Polymer. Polymeric materials were among the first candidates for imaging application. Figure 5.3 shows the spectral image plane at 1741cm-1 (C=O stretching) and two single pixel (62.5µm×62.5µm) spectra extracted from the polypropylene and the organic sheet domains respectively. The data were collected in the macro reflectance mode over 4×4mm2 illumination area of the sample with a 64×64 MCT array detector. The spatial distribution of the organic sheet (with C=O) is obvious from the image. The spectra at different spatial locations or images at different wavenumbers can be easily extracted or constructed by using the imaging software.
Principle component analysis (PCA) utilizes all of the spectral information to reveal sample heterogeneity. Figure 5.4 shows a PCA image of a polymer laminate that highlights a 10 micron layer in the polymer laminate and also a single frequency (1740cm-1) image that highlights inclusions in the polymer.
Biological Tissues. A composite chemical image for a native bovine nasal cartilage generated by a Euclidean distance multivariate approach shows the distribution of biochemical constituents. The red, blue and dark areas indicate the presence of chondroitin sulfate, collagen and chondrocytes, respectively. The Euclidean distance algorithm provides a more qualitative assessment, allowing for the visualization of relative concentration trends throughout the sample. Peak ratios are often used to generate images on relative concentration of species in the system. Figure 5.6 illustrates a 3D image of relative amounts of liquid and protein in a sectioned sample of a mouse brain cerebellum.
Pharmaceutical Samples. One important application area of FT-IR imaging is pharmaceutical sample analysis. Many pharmaceutical compounds are known to crystallize in different polymorphic forms. Physical properties of these compounds such as solubility, melting point, density, hardness, optical properties, vapor pressure and so forth may all vary with the polymerphic form. The polymorphic distribution of crystal film preparation as well as the molecular and crystal structure of a given pharmaceutical compound affect the efficacy of a drug and may carry enormous legal implications. FT-IR imaging provides a mean of obtaining detailed information on the distribution of molecular and crystal structures. Primidone (5-ethyl-5-phenyl-hexahydrophrimidine-4,6-dione) is frequently used as an anti-convulsant that is effective in the treatment of grand mal psychomotor epilepsy and Jacksonian seizures. Shown in Figure 5.7 are an IR image at 1671cm-1, corresponding to the amide I absorption band, and a visible image with visible polarized light for a primidone polycrystalline file. A quick comparison between the visible and the infrared images clearly indicates visual contrast enhancement on the IR image. Three single pixel spectra at different locations are shown, each representing a 6×6µm2 area on the sample. Comparisons between the individual spectra revealed distinct differences in the C-H stretching region, the stretching vibration of the amide I band (1706-1554cm-1), and the N-H deformation band (1650-1550cm-1), all indicative of at least two, possible three polymorphs.
Chapter 6 Polarization Modulation FT-IR Spectroscopy 6.1 Introduction Polarized IR spectroscopy provides a means to study molecular orientations and chiro-optical properties. Static polarized IR spectroscopy has been utilized to determine molecular orientations of polymeric materials in vibrational linear dichroic (VLD) measurements, as well as those of thin films on reflective surfaces in infrared reflection absorption spectroscopic measurements (IRRAS). In the static measurement, the dichroic difference is obtained from sequentially recorded absorbance spectra corresponding to parallel and perpendicular polarization directions, A‖ and A⊥. However, the static approach is subject to instrumental and sample fluctuations during the experiment and therefore lacks the sensitivity for the determination of small dichroic effects. In addition, the sequential measurement is relatively time-consuming. To overcome these problems, the polarization modulation technique is used to measure spectral dichroic difference spectrum is measured directly by modulating the polarization of the incident IR beam at high frequency with a photoelastic modulator (PEM). The modulation approach is essential for highly demanding experiments such as vibrational circular dichroism (VCD) where the dichroic difference is usually on the level of 10-4 to 10-5 absorbance unit. In this chapter, we focus on polarization modulation- based techniques for IRRAS, VLD and VCD measurements.
6.2 Photoelastic Modulation (PEM) The PEM is a device used for modulating or varying (at a fixed frequency) the polarization of a beam of light. The polarization of IR light can be modulated using a PEM with a ZnSe crystal. This crystal is periodically stressed by compression and stretch during operation, producing a birefringence that changes the polarization of IR light. Using a PEM-90TM photoelastic modulator (Hinds Instruments, Hillsboro, OR) operating at 50kHz, as an example, the polarization directions of the IR light at different phases of the mechanical stress are illustrated in Figure 6.1. The linear polarization modulation of this modulator, used for IRRAS and VLD, is produced at twice the frequency of the mechanical stress, i.e. 100kHz, whereas its circular polarization modulation, used for VCD, is produced at the same frequency of the mechanical stress.
6.3 Dual-channel Synchronous Sampling Technique (SSTTM) Research-grade Nicolet FT-IR spectrometers contain two independent, electronically matched digitizers on the SST module. Each digitizer has an associated software-controlled amplifier and electronic filters. The two digitizers can be used either independently or simultaneously. This dual-channel digitization capability allows simultaneous acquisition of two signals that are generated in double modulation experiments such as polarization modulation-based IRRAS, VLD and VCD. The Fourier modulation by the interferometer* gives the “static” sum reference or background spectrum, and the polarization modulation by a PEM produces the differential spectrum between different polarization directions. The dual-channel capability allows both spectra to be recorded simultaneously. Figure 6.2 illustrates a generic dual-channel electronic setup on the PEM module for polarization modulation experiments. Channels A and B of the spectrometer digitize the sum and difference signals from the demodulator, respectrively.
In principle, a polarization modulation experiment (IRRAS, VLD or VCD) can be conducted sequentially in two separate experiments on a spectrometer with just one digitizer. However, since the differential absorption signals are typically very weak, the demodulated dichroic difference signals can be masked by any instant changes in the environment such as water vapor and CO2 in the system or any change in the spectrometer between the two measurements.
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