FT‐IR Imaging in ATR and Transmission Modes: Practical Considerations and Emerging Applications

Abstract

Fourier transform infrared (FT-IR) spectroscopic imaging is a powerful chemical imaging technique. A range of approaches and applications of FT-IR imaging have been developed in both attenuated total reflection (ATR) and transmission modes for a broad range of materials and chemical systems. ATR and transmission approaches are often complementary -providing different advantages depending on the requirements of the system under study with respect to chemical composition, field of view (FOV), or spatial resolution, for instance.FT-IR spectroscopic imaging is achieved by coupling an FT-IR spectrometer with a focal plane array (FPA) detector. Unlike single-element detectors, which provide an average signal from the illuminated sample area, FPA detectors allow the simultaneous detection of spatially resolved spectra across the whole imaged area as shown schematically in Figure 9.1. This powerful approach of coupling FT-IR spectroscopy with FPA detectors and thereby obtaining simultaneous, spatially resolved spectra allows systems to be studied as a function of spectral, spatial, and temporal domains. Since a full spectrum is recorded at each pixel, different chemical species and their relationship with one another can be revealed by studying the resulting images. Generally, the images are prepared by plotting the distribution of the integrated absorbance of a vibrational band of interest and assigning a false color to indicate its relative absorbance. Many images showing the distribution of different components in the sample can be generated from a single imaging measurement simply by post-measurement data analysis. Other parameters such as band position or width can be used to reveal chemical information about the system.Single-element detectors or linear array detectors are capable of building chemical images through point-by-point mapping or raster scanning, respectively. Although this may be sufficient for some applications, the increased time required and the consecutive collection of spectra to build an image may not permit the study of dynamic systems nor is it particularly suitable for high-throughput studies.

Fiber-Array-Based Raman Hyperspectral Imaging for Simultaneous, Chemically-Selective Monitoring of Particle Size and Shape of Active Ingredients in Analgesic Tablets

“…This could include for example the monitoring of the particle distribution before and after compaction, to understand the effect of different parameters of the pressing process on the API morphologies. For other settings, the FOV dimensions can be easily adjusted, simply by proper choice of the magnification of the objective lens (in contrast to ATR imaging where the FOV is given by the lens dimensions [63]). It could be also applied for monitoring (unwanted) crystallization processes at different physical conditions, e.g., during hot-melt extrusion processes, or for testing the effect of different storage conditions on the APIs in the formulation.…”

Nontoxic ammunition: Challenges and perspectives for GSR identification

“…), wavelength of incident light, and refraction index of the particle analyzed (Dougan et al, 2014). However, a typical diamond ATR setup provides a spatial resolution of 15-20 μm, being able to reach 4 μm if a micro-ATR setup is employed (Dougan et al, 2014). In that scenario, it is clear that there are certain limitations regarding the spatial resolution of IR microscopy to obtain a reliable spectrum if a small particle is analyzed.…”

“…This is because spatial resolution in infrared is related to the limit of diffraction, which depends on the microscope aperture, imaging setup (e.g., macro‐ATR, diamond ATR, etc. ), wavelength of incident light, and refraction index of the particle analyzed (Dougan et al, 2014). However, a typical diamond ATR setup provides a spatial resolution of 15–20 μm, being able to reach 4 μm if a micro‐ATR setup is employed (Dougan et al, 2014).…”

Infrared Spectroscopic Imaging Advances as an Analytical Technology for Biomedical Sciences

“…Another experimental approach was aimed at minimizing chromatic aberration caused by thick CaF 2 windows in a flow cell by introducing additional optical elements on the top and bottom of the windows to form a pseudohemisphere. 193–195 Such an approach not only forces all of the different light wavelengths to focus in one spot but also additionally increases the effective NA of the system, allowing more efficient light collection and reducing scattering contributions to the recorded spectra. Another aspect of imaging is not to push the limits of pixel size but to expand the field of view and enable new applications.…”