm) and large- (4 × 4 mm) area detectors are found to have similar signal quality.

Digital Signal Processing for Step-Scan Fourier Transform Infrared Photoacoustic Spectroscopy

1997 ◽  
Vol 51 (4) ◽  
pp. 453-460 ◽  
Author(s):  
David L. Drapcho ◽  
Raul Curbelo ◽  
Eric Y. Jiang ◽  
Richard A. Crocombe ◽  
William J. McCarthy
Keyword(s):  
Signal Processing ◽  
Fourier Transform ◽  
Digital Signal Processing ◽  
Phase Modulation ◽  
Modulation Frequency ◽  
Digital Signal ◽  
Sampling Depth ◽  
Ft Ir ◽  
Infrared Photoacoustic Spectroscopy ◽  
Additional Hardware

A software-based digital signal processing (DSP) method using the data system processor has been developed to demodulate the photoacoustic responses of a sample to the fundamental phase modulation frequency and its harmonic frequencies (up to the ninth harmonic) in step-scan FT-IR photoacoustic measurements, without the use of any additional hardware. The DSP algorithm and its sampling depth multiplexing advantages are compared to conventional hardware demodulation. Comparison of results from the DSP method to those from hardware demodulators are shown at both the phase modulation frequency and the harmonics, and application of the DSP method to step-scan photoacoustic measurements with phase modulation is discussed as it applies to obtaining depth profile information in heterogeneous materials.

New application of the step-scan lock-in technique to vibrational circular dichroism

1995 ◽  
Vol 349 ◽  
pp. 451-454 ◽  
Author(s):  
Manfred Niemeyer ◽  
Günter G. Hoffmann ◽  
Bernhard Schrader
Keyword(s):  
Circular Dichroism ◽  
Vibrational Circular Dichroism ◽  
Lock In ◽  
Circular Dichroïsm

Observations on the Measurement of Vibrational Circular Dichroism with Rapid-Scan and Step-Scan FT-IR Techniques

1995 ◽  
Vol 49 (9) ◽  
pp. 1347-1355 ◽  
Author(s):  
Baoliang Wang ◽  
Timothy A. Keiderling
Keyword(s):  
Circular Dichroism ◽  
Phase Modulation ◽  
Optimal Parameter ◽  
Response Spectra ◽  
Vibrational Circular Dichroism ◽  
Slow Step ◽  
Optimal Method ◽  
Near Ir ◽  
Rapid Scan ◽  
Circular Dichroïsm

Extensive tests to determine an optimal method for using a Bio-Rad FTS-60A spectrometer for measurement of vibrational circular dichroism (VCD) in both step-scan and rapid-scan modes are reported. In the latter case, results of “fast-scan” and “slow-scan” experiments are also presented for comparison. In step-scan mode, phase modulation (PM) during the polarization-modulation measurements is found to be detrimental to the signal-to-noise ratio (S/N), but is very useful for the normal IR measurements at slow step speeds. VCD S/N is improved by doing sequential spectral collection of the polarization-modulated spectrum without PM and the instrument response spectra with phase modulation. Systematic comparative studies are used to identify optimal parameter sets for doing the step-scan VCD measurements. These conditions use more rapid data collection with faster steps than previously suggested, which permits a more realistic evaluation of S/N and reproducibility of the spectra through comparison of spectra collected in blocks. Example VCD spectra are presented for small molecules in nonaqueous solution measured in short times and for proteins in aqueous solution measured over longer times scales. Near-IR FT-VCD measured in step-scan mode at both moderate resolution and high S/N are presented. At this stage, fast-scan, slow-scan, and step-scan techniques are shown to be virtually equivalent in the mid-IR, but the reduction in frequency dependence of the phase correction and the new capability for VCD measurement in the near-IR favor continued development of step-scan techniques.

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