Raman techniques introduction

Raman spectroscopy failed to live up to its original expectation when the technique was discovered. This was due to instrumental problems, high cost of the instrument, and the fluorescence problem. However, with improvement in instrumentation, the use of a near infrared laser with FT-Raman, the introduction of fiber optics, the number of applications (some of which were discussed in Chapter 3) has escalated. The applications are expanded in this chapter, which deals with materials applications involving structural chemistry, solid state, and surfaces. Additional applications are presented in Chapter 5 (analytical applications), Chapter 6 (biochemical and medical applications) and Chapter 7 (industrial applications). 

There is a real chance of a breakthrough of Raman spectroscopy in routine analytics. Excitation of Raman spectra by near-infrared radiation and recording with interferometers, followed by the Fourier transformation of the interferogram into a spectrum -the so-called NIR-FT-Raman technique - has made it possible to obtain Raman spectra of most samples uninhibited by fluorescence. In addition, the introduction of dispersive spectrometers with multi-channel detectors and the development of several varieties of Raman spectroscopy has made it possible to combine infrared and Raman spectroscopy whenever this appears to be advantageous. 

With the introduction of Fourier-Transform (FT) Raman instruments (19,20), near-infirared (NIR) Raman spectroscopy has become an excellent technique for eliminating sample fluorescence and photochemistry in Raman measurements. Recently, Ae range of NIR Raman techniques was extended to include NIR SERS (6,21). Most SERS studies to date have been performed using visible excitation sources such as Ar-ion lasers the demonstration of NIR SERS offers the possibility of using solid-state Nd YAG and diode lasers. 

The objective of this chapter is to provide an introduction to Raman spectroscopic microscopy and its potential for biochemical analysis and clinical diagnostic applications, such that it can be compared and contrasted to the techniques of synchrotron and bench-top mid-FTIR spectroscopy discussed elsewhere in this book. Raman spectroscopy is a complementary technique to mid-IR absorption spectroscopy with established capabilities for materials and process analysis. As a bioanalytical and diagnostic technique, similar to FTIR spectroscopy, its potential has been demonstrated although there are many differing technical considerations to be addressed. Raman has potentially significant advantages as well as drawbacks compared to FTIR techniques. Here we endeavour to outline these benefits and pitfalls and project the complementary and competitive usage of Raman techniques. 

In 1994, we proposed that a metallic needle having a nano-tip at its apex be employed as a nano-light-source for microscopy attaining nanometric spatial resolution [2]. Later, we expanded the technique to Raman spectroscopy for molecular nano-identification, nano-analysis and nano-imaging. In this chapter, we give a brief introduction to local plasmons and microscopy using a metallic nano-needle to produce the local plasmons. Then, we describe the microscope that we built and... 

A nano-light-source generated on the metallic nano-tip induces a variety of optical phenomena in a nano-volume. Hence, nano-analysis, nano-identification and nanoimaging are achieved by combining the near-field technique with many kinds of spectroscopy. The use of a metallic nano-tip applied to nanoscale spectroscopy, for example, Raman spectroscopy [9], two-photon fluorescence spectroscopy [13] and infrared absorption spectroscopy [14], was reported in 1999. We have incorporated Raman spectroscopy with tip-enhanced near-field microscopy for the direct observation of molecules. In this section, we will give a brief introduction to Raman spectroscopy and demonstrate our experimental nano-Raman spectroscopy and imaging results. Furthermore, we will describe the improvement of spatial resolution... 

Various techniques have been introduced which still lack specific applications in polymer/additive analysis, but which may reasonably be expected to lead to significant contributions in the future. Examples are LC-QToFMS, LC-multi-API-MS, GC-ToFMS, Raman spectroscopy (to a minor extent), etc. Expectations for DIP-ToFMS [132], PTV-GC-ToFMS [133] and ASE are high. The advantages of SFC [134,135], on-line multidimensional chromatographic techniques [136,137] and laser-based methods for polymer/additive analysis appear to be more distant. Table 10.33 lists some innovative polymer/additive analysis protocols. As in all endeavours, the introduction of new technology needs a champion. 

A promising recent development in the study of nitrenium ions has been the introduction of time-resolved vibrational spectroscopy for their characterization. These methods are based on pulsed laser photolysis. However, they employ either time resolved IR (TRIR) or time-resolved resonance Raman (TRRR) spectroscopy as the mode of detection. While these detection techniques are inherently less sensitive than UV-vis absorption, they provide more detailed and readily interpretable spectral information. In fact, it is possible to directly calculate these spectra using relatively fast and inexpensive DFT and MP2 methods. Thus, spectra derived from experiment can be used to validate (or falsify) various computational treatments of nitrenium ion stmctures and reactivity. In contrast, UV-vis spectra do not lend themselves to detailed structural analysis and, moreover, calculating these spectra from first principles is still expensive and highly approximate. 

