Raman spectroscopy sampling techniques

Marked differences are seen between IR and Raman spectroscopy in sampling techniques. In IR spectroscopy, sampling techniques for routine measurements are relatively simple. In contrast, Raman sampling techniques are intricate and versatile, and individual workers employ a variety of sampling techniques developed for their needs. Some of these techniques are described below. 

Heretofore, Raman spectroscopy has not played a role in forensic science because of the fluorescent problems and the sample alignment, which is time-consuming. As a consequence, the technique was never seriously considered as a routine tool to study forensic materials. However, with the development of FT-Raman spectroscopy, the technique is now being reexamined. One such application in forensic science follows (15). 

Joint interpretation of the IR and Raman spectra of biomolecules, which frequently lack symmetry properties, may afford more extensive information concerning the primary, secondary and tertiary structure, than does the interpretation of one type of spectrum only. Many systems can only be investigated in aqueous solution, a good solvent for Raman spectroscopy. The technique of resonance Raman spectroscopy facilitates investigations of pigments and the NIR FT Raman spectroscopy allows the investigation of nearly all samples which has not been possible before due to absorption and fluorescence. Spectra of biomolecules are exhaustively discussed in Sec. 4.7. Here only some general features are discussed. 

Cobalt complexes are the specific area of interest for a paper dealing with the technology of spinning-cell Fourier transform Raman spectroscopy. The technique uses near-IR light and, because of the rapidly spinning cell, avoids the problems of sample burning and should be most useful for the study of delicate carbonyl species. Clusters are studied somewhat differently, following UV laser photolysis, in a paper published by Belyaev et. ... 

The future of Raman microspectroscopy is probably imaging and optical near-field nano-Raman spectroscopy [529], cfr. Chp. 5.5.2. While conventional laser Raman spectroscopy samples 10 g (mm ), /zRS handles 10 g (nm ) and near-field Raman spectroscopy 10 g (nm ). Mobile Raman microscopy (MRM) allows in situ Raman analysis [530]. One can expect further developments in the field of NIR multichannel Raman spectroscopy with the advent of 2D array detectors offering extended response in the NIR. With these 2D sensors it wiU become possible to apply in the NIR region the powerful techniques already developed in the visible, such as confocal line imaging techniques or multisite remote analysis with optical fibres. 

Ideally, a mass spectmm contains a molecular ion, corresponding to the molecular mass of the analyte, as well as stmcturaHy significant fragment ions which allow either the direct deterrnination of stmcture or a comparison to Hbraries of spectra of known compounds. Mass spectrometry (ms) is unique in its abiUty to determine direcdy the molecular mass of a sample. Other techniques such as nuclear magnetic resonance (nmr) and infrared spectroscopy give stmctural information from which the molecular mass may be inferred (see Infrared technology and raman spectroscopy Magnetic spin resonance). 

Intensity enhancement takes place on rough silver surfaces. Under such conditions, Raman scattering can be measured from monolayers of molecular substances adsorbed on the silver (pyridine was the original test case), a technique known as surface-enhanced Raman spectroscopy. More recendy it has been found that sur-fiice enhancement also occurs when a thin layer of silver is sputtered onto a solid sample and the Raman scattering is observed through the silver. 

Because Raman spectroscopy requires one only to guide a laser beam to the sample and extract a scattered beam, the technique is easily adaptable to measurements as a function of temperature and pressure. High temperatures can be achieved by using a small furnace built into the sample compartment. Low temperatures, easily to 78 K (liquid nitrogen) and with some diflSculty to 4.2 K (liquid helium), can be achieved with various commercially available cryostats. Chambers suitable for Raman spectroscopy to pressures of a few hundred MPa can be constructed using sapphire windows for the laser and scattered beams. However, Raman spectroscopy is the characterizadon tool of choice in diamond-anvil high-pressure cells, which produce pressures well in excess of 100 GPa. ... 

Raman spectroscopy is a very convenient technique for the identification of crystalline or molecular phases, for obtaining structural information on noncrystalline solids, for identifying molecular species in aqueous solutions, and for characterizing solid—liquid interfaces. Backscattering geometries, especially with microfocus instruments, allow films, coatings, and surfaces to be easily measured. Ambient atmospheres can be used and no special sample preparation is needed. 

Since the vibrational spectra of sulfur allotropes are characteristic for their molecular and crystalline structure, vibrational spectroscopy has become a valuable tool in structural studies besides X-ray diffraction techniques. In particular, Raman spectroscopy on sulfur samples at high pressures is much easier to perform than IR spectroscopical studies due to technical demands (e.g., throughput of the IR beam, spectral range in the far-infrared). On the other hand, application of laser radiation for exciting the Raman spectrum may cause photo-induced structural changes. High-pressure phase transitions and structures of elemental sulfur at high pressures were already discussed in [1]. 

Vibrational spectroscopy and in particular Raman spectroscopy is by far the most useful spectroscopic technique to qualitatively characterize polysulfide samples. The fundamental vibrations of the polysulfide dianions with between 4 and 8 atoms have been calculated by Steudel and Schuster [96] using force constants derived partly from the vibrational spectra of NayS4 and (NH4)2Ss and partly from cydo-Sg. It turned out that not only species of differing molecular size but also rotational isomers like Ss of either Cy or Cs symmetry can be recognized from pronounced differences in their spectra. The latter two anions are present, for instance, in NaySg (Cs) and KySg (Cy), respectively (see Table 2). 

The identification of xanthophylls in vivo is a complex task and should be approached gradually with the increasing complexity of the sample. In the case of the antenna xanthophylls, the simplest sample is the isolated LHCII complex. Even here four xanthophylls are present, each having at least three major absorption transitions, 0-0, 0-1, and 0-2 (Figure 7.4). Heterogeneity in the xanthophyll environment and overlap with the chlorophyll absorption add additional complexity to the identification task. No single spectroscopic method seems suitable to resolve the overlapping spectra. However, the combination of two spectroscopic techniques, low-temperature absorption and resonance Raman spectroscopy, has proved to be fruitful (Ruban et al., 2001 Robert et al., 2004). 

Whereas several techniques may thus be used to study a certain characteristic of a polymer sample, for instance IR and Raman spectroscopy and X-ray diffraction as well as NMR may be used to determine or infer the crystallinity level of a sample, different techniques work differently and therefore usually do not measure the same. What this means is that crystallinity levels obtained from the same sample may differ when a different technique is applied, see, for example, ref. [23] and chapter 7 and references therein. However, these differences do not necessarily imply one technique being better than another. In fact these differences may contain useful information on the sample (see, for example, ref. [25]). 

When investigating opaque or transparent samples, where the laser light can penetrate the surface and be scattered into deeper regions, Raman light from these deeper zones also contributes to the collected signal and is of particular relevance with non-homogeneous samples, e.g., multilayer systems or blends. The above equation is only valid, if the beam is focused on the sample surface. Different considerations apply to confocal Raman spectroscopy, which is a very useful technique to probe (depth profile) samples below their surface. This nondestructive method is appropriate for studies on thin layers, inclusions and impurities buried within a matrix, and will be discussed below. 

Another technique of vibrational spectroscopy suited for the characterization of solids is that of Raman spectroscopy. In this methodology, the sample is irradiated with monochromatic laser radiation, and the inelastic scattering of the source energy is used to obtain a vibrational spectrum of the analyte [20]. Since... 

Sampling techniques for Raman spectroscopy are relatively general since the only requirement is that the monochromatic laser beam irradiate the sample of interest and the scattered radiation be focused upon the detector. 

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