Raman techniques instrumentation

I. R. Lewis and H. G. M. Edwards eds, Handbook of Raman Spectroscopy , Marcell Dekker Inc, New York, 2001. This comprehensive handbook lays out the richness and technical details of Raman techniques, instrumentation and measurements now employed in a variety of industrial and academic research fields. 

Most chemists tend to think of infrared (IR) spectroscopy as the only form of vibrational analysis for a molecular entity. In this framework, IR is typically used as an identification assay for various intermediates and final bulk drug products, and also as a quantitative technique for solution-phase studies. Full vibrational analysis of a molecule must also include Raman spectroscopy. Although IR and Raman spectroscopy are complementary techniques, widespread use of the Raman technique in pharmaceutical investigations has been limited. Before the advent of Fourier transform techniques and lasers, experimental difficulties limited the use of Raman spectroscopy. Over the last 20 years a renaissance of the Raman technique has been seen, however, due mainly to instrumentation development. 

Dramatic improvements in instrumentation (lasers, detectors, optics, computers, and so on) have during recent years raised the Raman spectroscopy technique to a level where it can be used for species specific quantitative chemical analysis. Although not as sensitive as, for example IR absorption, the Raman technique has the advantage that it can directly measure samples inside ampoules and other kinds of closed vials because of the transparency of many window materials. Furthermore, with the use of polarization techniques, one can derive molecular information that cannot be obtained from IR spectra. Good starting references dealing with Raman spectroscopy instruments and lasers are perhaps [34-38]. 

Raman microscopy was developed in the 1970s. Delhaye (34) in 1975 made the first micro Raman measurement. Simultaneously, Rosasco (35, 36) designed a Raman microprobe instrument at the National Bureau of Standards (now the NIST). This early work established the utility of Raman spectroscopy for microanalysis. The technique provides the capability of obtaining analytical-quality Raman spectra with 1 pm spatial resolution using samples in the picogram range. Commercial instruments are available. 

Such single-mode lasers, often pulse amplified by dye laser amplifiers pumped by injection-locked Nd YAG lasers, are used in nonlinear Raman techniques by which an instrumental resolution better than 0.001 cm is achieved (Esherick and Owyoung (1982), Schrotter et al. (1988a)). 

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. 

Near-infrared surface-enhanced Raman spectroscopy Some of the major irritants in Raman measurements are sample fluorescence and photochemistry. However, with the help of Fourier transform (FT) Raman instruments, near-infrared (near-IR) Raman spectroscopy has become an excellent technique for eliminating sample fluorescence and photochemistry in Raman measurements. As demonstrated recently, the range of near-IR Raman techniques can be extended to include near-IR SERS. Near-IR SERS reduces the magnitude of the fluorescence problem because near-IR excitation eliminates most sources of luminescence. Potential applications of near-IR SERS are in environmental monitoring and ultrasensitive detection of highly luminescent molecules [11]. 

SERS is not the only method to apply Raman techniques to surfaces. There are several other ways of increasing the sensitivity of the Raman technique, and all work together to offer a versatile and powerful instrument. Nowadays, Raman measurements can be performed with the high sensitivity of OMA techniques, which pose no special restrictions on the systems investigated (no specific metal or surface preparation). ... 

The authors record their appreciation of funding over many years for the basic analytical Raman spectroscopic studies of terrestrial extremophiles and Mars models from the European Space Agency and UK organisations (EPSRC, NERC and PPARC/STFC) which have enabled the preliminary work to be carried out to establish the viability of the Raman technique for incorporation into extraterrestrial life-detection instrumentation and for the opportunity to take novel analytical chemical science to the last frontier . Professor Edwards is especially grateful for... 

The Raman technique has been readily adapted for on-line process analysis, especially in the pharmaceutical industry ". It has the benefits of mid IR, e.g. the ability to identify compounds from the vibrational fundamentals, without the constraints of mid IR, e.g. the limitations of the optical materials that can be used. Its popularity is also due in part to the excellent throughput of optical fibres for the radiation required for Raman, i.e. in the Vis and NIR regions. This use of optical fibre probes (Figure 9.14) facilitates easy in-line analysis because the sample can be remote from the instrumentation, even to hundreds of metres in distance. Fibre optic multiplexers are also available, allowing many samples to be analysed sequentially. Small laser diode sources and CCD detectors can be attached to the optical fibres and changed as required, rendering the overall device small and flexible. Radiation from the laser diode light source is transmitted to the sample by optical fibre... 

