Applications of the Raman Microprobe

We will confine ourselves to those applications concerned with chemical analysis, although the Raman microprobe also enables the stress and strain imposed in a sample to be examined. Externally applied stress-induced changes in intramolecular distances of the lattice structures are reflected in changes in the Raman spectrum, so that the technique may be used, for example, to study the local stresses and strains in polymer fibre and ceramic fibre composite materials. 

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Only a few applications of the Raman microprobe will be presented. The interested reader can refer to the references cited at the end of this section for further applications. 

Application of the Raman Microprobe to Analytical Problems of Microelectronics... 

DC Smith, S Robin, Early-Roman Empire intaglios from rescue excavations in Paris An application of the Raman microprobe to the non-destructive characterisation of archaeological objects. J Raman Spectrosc 28(2 3) 189-193, 1997. 

DC Smith, F Gendron, F Gautier. Applications of the Raman microprobe (RM) to the non-destructive characterisation of archaeological objects II. Mexico Guatemala Eclogite, gabbro, jade axe-heads. Terra Nova 8(Suppl 2) 3, 1996. 

As the reader will see, the evolution in Raman instrumentation did not follow a linear path. The instrument evolution demonstrates significant interactions among the roles played by the sample properties, the excitation wavelength, evolution of the detectors, monochromator design driven to match the detectors, implementation of FT-Raman (Fourier transform Raman), and the surprising impact of the Raman microprobe. As each development is described, there will be small diversions included in square brackets, almost like a point-counterpoint musical exposition, that explain the interplay of these developments. The selection of topics to be included was motivated to expose the evolution of commercial instrumentation for applications in materials science and analytical spectroscopy. Traditionally, there have been numerous other Raman topics that have been quite interesting but not included here because of our selected focus. 

Skoog et al. [2] emphasized the instmmentation advances of the 1980s and early 1990s. Both Fourier-transform Raman (FT-Raman) spectrometers and single-stage spectrographs are discussed. There is some discussion of optical fiber probes, but none of the Raman microprobe. The authors sketch the theory of Raman scattering and present a classical (polarizability derivative) treatment of selection rules and intensity. Resonance enhancement and surface enhancement are treated briefly. In a textbook noted for its emphasis on instrumentation, there is little discussion of current applications. 

Raman Microspectroscopy. Raman spectra of small solids or small regions of solids can be obtained at a spatial resolution of about 1 J.m using a Raman microprobe. A widespread application is in the characterization of materials. For example, the Raman microprobe is used to measure lattice strain in semiconductors (30) and polymers (31,32), and to identify graphitic regions in diamond films (33). The microprobe has long been employed to identify fluid inclusions in minerals (34), and is increasingly popular for identification of inclusions in glass (qv) (35). 

The Raman microprobe has provided applications in a number of diverse areas of science. Generally, the areas of applications fall into two major... 

The Raman microprobe has been used to detect foreign bodies in various tissues (38). Figure 3-9 shows spectra of lymph node tissue of 5 pm size, which was obtained by biopsy from a patient. The foreign body was identified as a particle of silicon rubber (dimethyl siloxane). For more biological and medical applications, see Section 6.2.4. 

In the study of minerals and other geological materials, Raman spectroscopy has been applied for chemical analysis and in studies of molecular and crystal structure, and of elastic and thermodynamic properties. A particularly important field for the application of Raman spectroscopy in chemical analysis is in the study of fluid inclusions in minerals, where the Raman microprobe has been developed to enable nondestructive in... 

An important development in Raman spectroscopy has been the coupling cf the spectrometer to an optical microscope. This allows the chemical and structural analysis described above to be applied to sample volumes only 1 across [38]. No more sample preparation is required than that for optical microscopy, and the microscope itself can be used to locate and record the area which is analyzed. This has obvious practical application to the characterization of small impurities or dispersed phases in polymer samples. This instrument, which may be called the micro-Raman spectrometer, the Raman microprobe or the Molecular Optics Laser Examiner [39] has also been applied to the study of mechanical properties in polymer fibers and composites. It can act as a non-invasive strain gauge with 1 fim resolution, and this type of work has recently been reviewed by Meier and Kip [40]. Even if the sample is large and homogeneous, there may be advantages in using the micro-Raman instrument. The microscope... 

Before 1975, Raman spectroscopy applied to minerals and glasses was limited by the intrinsic weakness of the Raman signal and by the requirement of optically high-quality samples [58,59]. The situation changed with the advent of Raman microprobes [60,61] capable of a spatial resolution of the order of 1 tm on samples of 10 -10 g. For this reason, the application of Raman spectroscopy developed at full speed since 1975. 

