Solid imaging techniques

An alternative approach to solid imaging is the CW-NMRI technique, which uses continuous-RF irradiation and detection in the presence of continuously applied, moderately strong gradients. This technique offers a number of advantages compared to other solid imaging techniques ... 

The RF power requirement in CW-NMRI is a factor of 103 to 106 lower compared to the other solid imaging techniques, thus making it practicable to envisage examination of full-size structural components which might prove problematic for conventional pulsed techniques due to excessive RF power requirements. 

In the case of Langmuir monolayers, film thickness and index of refraction have not been given much attention. While several groups have measured A versus a, [143-145], calculations by Knoll and co-workers [146] call into question the ability of ellipsometry to unambiguously determine thickness and refractive index of a Langmuir monolayer. A small error in the chosen index of refraction produces a large error in thickness. A new microscopic imaging technique described in section IV-3E uses ellipsometric contrast but does not require absolute determination of thickness and refractive index. Ellipsometry is routinely used to successfully characterize thin films on solid supports as described in Sections X-7, XI-2, and XV-7. 

The Sccmning Electron Microscope (SEM) is a standard imaging technique based on electron back-scattering from the sample surface. It analyses the surfaces of solid objects, producing images with the resolution which is about order of magnitude better than that of optical microscopy (typically 10 nm). The SEM avoids the problem of thin samples (TEM) but the SEM observation requires the deposition of a thin conductive metal film on the sample surface to prevent sample charging. 

Chemical compound homogeneity is an important issue for pharmaceutical sohd forms. A classical spectrometer integrates the spatial information. In solid form analysis, use of a mean spectrum on a surface can be a drawback. For example, in the pharmaceutical industry it is important to map the distribution of active ingredients and excipients in a tablet so as to reveal physical interaction between the compounds and help to solve homogeneity issues. Spectroscopic imaging techniques that visualize chemical component distribution are thus of great interest to the pharmaceutical community. 

Thus Raman imaging is a useful tool for detecting small API particles on the surface of pharmaceutical solid forms. It may even be the most suitable chemical imaging technique for API mapping due to its low spatial resolution (up to 0.5 pm/ pixel) and the polymorphism of the spectral information. 

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

In the present volume. Gladden, Mantle, and Sederman summarize the application of magnetic resonance imaging techniques to represent both local flow fields in reactors containing solid catalyst particles and conversions within model reactors. The techniques provide a non-invasive, chemically specific measurement technique that leads to representation of a reactor over length scales ranging from Angstroms to centimeters. 

Note finally that, due to its high specificity and sensitivity to the local degree of constraints in a rubbery material, 2H NMR has been used to obtain a spatial map of the constraints in a material by imaging techniques [106,107]. These experiments are described in another chapter of this book. 2H NMR has been used as well to investigate other related polymer systems, such as gels [108], thermosets [109], fibres [110, 111, 112, 113] and various solid (glassy and/or semi crystalline) polymers [114]. 

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