Reflection microspectrometry

UV and visible absorption (also reflectance) microspectrometry has an extensive use in screening pigments and colors in paints, inks, and fibers, whereas it has found applications mostly for quantitative measurements in drug analysis (in particular for barbiturates). The development of sensitive IR spectrometers with Fourier transformation and... 

FT-IR spectrometers specifically manufactured for reflection microspectrometry are commercially available. Such spectrometers can be combined with most microscopes having infinity-corrected optics (in which the output from the Cassegrain objective mirror becomes parallel radiation). Since, in the infinity-corrected design, the output... 

The frequencies of absorption bands present gives diagnostic information on the nature of functional groups in materials as well as information from any observed frequency shifts on aspects such as hydrogen bonding and crystallinity. In many cases, spectra can be recorded non-destructively using either reflection or transmission procedures. IR spectra of small samples can also be obtained through microscopes (IR microspectrometry). Chalmers and Dent [8] discuss the theory and practice of IR spectroscopy in their book on industrial analysis with vibrational spectroscopy. Standard spectra of additives for polymeric materials include the major collection by Hummel and Scholl [9]. 

Microspectrometry is an indispensable technique in criminalistic analyses, being a combination of optical microscopy and spectrometry. Microscopy creates, records and interprets magnified images, whereas spectrometry uses emission, absorption and reflection of radiant energy by matter to determine its structure, properties and composition. On the basis of the type of energy applied, microspectrometry can be divided into IR, visual and ultraviolet (UV-vis), and Raman microspectrometry. This group also includes X-ray microspectrometry, in which an electron microscope takes the place of an optical microscope. Infrared and Raman microspectrometry enable determination and comparison of the chemical composition of studied samples UV-vis microspectrometry serves to compare the colour of samples in an objective way that is independent of the observer and X-ray microspectrometry allows determination of the elemental composition. 

FTIR spectroscopy was used for the analysis of ultrathin organic films on metals. FTIR in the reflection mode (IRRAS) was used to study the interaction of ultrathin films of dicyandiamide (hardener of most one-pack epoxy resins) with various substrates, model ones such as gold or zinc and industrial ones such as steel and zinc-coated steels. Pure zinc surfaces and, to a lesser extent, zinc-coated steels are shown to react with dicyandiamide after heating at 180 C, as evidenced by the frequency shift of the absorption band characteristic for nitrile groups. Some mechanically tested specimens are then analysed, after failure, by FTIR microspectrometry. The spectra obtained, corresponding to the fracture initiation zone which is about 100 micrometers in diameter, indicate the presence of an ultrathin layer of modified polymer still covering the substrate. 28 refs. 

Vibrational microspectrometry will undoubtedly be applied to medical diagnosis in the near future. One particularly important application of microspectrometry is for the characterization of tissue samples. Tissue samples can be mounted on a water-insoluble infrared-transparent window such as ZnSe, but these windows are expensive and not conducive to visual examination (e.g., after staining of the tissue). A convenient alternative to transmission spectrometry is the measurement of the transflectance spectrum (see Section 13.5) of tissue samples mounted on low-emissivity glass slides [4]. These slides are transparent to visible light but highly reflective to mid-infrared radiation. 

Attenuated total reflection (ATR) spectrometry is becoming an increasingly important sampling technique for infrared microspectrometry using both single-element and array detectors. This topic is covered in Chapter 15. 

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