The PEM Module from Thermo Fisher Scientific effectively meets the needs of all PM experiments when combined with a Nicolet FT-IR spectrometer equipped with the Synchronous Sampling Techniques (SST TM). Typical PM experiments include infrared reflection absorbtion spectroscopy (IRRAS), vibrational linear dichroism (VLD), and vibrational cirucular dichroism(VCD). For IRRAS experiments, angles of incidence on the PEM Module can be changed continuously from 45-89° since the detector is placed on a sliding baseplate system.
The spectrometer contains a high frequency (187.5 kHz), electronically balanced, multi-channel analog-to-digital converter, which allows the simultaneous acquisition of signals generated from two channels in PM experiments, such as IRRAS, VLD, and VCD. The Fourier modulation by the interferometer gives the static reference of background single-beam spectrum. The demodulated results of polarization modulated signal give the differential absorption of the IR light in different polarization directions.
This dual-channel capability ratios out almost all isotropic absorptions, such as that of water vapor and carbon dioxide by allowing researchers to record the sample and background spectra simultaneously. Interference from water and carbon dioxide is often difficult to remove when measuring small changes in absorptivity, which are typical in single channel experiments.
Angles for optimized measurements of thin films on metal, glass, semiconductor, and dielectric substrates can be chosen conveniently. The "reflection angle" of 90° for the detector is in essence the transmission position for VLD and VCD experiments. The VCD optics are based on a patent pending all-reflective collimated beam design, allowing minimum distortion of polarized light and maximum throughput for highly demanding VCD experiments. The overall design of the module allows researchers to spend much more time to measure samples and much less time to adjust and align the instrument.
Applications
PM-IRRAS
IRRAS is used as a primary method for monitoring chemical structure and molecular oreintation of ultra-thin films and monolayers adsorbed onto metal and dielectric substrates. Polarization modulation used in IRRAS substantially increases the sensitivity of the measurement. It is particularily useful for suppressing spectral interference from molecules in the adjacent gas or liquid phase.
PM-VLD
VLD spectroscopy measures the linear polarization nature of molecules. A VLD spectrum is a measurement of the difference in molecular absorption of plane-parallel and plane-perpendicular linear polarized light. An example of an application with VLD experiments is the characterization of the orientation nature of polymer films. PM-VLD allows direct measurement of the dichroic difference with high sensitivity, and it can be performed conveniently in transmission mode.
PM-VCD*
The PM-VCD is a highly sensitive method for chirality studies of enantiomers (optically active species) and conformational analysis of biomolecules (e.g., the secondary structure elucidation of proteins) given the small absorbtion difference measurements, typically between 10-4 and 10-5 absorbance unit. This technique offers a broad range of applications in organic and biological chemistry, as well as in pharmaceutical research and development. The new patent-pending optical design has significantly enhanced the sensitivity of the PEM module for VCD. The data collection time has been reduced to two minutes or less for a standard performance test sample, such as (+) or (-) a-pinene neat sample, with a noise level of 10-5 Abs. units over 1900 - 900 cm-1 spectral region.
*patent pending
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