Dispersive Raman Applications

Various Dispersive Raman applications for surface analysis Table 13.6... 

The list below defines most of the detector properties important to Raman applications, particularly for dispersive spectrometers. Specifications of particular devices will be discussed in subsequent sections. More specific definitions are provided for CCD detectors in Section 8.5.2. 

Various Dispersive Raman spectroscopy applications in microscopy and imaging Table 11.3... 

Various Applications of dispersive Raman coupled with fiber-optic sampling Table 12.4... 

Raman microscopy is a valuable analytical tool for the characterization of solid-state forms where applications may be extended by combination with a hot stage. This has been demonstrated by Szelagiewicz etal. [50] using a dispersive Raman microscope as well as by Griesser etal. [107] applying FT-NIR microscopy (see Fig. 7.12). Note that Raman spectra also record the far-IR region below 200cm where the phonon vibrations of the crystal lattice can be observed. 

Unlike IR spectroscopy where nowadays FT instrumentation is solely used, in Raman spectroscopy both conventional dispersive and FT techniques have their applications, the choice being governed by several factors. The two techniques differ significantly in several performance criteria, and neither one is best for all applications. Contemporary dispersive Raman spectrometers are often equipped with silicon-based charge coupled device (CCD) multichannel detector systems, and laser sources with operating wavelength in the ultraviolet, visible or near-infrared region are employed. In FT Raman spectroscopy, the excitation is provided exclusively by near-infrared lasers (1064 nm or 780 nm). 

It is difficult to say with certainty how Raman instrumentation will evolve over the next 10 years. However, there are several instrumentation developments that are beginning to appear as commercial products. These include more systems engineered for dedicated process and/or QA/QC applications, NIR multichannel detectors for use on a dispersive Raman instrument with long-wavelength lasers, UV microprobes, and near-field Raman microprobes that can complement atomic force microscopes (AFMs) or scanning tunneling microscopes (STMs), or their variants. It is an exciting time to be a Raman researcher. 

Since 1970, researchers have recognized that RS offers great promise for environmental analysis because measurements can be made directly on contaminants in water [9]. Unfortunately, developments in this area were slow until recently because of the historical problems associated with RS, namely high detection levels, severe problems with fluorescence backgrounds, and the need for large and complex instrumentation. However, as we have described, the sensitivity of NRS has increased considerably over the decade prior to this writing. This improvement is due to advances in instrumental components that make up the modem dispersive Raman instmment. Also, advances in instmmental components have made these spectrometers more compact and portable, able to record spectra much quicker, less hindered by fluorescence, much simpler and less dependent on specialized facilities, more amenable to remote application, and much easier to operate. All of these improvements have led recently to more widespread application to environmental problems in aqueous systems, as illustrated in Table 1. However, further improvement will be needed for the full potential of RS to be realized in this arena. 

The same criteria that are applicable in the laboratory can be used for the on-line analysis. In the model-building step, it is important to tolerance the model however, the level of calibration for accuracy and precision and the frequency of recalibration all need to be addressed on an application-by-application basis. As in all modeling situations, it is important that the model be adequate for the questions being raised (see Chapter 7 and the earlier Section II.B for further information). For all process applications, it is desirable to minimize calibration frequency because time calibrating the analyzer is, for all intents and purposes, time lost. Calibration of a dispersive Raman spectrometer is discussed in detail in Chapters 3 and 6 and thus it will only be briefly mention here. The major concern in the trial phase revolves around environmental issues such as temperature and vibration. If the analyzer is being installed in a control room, then a process-hardened analyzer is not necessarily needed. If, however, the analyzer is being installed on or near the plant floor, then it is essential to use a hardened analyzer. If not, then a significant amount of time can be taken up only to discover that laboratory analyzers are not as environmentally... 

This application was developed at Monsanto in order to characterize the manufacture of phosphorus trichloride (PCI3) [18-20]. It is probably one of the most extensively documented Raman applications and is included here as a model for future applications. In addition, it is of historical interest, as it was one of the few application where a FT-Raman spectrometer was used for on-line analysis. Most recently, dispersive Raman spectroscopy based on charge-coupled device (CCD) spectrometers have displaced the original FT-Raman analyzer [21] for several reasons, as follows. 

Currently, both FT-Raman and dispersive Raman spectrometers are being used within the pharmaceutical industry. Dispersive Raman spectroscopy in the form of Raman microprobes are heavily employed in the research area to map active-excipient distribution using the diffraction limited spatial resolution attainable with the microprobe. In this subsection, it is inappropriate to describe the varied applications of Raman microscopy to the study of pharmaceuticals thus, the reader is referred to the literature [108,109] and Chapter 14. Dispersive Raman analyzers are also being used for reaction analysis, pilot-plant batch analysis, and process monitoring. FT-Raman spectrometers have been adopted for formulated product analysis and for incoming goods testing. 

For trace analysis in fluids, some Raman sensors (try to) make use of the SERS effect to increase their sensitivity. While the basic sensor layout for SERS sensors is similar to non-enhanced Raman sensors, somehow the metal particles have to be added. Other than in the laboratory, where the necessary metal particles can be added as colloidal solution to the sample, for sensor applications the particles must be suitably immobilised. In most cases, this is achieved by depositing the metal particles onto the surfaces of the excitation waveguide or the interface window and covering them with a suitable protection layer. The additional layer is required as otherwise washout effects or chemical reactions between e.g. sulphur-compounds and the particles reduce the enhancement effect. Alternatively, it is also possible to disperse the metal particles in the layer material before coating and apply them in one step with the coating. Suitable protection or matrix materials for SERS substrates could be e.g. sol-gel layers or polymer coatings. In either... 

In one application, Raman spectroscopy was used to identify and quantitate various drugs present in polymer matrices [21]. In Fig. 2, Raman spectra obtained within the fingerprint region for diclofenac, sodium alginate, and a 20% dispersion of diclofenac in sodium alginate are shown. It is evident in the spectra... 

The technique of stimulated Raman scattering (SRS) has been demonstrated as a practical method for the simultaneous measurement of diameter, number density and constituent material of micrometer-sized droplets. 709 The SRS method is applicable to all Raman active materials and to droplets larger than 8 pm in diameter. Experimental studies were conducted for water and ethanol mono-disperse droplets in the diameter range of 40-90 pm. Results with a single laser pulse and multiple pulses showed that the SRS method can be used to diagnose droplets of mixed liquids and ensembles of polydisperse droplets. 

Professors Alex Bell and Enrique Iglesia of the University of California, Berkeley have used UV-Vis DRIFTS and Raman spectroscopy to elucidate the role of many catalytic systems ranging from mixed metal oxides to precious metal formulations for applications ranging from dehydrogenation of hydrocarbons to oxidation of alkanes to the role of exposed species on dispersed surfaces. ... 

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