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The use of an acousto-optic tunable filter spectrometer can simplify the instrumental requirements for the collection of time resolved spectra to measure the difference in absorbance of polarized radiation due to chemical functional group orientation of a polymer as it undergoes repetitive oscillatory strain. The rapid data collection achieved in the near-infrared spectral region rivals that of current interferometric instruments. Achieving a good signal to noise ratio requires only a singly modulated optical signal, and the use of digital filtering eliminates the requirement of phase sensitive detection. The usefulness of this instrument for routine testing of polymers is demonstrated by differentiating polymers of the same chemical composition but different rheology and by determining the effect of varying the concentration of monomers in a copolymer.

A 64×64 Mercury–Cadmium–Telluride (MCT) focal-plane array detector attached to a Fourier transform infrared (FT-IR) microscope was used to spectroscopically image 8-μm-thick cross-sections of wheat kernels in the fingerprint region of the infrared spectrum. After fast-Fourier transformation of the raw image interferograms, the data can be displayed as either a series of spectroscopic images collected at individual wavelengths, or as a collection of IR spectra obtained at each pixel position in the image. Image contrast is achieved due to the intrinsic chemical nature of the sample at each pixel location in the image. Individual cell layers near the outer portion of the wheat kernel, as well as the primary root within the germ, can be clearly differentiated in the IR images as a result of this enhanced chemical contrast.

The sections in this article are Near-Infrared Filter Spectrometers Acousto-Optic Tunable Filters Introduction Background and Theory Features of an Acousto-Optic Tunable Filter Spectrometer A Typical Acousto-Optic Tunable Filter Spectroscopy Scheme Operation of an Acousto-Optic TFS Performance of an Acousto-Optic TFS Designed for Quantitative Analysis Temporal Resolution with an Acousto-Optic TFS Liquid Crystal Tunable Filters Introduction Nematic Liquid Crystal Tunable Filter Theory Imaging with Liquid Crystal Tunable Filters Discrete Multilayer Interference Filter Photometers

Fourier transform infrared (FT-IR) microspectroscopy is a powerful technique that can be used to collect infrared spectra from microscopic regions of tissue sections. The infrared spectra are evaluated to chemically characterize the absorbing molecules. This technique can be applied to normal or diseased tissues. In the latter case, FT-IR microspectroscopy can reveal chemical changes that are associated with discrete regions of lesion sites, which can provide insights into the chemical mechanisms of disease processes. In the present study, FT-IR microspectroscopy was used to analyze sections of retina from normal (pigmented) and albino rats. The outer segments of retinas from pigmented animals were found to have unusually strong absorption values for Cz.dbnd6;C-H unsaturation and carbonyl functional groups. Docosahexaenoic acid (DHA), a major constituent of lipids in the outer segments, also had particularly high absorption values for these functional groups, which suggests that it is responsible for those enhanced absorption values. Absorbance values for the unsaturation and carbonyl functional groups were substantially reduced in the outer segments of retinas from albino animals. This finding, together with data from other studies on light-induced oxidative events in the retina, indicates a loss of DHA by a light-induced mechanism in albino animals. The outer nuclear layer had strong absorbance values for H-C-OH and Pz. dbnd6;O functional groups, which is likely due to the sugar phosphate backbone of DNA. The outer and inner plexiform layers were found to contain greater concentrations of CH(2) and Cz.dbnd6;O functional groups than the outer and inner nuclear layers, which is due to the high concentration of synaptic connections in the former layers. In summary, FT-IR microspectroscopy revealed a unique chemical profile in the outer segments compared to other retinal layers, and this profile was altered in albino animals.

Cross sections of grain kernels, leaves, other plant material, and their products have been examined routinely in our own laboratory with an integrated FT-IR microspectrometer equipped with a conventional (thermal) globar source. With plant material, scattering is often a problem. Representative (low density) mapping requires interpolation between spots on the tissue actually interrogated. High density (100%) mapping with a small pixel size is typically painstakingly done and requires coaddition of many scans. With the synchrotron source (National Synchrotron Light Source, Beamline U2B) of the U.S. Department of Energy’s Brookhaven National Laboratory, Upton, New York, nearly all of these problems are solved. Low thermal noise and brightness of the beam provide high S/N. The non-divergence of the synchrotron microbeam allows the high S/N to be retained even with aperturing of 6 μm or 12 μm sizes. Diffraction influences the practical limit. Step sizes corresponding to the small aperture dimension reveal highly localized chemical differences between adjacent pixels of a tissue specimen.