The extension of the spectral range to the vacuum uv region (below 180 nm) would make possible the study of additional n-S and ti-ti transitions of amino acids and peptides not yet accessible with existing commercial instruments. On the longer-wavelength side, it has become technically possible to measure vibrational optical activity via ir and Raman techniques, and one may hope that commercial equipment for such measurements will eventually become available. Present ORD/CD techniques that are used to measure electronic optical activity require the presence of a chromophoric group in the molecule. In contrast, any vibrationally excitable bond in an assymmetric molecule will give rise to vibrational optical activity. 

Wide spectral range. In the far- and middle-infrared ranges spectra are measured using different optical elements while the Raman technique covers all this range of vibrational frequencies using a single instrument. 

This chapter has been organized by considering several aspects. An introduction concerning the relevance of the electronic properties and applications of the azamacrocycles related to surface phenomena as well as the general aspects and characteristics of the vibrational techniques, instruments and surfaces normally used in the study of the adsorbate-surface interaction. The vibrational enhanced Raman and infrared surface spectroscopies, along with the reflection-absorption infrared spectroscopy to the study of the interaction of several azamacrocycles with different metal surfaces are discussed. The analysis of the most recent publications concerning data on bands assignment, normal coordinate analysis, surface-enhanced Raman and infrared spectroscopies, reflection-absorption infrared spectra and theoretical calculations on models of the adsorbate-substrate interaction is performed. Finally, new trends about modified metal surfaces for surface-enhanced vibrational studies of new macrocycles and different molecular systems are commented. 

Classical, spontaneous Raman scattering is a powerful analytical tool that allows for the investigation of the qualitative and quantitative composition of biological, pharmaceutical, and environmental samples. The following discussion of NIR-Raman spectroscopy will begin with a general review of Raman spectroscopy, followed by a description of NIR-Raman, with further discussion about instrumentation and applications of the NIR-Raman technique. 

See also Blood and Plasma. Clinical Analysis Glucose. DNA Sequencing. Fluorescence Overview. Forensic Sciences Drug Screening in Sport. Microscopy Techniques Electron Microscopy Scanning Electron Microscopy Atomic Force and Scanning Tunneling Microscopy. Nucleic Acids Spectroscopic Methods. Raman Spectroscopy Instrumentation. Sensors Overview. 

Infrared (IR) and Raman are both well established as methods of vibrational spectroscopy. Both have been used for decades as tools for the identification and characterization of polymeric materials in fact, the requirement for a method of analysis synthetic polymers was the basis for the original development of analytical infrared instrumentation during World War II. It is assumed that the reader has a general understanding of analytical chemistry, and a basic understanding of the principles of spectroscopy. A general overview of vibrational spectroscopy is provided in Sec. 5 for those unfamiliar with the infrared and Raman techniques. 

Prior to the much-vaunted renaissance of the Raman technique with the advent of FT instrumentation or the availability of CCD systems, there were few literature reports on the use of Raman spectroscopy for investigating pharmaceutical systems. The technique has been used to characterize drugs in much the same way that infrared has been used for identification testing. Thus, the infrared (IR) and Raman spectra of Dapsone, used in the treatment of leprosy, have been reported [1]. 

Figure 1 Number of journal publications devoted to environmental applications of various Raman techniques as a function of years. The resurgence of normal Raman spectroscopy in the late 1990s due to the improvements in Raman instrumentation is obvious. Also, a recent increase in SERS applications can clearly be seen. Moreover, the maturity of Raman spectroscopy as an important analytical tool in the analysis of complex environmental mixtures is seen in the steady growth of hyphenated techniques involving Raman spectroscopy. Note that publications reporting the use of hyphenated techniques were included in the tally for both hyphenated techniques as well as the specific Raman method employed (i.e., NRS, RRS, and SERS). In addition, publications reporting the use of the SERRS technique were included in the tally for both RRS and SERS. Hence, the Total category is not always equal to the sum of the NRS, RRS, and SERS applications.

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