For mixed materials with different crystal lattice parameters, poor manufacturing conditions can lead to island formation and strain-induced cracking. The use of a Raman microprobe for mapping or a microscope for imaging can be used to identify areas exhibiting strain. The reader is referred to Chapters 2, 5, and 12 for further examples of this application. 

In summary, the utility of micro-SERS spectroscopy for the evaluation of potential-dependent interfacial com-petititve and displacement reactions at chargwl surface has been demonstrated. The data obtained allow the determination of the chemical identity, structure, orientation, competitive and displacement adsorption of cationic surfactants and nitrophenol in the first adsorption layer. The examples of these measurements in the field of surfactants and organic pollutants reviewed in this article were selected to illustrate the sensitivity, molecular specificity of adsorption processes, accuracy, ease of substrate preparation, and manifold applications of Raman analysis. The spatial resolution of the laser microprobe, coupled with the 10 enhancement of the Raman cross-section, means that picogram quantities of material localized to pm-sized surfaces areas can be detected and identified by SERS vibrational spectroscopy. 

Infrared and ultraviolet probes for surface analysis are then considered.The applications of IR spectroscopy and Raman microscopy are discussed, and a brief account is also given of laser-microprobe mass spectrometry (LAMMA). 

Conventionally, wide-field Raman microprobes are applied for such mappings, but, recently, confocal microscope systems have also been used (Bridges et al., 2004 Puppels et al., 1990, 1991 Schliicker et al., 2003). Confocal microscopy originated from biological applications with the goal of analysis of the insides of cells without destruction of the cell membrane. Confocal microscopy selectively rejects any information from planes closer or further from the focal plane. Confocal microscopy is a... 

In many industrial laboratories, Raman spectrscopy is routinely used, together with infrared spectroscopy, for acquisition of vibrational spectra. Raman spectrometer systems for routine analytical applications are commercially available. An important expansion of the potential of the technique has arisen from the use of the microprobe, which permits acquisition of spectra from domains as small as one micron. 

Applications of IR and Raman spectroscopy to the study of clinkers and unhydrated cements have been reviewed (B39,B40). The laser Raman microprobe, with which regions of micrometre dimensions on a polished surface may be examined, has been used to investigate structure and crystallinity, especially of the alite and belite (Cl9). Spectroscopic methods for studying the surface structures and compositions of cements are considered in Section 5.6.2. 

At this juncture, it is necessary to define what will be considered under the title of process Raman spectroscopy in the industrial environment. Quality assurance/quality control (QA/QC) applications and failure analysis are important areas of industrial interest however, these areas, in general, can be adequately addressed with standard laboratory Raman spectrometers including FT-Raman spectrometers and dispersive microprobes. For the purpose of the remainder of this chapter, the industrial environment and the process Raman analyzer will be restricted to instrumentation and protocols usable for on-line measurements. At this point, it is necessary to outline a definition of what the requirements for an on-line process Raman analyzer are. A process Raman analyzer should be composed of components which have the following properties ... 

Figure 19 Specific area of the D band approximated by the area of the D peak over sum of the areas of the D and G peaks, multiplied by 100, versus the in-plane crystallite size of La in angstroms. Points are for anthracene, thin and thick C films, and the Tuinstra and Koenig correlation for comparison [55]. (Reproduced from American Mineralogist, 78, Wopenka, B., et al., Structural characterization of kerogens to granulite-facies graphite Applicability of Raman microprobe spectroscopy, pp. 533-557. Copyright 1993, with permission from American Mineralogical Society.)...

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. 

A number of reports have been made concerning the application of Raman spectroscopy to the study of gas bubbles in materials (similar studies are possible for liquids in minerals see fluid inclusions in Chapter 10 and in Ref. 125). In this application, the Raman spectrum of the gas is excited through the host material and the instrumental platform is normally a microprobe. In this application, Raman spectroscopy is used to qualitatively identify the gas composition of a bubble [126,127], In Fig. 27, the in situ Raman spectrum of a bubble found in a sample of glass is shown. The spectrum of the gas sample can be identified [127] as carbon dioxide (1388 and 1286 cm ). 

Initial experiments on DLC were conducted with Raman microprobes however, the opportunity afforded by the application of Raman spectroscopy for the measurement of DLC film properties have led to a number of instrument companies introducing nonmicroscope-based analyzers tailored specifically for this market. Raman microscope systems are currently extensively used in the area of failure analysis. In this application, the high spatial resolution obtainable with a Raman microprobe is useful for the identification of small defects and contaminants. 

The Raman mlcroprobe has several important areas of application. Although the principal use is microspectroscopy, the microprobe is practical for rough mapping, particularly when only linear or radial distributions are needed. In such cases, 10-20 spectra are used to define the spatial features, and the microscope stage may be manually scanned. 

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