Microspectroscopy

Abstract

AbstractThe essential features of an infrared (IR) microscope accessory includereflective 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, anda 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 IR (FTIR) spectrometer by a set of transfer optics from the interferometer into the microscope.IR microspectroscopy provides the analyst 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, spatial and spectral information can be collected simultaneously using a step‐scan interferometer coupled with an IR microscope and a focal plane array (FPA) detector. In either case, 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 using the IR microscope, 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 take into account a sample's thickness, diameter, refractive index, and shape.