Dispersive Raman techniques

In the case of the sulphur triimide S(NBu-f)3, the dispersive Raman technique applying a double monochromator and a CCD camera was employed to obtain the information from polarized measurements (solution studies) and also to obtain high-resolution spectra by low-temperature measurements. In the case of the main group metal complex, only FT-Raman studies with long-wavenumber excitation were successful, since visible-light excitation caused strong fluorescence. The FT-Raman spectra of the tetraimidosulphate residue were similar to those obtained from excitation with visible laser lines. 

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

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. 

Dispersive Raman spectroscopy This spectroscopy technique uses grating and/or prism dispersing elements. 

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. 

Dispersive Raman spectrometers are used with excitation in the visible range (typically He—Ne or Ar+ lasers are used), Fourier transform Raman spectrometers are used with excitation in the near infrared range (Nd YAG laser). For both ranges, microscopic techniques working with a laser beam diameter of micrometer size, are commercially available. 

Unlike IR spectroscopy where nowadays FT instrumentation is solely used, in Raman spectroscopy both conventional dispersive and FT techniques have their applications, the choice being governed by several factors. The two techniques differ significantly in several performance criteria, and neither one is best for all applications. Contemporary dispersive Raman spectrometers are often equipped with silicon-based charge coupled device (CCD) multichannel detector systems, and laser sources with operating wavelength in the ultraviolet, visible or near-infrared region are employed. In FT Raman spectroscopy, the excitation is provided exclusively by near-infrared lasers (1064 nm or 780 nm). 

Although the Raman effect was discovered in 1928 by Sir Chandrasekhara Venkata Raman, it has not until recently been applied to food adulteration problems (Baeten et al., 1996 Li-Chan, 1994 Ozaki et aL, 1992 Sadeghi-Jorabchi et al., 1990, 1991). Baeten et al. (1996) used FT-Raman which, they claim, produces fluorescence-free spectra, using a 1.064 pm laser. They were able to detect adulteration with soybean, corn and olive pomace with 100% accuracy down to 1% adulterant. In fact 780 nm excitation in a confocal instrument (Williams, 1994 Williams et al., 1994) produces excellent dispersive Raman spectra from olive oils in a wholly non-invasive fashion (N. Kaderbhai and the authors, unpublished observations). Baeten et al. (1996) comment that at present liquid and gas chromatography is the most accurate technique to determine adulteration, and it is this method that is the European Union adulteration standard (EC, 1991), but that FT-Raman has the potential for detecting adulterants beyond the limits of liquid and gas chromatography. 

Karavas E, Georgarakis M, Docoslis A, Bikiaris D (2007b) Combining SEM, TEM, and micro-Raman techniques to differentiate between the amorphous molecular level dispersions and nanodispersions of a poorly water-soluble drug within a polymer matrix. Int J Pharm 340 76-83 Karmwar P, Graeser K, Gordon KC, Strachan CJ, Rades T (2012) Effect of different preparation methods on the dissolution behaviour of amorphous indomethacin. Eur J Pharm Biopharm 80 459-464... 

RL McCreery. Instrumentation for dispersive Raman spectroscopy. In JJ Lasema, ed. Modern Techniques on Raman Spectroscopy. Chichester Wiley, 1996. 

Besides the established dispersive techniques using monochromators and polychro-mators, also the Fourier transform (FT)-Raman technique with near-infrared excitation has... 

Raman and infrared spectra have also been compared for a series of 1,4-benzodiazepines, including diazepam (Vallium) and of closely related compounds [16,17]. The complementary nature of these two vibrational spectroscopic techniques was highlighted and the data provided spectral features that allowed identification of the drugs. The value of Fourier transform Raman spectroscopy using a near-infrared excitation source was also demonstrated for these heterocyclic molecules which have a tendency to fluoresce with visible radiation from conventional dispersive Raman spectrometers. 

Raman spectroscopy is an emission-based technique. Although conventional dispersive Raman spectroscopy (laser wavelengths between 500 and 700 nm) has not been successfully used to monitor polymerization reactions due to the tremendous effect of fluorescence on the spectra, FT-Raman (laser wavelength in the NIR region, 1034 nm) or modem dispersive Raman equipments (laser wavelengths over 800 nm) overcome this difficulty. Currently, Raman spectroscopy can be considered as the spectroscopic technique with the greater potential to monitor polymerization reactors, and especially emulsion polymerization reactors, in situ. Raman spectroscopy presents several advantages over the absorption techniques (MIR and NIR). The most important ones are ... 

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