Microspectroscopy in the Mid‐Infrared

Abstract

Abstract The first type of infrared (IR) microscope to be developed, and still in general use, provides optical visualization plus IR spectroscopic data collection. Its essential features include reflective optics for focusing, collecting, and imaging transmitted or reflected IR radiation from the sample onto a detector, one or more variable apertures located in an image plane of the sample, which serve to define the area to be recorded, and a visible light path, which is parfocal and collinear with the IR light path so that the sample can be viewed and positioned. The accessory is coupled to a Fourier transform infrared (FTIR) spectrometer by a set of transfer optics from the interferometer into the microscope. The analyst is thus provided with the capability to perform IR transmission, absorption–reflection, specular reflection, and diffuse reflection experiments on microscopic samples. Using special microscope objectives or accessories, attenuated total reflection (ATR) and grazing angle reflection experiments can be performed as well. IR microspectroscopy can also be used to map a sample's heterogeneity using a motorized stage in conjunction with a standard mercury cadmium telluride (MCT) detector. Alternatively, the second (more recent and more advanced) technique employs focal‐plane array (FPA) detection coupled with step‐scan interferometry for IR chemical imaging, where the image contrast is determined by the response of individual sample regions to particular IR wavelengths selected by the user. Thus, spatial and spectral information can be collected simultaneously. The superior contrast attained by functional group imaging and the selectivity found in the mid‐ IR region has been shown to be a powerful analytical tool. However, diffraction ultimately limits the quality of the spectral information obtained, and its effects may include poor spatial resolution, a reduction in photometric accuracy, and unreliable band intensities. Furthermore, unwanted optical effects from microscopic samples can also affect the quality of the spectrum, and sample preparation techniques must often consider a sample's thickness, diameter, refractive index, and shape. These limitations have to some extent been circumvented by the development of near‐field techniques such as scanning near‐field optical microscopy (SNOM), IR apertureless scattering scanning near‐field optical microscopy (s‐SNOM), photothermal microspectroscopy (PTMS), and atomic force microscopy‐infrared (AFM‐IR, alternatively known as photothermally induced resonance , PTIR). IR microscopes that rely on near‐field imaging have not yet become ‘standard’ technology but are increasingly coming into use.