Raman techniques imaging

Fig. 9.5. Raman chemical images of tablet formulations with different API form compositions to demonstrate technique limit of detection of 0.01% w/w for form D...

Raman microspectroscopy results from coupling of an optical microscope to a Raman spectrometer. The high spatial resolution of the confocal Raman microspectrometry allows the characterization of the structure of food sample at a micrometer scale. The principle of this imaging technique is based on specific vibration bands as markers of Raman technique, which permit the reconstruction of spectral images by surface scanning on an area. 

Thus far, we have reviewed basic theories and experimental techniques of Raman spectroscopy. In this chapter we shall discuss the principles, experimental design and typical applications of Raman spectroscopy that require special treatments. These include high pressure Raman spectroscopy, Raman microscopy, surface-enhanced Raman spectroscopy, Raman spectroelectro-chemistry, time-resolved Raman spectroscopy, matrix-isolation Raman spectroscopy, two-dimensional correlation Raman spectroscopy, Raman imaging spectrometry and non-linear Raman spectroscopy. The applications of Raman spectroscopy discussed in this chapter are brief in nature and are shown to illustrate the various techniques. Later chapters are devoted to a more extensive discussion of Raman applications to indicate the breadth and usefulness of the Raman technique. 

The Raman techniques combined with AEM microscopic imaging, as for instance TERS (tip-enhanced Raman scattering) spectroscopy [27], allow to analyze surface nanostructures beyond the diffraction limit, but the cost of the instrumental apparatus is not affordable for any research laboratory. Therefore, in this chapter, the results obtained with those techniques will not be presented, though they increased Raman enhancement factors by up to lO, with the possibility of single-molecule detection. Conversely, confocal micro-Raman apparatus is affordable to every research group allowing SERS investigations with more comparable results. 

MA Houghtaling, DE Bugay. Raman spectroscopic imaging A potential new technique for content uniformity testing. 14th Annual American Association of Pharmaceutical Scientists Annual Meeting and Exposition, New Orleans, LA, Nov 14-18, 1999. 

FT-IR and Raman spectroscopic imaging techniques may employ three general approaches to obtain spatially resolved chemical information mapping, imaging with a multi-element detector, and spatial encoding and decoding. 

Raman chemical imaging is performed from 1260 to 1400 cm in order to differentiate CPO, EPR, and PP [52]. Even though the CPO Raman band at 1300 cm significantly overlaps the EPR band, it is sufficiently broad to be detected using multivariate image analysis techniques, including CCA. 

Infrared spectroscopy performed both in the mid-IR [70] and near-IR [71] provides the potential of rapid determination with little or no sample preparation. Raman spectroscopy also has demonstrated capability for pharmaceutical analysis [72], Vibrational spectroscopic techniques are effective for compositional and structural characterization, as well as quantitation. However, bulk spectrosocpy is ineffective for measuring the spatial distribution and architecture of actives, which are heterogeneously distributed within intact tablets. Raman chemical imaging equipped with multivariate image processing capabilities is a powerful approach to the analysis of pharmaceutical tablet architecture. 

The subject of IR and Raman-spectroscopic imaging has been covered comprehensively in a recent book [86]. In contrast to mapping, modern imaging techniques are based on the use of focal plane array detectors which allow the rapid characterization of multicomponent polymer samples such as blends of PMMA or poly(ethylacrylate) with polystyrene, or of poly(3-hydroxybutyrate) with poly(lactic acid). 

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