Figure 4.10 shows the step-scan time-resolved FT-IR setup for a polymer stretching experiment. In this diagram, the reference input of the Manning polymer stretcher control is from a WaveTek function generator that is triggered by the spectrometer. Thus, the initiation of the stretcher is synchronized with the interferometer stepping, and TRS stretching data are then collected. Since the time scale of this experiment is on the level of ms, a normal AC-coupled photoconductive MCT detector with a preamplifier bandwidth of 175-200kHz will be fast enough. The high pass filter setting of a few to tens of Hz of the MCT preamplifier usually allows both static and dynamic signals to pass through, however the signal will die out eventually if there is no modulation. This dying-out effect can be projected onto the TRS data over longer time scale (say 100ms) with significant difference. Thus a relatively short average time (static sampling) should be used for the static reference spectrum when an AC-coupled detector is used. The other option I used a DC-coupled photoconductive TRS MCT detector that allows the detector response to stay constant when a dynamic perturbation is kept unchanged over a certain time frame as in a square wave perturbation experiment. One major drawback of a DC-coupled detector for this experiment is the classic dynamic range problem that limits the sensitivity of the dynamic measurement.
The signal phase relationship between the stretcher reference function and its stress and strain output is illustrated in Figure 4.11 with delta and sinusoidal functions. It can be seen that the stress output of the stretcher is almost in-phase with the reference function whereas the strain output is delayed by 180 degrees with respect to the reference phase.
S2TRS spectra of an isotactic polypropylene film during the first 6ms of a delta function stretch are shown in Figure 4.12. As the strain increases, the dynamic signal follows the same pattern to reach the maximum. The differential spectral features over the first few ms are similar to those obtained in the phase-resolved mode with a sinusoidal modulation. When a sinusoidal stretch is used in the TRS mode, the dynamic signal intensity profile exhibits a sinusoidal feature over the time scale of measurement, as illustrated in Figure 4.13 and 4.14. This result clearly confirms the theoretical relationship between phase-resolved spectroscopy, ΔФ=ωΔt=2πfΔt, where, in a polymer stretching experiment, ΔФ and Δt are the dynamic signal phase and time differences, and f is the sample modulation frequency.
Chapter 5 Step-Scan Space-Resolved FT-IR Spectroscopy (S3RS) –Focal Plane Array Detector-Based IR Imaging 5.1 Introduction In the mid-1990’s, a new application of the step-scan interferometry was found in infrared imaging. In this application, a step-scan FT-IR spectrometer is coupled to a microscope (or a “macroscope”) that contains a focal plane array (FPA) photovoltaic MCT or InSb detector that has 64×64 or 128×128 detector elements. Combining the FPA multi-channel IR detector technology with step-scan interferometry adds another multiplex advantage in the space domain for FT-IR. This combination allows rapid imaging data acquisition with high spatial resolution in just a few minutes. The step-scan imaging technology has been proven to be an effective approach for heterogeneous sample analysis encountered in many areas ranging from characterizing biological tissues and pharmaceutical sample to studying contaminants, inclusions and diffusions in materials.
5.2 Focal Plane Array Detector-Based FT-IR Imaging System The Nicolet ImageMax IR imaging system is based upon the use of step-scan interferometry, the focal plane array detector (FPA) technology, and infrared microscopy. As shown in Figure 5.1, a DC-coupled 64×64 focal plane array MCT detector is mounted on the Nicolet Nexus ImageMax microscope that is driven by a Nicolet Nexus 870 step-scan spectrometer. At each interferometer mirror step (retardation point), a spectrometer trigger signal is sent to the array detector to collect a series of interferometric frames from all pixels. The image frames are then averaged for each retardation position. Multiple scans can also be used to improve overall SNR. The interferometric image data are then sorted or resolved by the detector element location and then Fourier transformed. In this manner, the space-resolved spectra or spectral images over the entire detection area at different wavenumbers can be generated. Figure 5.2 shows data acquisition in a step-scan FT-IR imaging system with a focal plane array detector.
This imaging system is designed for both routine and advanced imaging data collection, processing and quantization. It can be used to visualize the spatial distribution of distinct chemical species in a variety of systems. A great number of imaging data sets can be recorded with the system in a fraction of the time required by a traditional mapping experiment. The Nicolet ImageMax accommodates both transmission and reflectance illumination modes for imaging either a small sample area (400µm×400µm) or a large area (4mm×4mm), without having to relocate the FPA detector. The most common FPA MCT detector has 64×64 pixels with a long wavelength cutoff around 900cm-1. The spatial resolution for mid-IR can be as high as 6.25 microns for macrosampling. The versatile imaging software package, with univariate and multivariate approaches as well as statistical representations, offers power and convenience for sophisticated quantitative imaging analysis. These tools include PCA (principal component analysis), PLS (partial least squares) regression, fuzzy C-means clustering and factor analysis.
5.3 Application Examples The powerful FPA detector-based imaging technique can be utilized to study a variety of heterogeneous samples in many fields. In this section we present only a few application possibilities.
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