Similarly to FTIR spectroscopy, Raman spectroscopy is a versatile technique of analyzing both organic and inorganic materials that has experienced noticeable growth in the field of art and art conservation, in parallel to the improvement of the instrumentation [38]. In particular, the introduction of fiber optic devices has made feasible the development of mobile Raman equipments, enabling nondestructive in situ analyses [39]. On the other hand, the coupling of Raman spectroscopes with optical microscopes has given rise to Raman microscopy vide infra). 

Lasers come next, not because of their intrinsic construction and mode of operation, but because they open up new dimensions of technique, precision, and scale. The experimental technique of physical chemistry that has benefited most from the laser is Raman spectroscopy, which barely existed before their introduction and is now in full flower, showing enormously detailed and interesting information about bulk matter and surfaces. A technique that was essentially invented by the laser is femtochemistry, where we can catch atoms red-handed in the act of reaction. Lasers have brought us right to the heart of reactions, and as such we must build them into our courses. 

Progress in the Raman spectroscopic study of carbohydrates became possible during the past few years owing to the introduction of laser sources. Before discussing the results of laser-Raman spectroscopy applied to carbohydrates, we shall give a brief recapitulation of the physical principles of the Raman effect. Experimental techniques of infrared spectroscopy have been described in previous reviews,116,17 but no such description has been given for the Raman method. That is why the Description Section, which follows, will include the physical fundamentals of the method, as well as the sampling techniques. 

Abstract The introduction of Raman spectroscopy into new fields has been driven largely by advances in the underlying technology. While the spectrometer is still comprised of a light source, a wavelength selector, and a detector, the improvement in functionality of each of these components has had dramatic impacts on areas where Raman was once thought impractical, if not impossible. In addition, esoteric techniques once confined to academic spectroscopy labs are now finding wide application. 

Spectra differ markedly in appearance depending on the particular technique. UV-vis and near-infrared (NIR) spectra typically consist of a few broad bands that make unambiguous identification difficult. By contrast, IR and Raman spectra contain a number of sharp bands at well-defined positions that facilitate the identification of specific compounds. Recent improvements in sensitivity and reproducibility of spectroscopic equipment have substantially expanded the scope of these techniques. The introduction of... 

Identifying pharmaceuticals, whether APIs or excipients used to manufacture products, and the end products themselves is among the routine tests needed to control pharmaceutical manufacturing processes. Pharmacopoeias have compiled a wide range of analytical methods for the identification of pharmaceutical APIs and usually several tests for a product are recommended. The process can be labor-intensive and time-consuming with these conventional methods. This has raised the need for alternative, faster methods also ensuring reliable identification. Of the four spectroscopic techniques reviewed in this book, IR and Raman spectroscopy are suitable for the unequivocal identification of pharmaceuticals as their spectra are compound-specific no two compounds other than pairs of enantiomers or oligomers possess the same IR spectrum. However, IR spectrometry is confronted with some practical constraints such as the need to pretreat the sample. The introduction of substantial instrumental improvements and the spread of attenuated total reflectance (ATR) and IR microscopy techniques have considerably expanded the scope of IR spectroscopy in the pharmaceutical field. Raman spectroscopy,... 

Vibrational spectroscopy is an important tool for the characterization of various chemical species. Valuable information regarding molecular structures as well as intra- and intermolecular forces can be extracted from vibrational spectral data. Recent advances, such as the introduction of laser sources to Raman spectroscopy, the commercial availability of Fourier transform infrared spectrometers, and the continuing development and application of the matrix-isolation technique to a variety of chemical systems, have greatly enhanced the utility of vibrational spectroscopy to chemists. 

The area in which matrix isolation is perhaps of greatest value is the stabilization of transient species such as free radicals and high-temperature vapors. Until quite recently, infrared spectroscopy was utilized almost exclusively for the vibrational studies of matrix-isolated species. With the introduction of laser sources and the development of more sensitive, electronic, light detection systems, Raman matrix-isolation studies are now feasible and have recently been applied to a limited number of unstable inorganic fluoride species including the molecules OF (5) and C1F2 (6). Both of these species were formed for Raman study by a novel technique that utilizes the... 

In the weakly anharmonic molecular crystal the natural modes of vibration are collective, with each internal vibrational state of the molecules forming a band of elementary excitations called vibrons, in order to distinguish them from low-frequency lattice vibrations known as phonons. Unlike isolated impurities in matrices, vibrons may be studied by Raman spectroscopy, which has lead to the establishment of a large body of data. We will briefly attempt to summarize some of the salient experimental and theoretical results as an introduction to some new developments in this field, which have mainly been incited by picosecond coherent techniques. 

The audience for this book should include graduate students, practicing chemists, and Raman spectroscopists who seek information on recent instrumentation developments. It is not a comprehensive review but more of a textbook intended as an introduction to modern Raman spectroscopy. In most cases, the techniques discussed are available in commercially available spectrometers, and the book should be useful to chemists who are implementing Raman spectroscopy in industrial or academic laboratories. Although a large number of useful Raman applications involve custom-built instrumentation, the book emphasizes configurations and components used by current vendors of integrated Raman spectrometers. 

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