Dispersive Raman Instrumentation

The Raman spectrum is given by the detection of the intensity of the scattered, frequency-shifted light by a photoelectric system. The resulting signal of the detector is amplified and converted to a form appropriate for plotting as a function of frequency. 

IR-transmitting optical fibres are evanescent wave sensors using a mathematical deconvolution technique to extract the absorbances and follow the concentrations of the components as they occur in both laboratory scale and process production. The fibre-optic probe used can be placed at specific locations within the samples or at the surface. The specificity of the technique, the speed of data acquisition and the portability of equipment make this method ideal as a tool to fundamentally probe polymer reactions and processes. Chalcogenide optical fibres are used to direct IR radiation from an FUR spectrometer through an attenuated total reflection (ATR) probe immersed in a reactor and back to the spectrometer. 

The evolution of Raman instrumentation has been dramatic. After making difficult measurements with uncertain sources, the laser came along and Raman 

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Raman microspectroscopy (often called micro-Raman), like most Raman spectrometry, is of the dispersive type. It requires collecting a spectrum at each wavenumber separately, not like the FUR type that collects a spectrum in a range of wavenumbers simultaneously. Although this chapter only describes the instrumentation for Raman microscopy, its working principles and spectra are basically the same as those of conventional dispersive Raman instruments, which consist of the following elements ... 

The FT-Raman spectrometer has a number of unique advantages for Raman spectrometry. TIowever, the limitations previously noted mean that dispersive Raman instruments will be widely used for some time. 

Table 4.2 Lasers used with dispersive Raman instruments.

Raman spectroscopy may be performed on either a dispersive instrument or a FT instrument. All of the spectra provided in this chapter were obtained from a FT-Raman instrument (see Fig. 62), featuring a NdiYAG solid-state laser, and a InGaAs (indium-gallium arsenide) detector, combined with a silicon on quartz beam splitter. Note that in the FT-Raman experiment, the sample effectively becomes the source to the FT spectrometer. Dispersive Raman instruments are also popular, and these usually feature a silicon-based array detector (CCD array) in combination with either a visible laser (doubled YAG or HeNe) or a short-wavelength solid-state NIR laser. 

Raman spectroscopy is fast becoming a valuable process measuring tool. Several companies have introduced Fourier transform (FT) Raman and dispersive Raman instruments designed for process control. This is important because laboratory instruments seldom are robust enough for the process... 

Di.scuss the advantages and disadvantages of FT-Raman spectrometers compared to conventional dispersive Raman instruments. 

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. 

McCreery R L 1996 Instrumentation for dispersive Raman spectroscopy Modern Techniques in Raman Spectroscopy ed J J Laserna (New York Wiley)... 

Due to limitations in signal-to-noise ratio available for the then common dispersive IR instruments, peptide and protein vibrational spectroscopic studies shifted to emphasize Raman measurements in the 1970s 29-32 Qualitatively the same sorts of empirical correlations as discussed above have been found between frequencies of amide bands in the Raman and secondary structure. However, due to the complementary selection rules for Raman as compared to IR and to the multi-component nature of these polymeric spectral bands, the... 

Although there are a variety of wavelength selection methods available, the vast majority of Raman instruments utilize either dispersive or Fourier transform spectrometers. These are shown schematically in Fig. 1.6. The high throughput and spectral resolution obtainable from these instruments make them obvious choices for Raman spectroscopy however, each has specific strengths and drawbacks which make them more suitable in specific applications. 

The expression for Raman signal from Chapter 3 may be combined with Eq. (4.11) to arrive at the dependence of experimental SNR on various sample and measurement variables. SNR is a generally more important indicator of the utility of the measurement than raw signal, since SNR determines the detection limit and overall information content. In addition, SNR may be compared for spectra with quite different intensity units, such as dispersive/CCD and FT-Raman instruments. In the remainder of this chapter, we will derive SNR expressions for several situations, and define a figure of merit for SNR. 

At the time of this writing, the Raman spectrometer market is approximately split between dispersive (spectrograph/CCD) and nondispersive (FT-Raman) instruments. Both types have their pros and cons, which enter into a selection for a given application. Several generalizations are listed in Table 5.3. These... 

In part because many FT-Raman instruments were adaptations of existing FTIR spectrometer, there is a fairly wide variety of instrument configurations in current use. However, they all share the components shown in the block diagram of Figure 9.10. While all of the components shown are represented in the generic spectrometer of Figure 1.7, there are some important differences between the FT and dispersive spectrometers, heyond the obvious case of the wavelength analyzer itself. 

An optical multichannel Raman instrument functions as a spectrograph rather than as a monochromator due to the absence of an exit slit. The Raman light at the output of a grating instrument is dispersed across a detector consisting of an electronic image sensor that functions as an electronic photographic plate . For optical activity measurements, the components between the laser and the sample are essentially the same as described in the previous section for the scanning instrument. 

Advancements in dispersive Raman spectroscopy, including improvements in detectors, filters, optical fibers, and instrument designs, have made this tech-... 

The Raman effect was discovered in 1928, but the first commercial Raman instruments did not start to appear until the early 1950s. These instruments did not use laser sources, but used elemental sources and arc lamps. In 1962, laser sources started to become available for Raman instruments, and the first commercial laser Raman instruments appeared in 1964-1965. The first commercial FT-Raman instruments were available starting in 1988, and by the next year, FT-Raman microscopy was possible (32). Due to the various complexities when one compares dispersive Raman spectrometers with FT-based systems (33), only sampling techniques will be discussed here. 

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