Typical PM experiments include:
• Infrared reflection absorption spectroscopy (IRRAS)
• Vibrational linear dichroism (VLD)
• Dynamic infrared linear dichroism (DIRLD)
• Vibrational circular dichroism (VCD)
The TOM effectively meets the needs of all PM experiments when combined with a Nicolet FT-IR spectrometer that is equipped with SST, or step-scan SMM technology. The spectrometer contains two independent, electronically equivalent analog-to-digital converters. This dual-channel digitizing capability allows simultaneous acquisition of two signals generated in typical PM experiments. Each digitizer has an associated software-controlled electronic filter which can be used either independently or simultaneously.
The Fourier modulation by the interferometer gives the static reference orbackground single-beam spectrum. The polarization modulation by a photoelastic modulator gives the differential absorption of the IR light of different polarizations. By allowing researchers to record the sample and background spectra simultaneously, this dual-channel capability ratios out almost all isotropic absorptions, such as that of water vapor and carbon dioxide. Interference from these species is often difficult to remove when measuring small changes in absorptivity, which is typical in single-channel PM experiments.
The TOM has built-in, pre-aligned optical components, detector assembly and a pinned-in-place universal adapter for sampling accessories, requiring minimum optical installation and adjustment on the customer’s site. Its MCT detector is automatically activated through selection of the TOM in the OMNIC™ software. The wide range of continuously varying reflection angle (useful range 33°– 85°, optically available range 33°– 89°) for IRRAS is achieved by placing the detector assembly on an optical rail. The universal adapter allows easy coupling of a variety of standard and special sampling accessories such as standard vertical IRRAS holder, 6" disk (wafer) IRRAS holder, horizontal grazing angle accessory for mono-layer samples on liquid substrates, and transmission based-VLD, VCD and DIRLD (e.g., polymer stretcher) accessories.
IRRAS is used as a primary method of monitoring chemical structure and molecular orientation of ultra-thin films and monolayers absorbed onto metal, dielectric or liquid substrates. PM used in IRRAS substantially increases the sensitivity of the measurement. It is particularly useful for suppressing spectral interference from molecules in the adjacent gas or liquid phase.
Vibrational linear dichroism (VLD) spectroscopy measures the linear polarization nature of molecules. Typically, a VLD spectrum is a measurement of molecular absorbance difference between planeparallel and plane-perpendicular linear polarized light in transmission mode. An example of VLD experiments is the characterization of the orientation nature of polymer films. Dual-Channel PM used in VLD allows measuring the dichroic difference directly with high sensitivity and a single scan.
The IR spectral measurement of orientation change of a material under a sinusoidal strain perturbation is referred to as DIRLD. DIRLD can be measured by two step-scan FT-IR experiments with a wire-grid polarizer for determining the dynamic dichroic difference spectrum, as seen in the classic dynamic polymer stretching experiment. When PM is used with DIRLD, the dynamic dichroic difference spectrum can be measured directly from a single triple modulation experiment (phase, sample and polarization modulations). This triple-modulation experiment can be performed by coupling a TOM box with step-scan DSP synchronous multiple modulation (SMM) technology. Use of PM for DIRLD significantly increases the dynamic range and thus the sensitivity of the measurement.
VCD is a spectroscopic method for chirality studies of enantiomers (optically active species), and conformational analysis of biomolecules (e.g., the secondary structure elucidation of proteins). The technique offers a broad range of applications in organic and biological chemistry, as well as in research and development of pharmaceutical and life science industries. Given the small absorption difference measurements – typically between 10-4 and 10-5 absorbance units in VCD, PM is a must for achieving necessary sensitivity in VCD measurements.
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