Theory of Midinfrared Absorption Microspectroscopy: I. Homogeneous Samples

Abstract

Midinfrared (IR) microspectroscopy is widely employed for spatially localized spectral analyses. A comprehensive theoretical model for the technique, however, has not been previously proposed. In this paper, rigorous theory is presented for IR absorption microspectroscopy by using Maxwell's equations to model beam propagation. Focusing effects, material dispersion, and the geometry of the sample are accounted to predict spectral response for homogeneous samples. Predictions are validated experimentally using Fourier transform IR (FT-IR) microspectroscopic examination of a photoresist. The results emphasize that meaningful interpretation of IR microspectroscopic data must involve an understanding of the coupled optical effects associated with the sample, substrate properties, and microscopy configuration. Simulations provide guidance for developing experimental methods and future instrument design by quantifying distortions in the recorded data. Distortions are especially severe for transflection mode and for samples mounted on certain substrates. Last, the model generalizes to rigorously consider the effects of focusing. While spectral analyses range from examining gross spectral features to assessing subtle features using advanced chemometrics, the limitations imposed by these effects in the data acquisition on the information available are less clear. The distorting effects are shown to be larger than noise levels seen in modern spectrometers. Hence, the model provides a framework to quantify spectral distortions that may limit the accuracy of information or present confounding effects in microspectroscopy.

Application of infrared spectroscopic imaging in specular reflection mode for determination of distribution of chemical components in urinary stones

“…However, frequently urinary stones are too fragile and the polishing leads to fragmentation. Heterogeneity of the sample cannot be neglected as well [18,19]; thus the spectra should be processed and the diffuse reflection component separated from the specular reflection component before applying the Kramers-Kronig relations.…”

Medical Applications of Infrared Spectral Imaging of Individual Cells

“…May it suffice here to state that these effects are due to the morphology of the sample; in particular, spherical particles exhibit these effects particularly strongly. However, as demonstrated theoretically by Bhargava and coworkers [11,12], any sample with strong gradients of the refractive index (for example, a straight edge of a particle boundary) will exhibit these effects as well. At present, there exist several computational approaches to correct these phenomena, based on either an iterative numerical process [14], a step-wise approximation method [15], or a phase-correction approach that was suggested as early as 2005 [9,16,24].…”

BIM-Sim: Interactive Simulation of Broadband Imaging Using Mie Theory

“…Recent methods have been proposed for creating a general framework for simulating scattering in microspectroscopy [16,19], with the goal of correcting artifacts. These methods evaluate cross-sections of the EM field near the sample.…”