Fourier transform Raman techniques

Hendra P J 1996 Fourier transform Raman spectroscopy Modern Techniques in Raman Spectroscopy ed J J Laserna (New York Wley)... 

The infrared laser which is mosf often used in this technique of Fourier transform Raman, or FT-Raman, spectroscopy is the Nd-YAG laser (see Section 9.2.3) operating at a wavelength of 1064 nm. 

Fourier transformation techniques in spectroscopy are now quite common—the latest to arrive on the scene is Fourier transform Raman spectroscopy. In Chapter 3 1 have expanded considerably the discussion of these techniques and included Fourier transform Raman spectroscopy for the first time. 

However, the high frequency of the laser irradiation in the visible region may lead to photochemical reactions in the laser focus. Besides, fluorescence can often cover the whole Raman spectrum. Such problems can be avoided by using an excitation wavelength in the near-infrared (NIR) region, e.g. with an Nd YAG laser operating at 1064 nm. Deficits arising from the v dependence of the Raman intensity and the lower sensitivity of NIR detectors are compensated by the Fourier-Transform (IT) technique, which is widespread in IR spectroscopy . ... 

Raman and infrared vibrations are mutually exclusive and consequently use of both techniques is required in order to obtain a set of vibrational bands for a molecule. The advent of powerful computer-controlled instrumentation has greatly enhanced the sensitivity of these vibrational spectroscopies by the use of Fourier transform (FT) techniques, whereby spectra are recorded at all frequencies simultaneously in the time domain and then Fourier transformed to give conventional plots of absorbance versus frequency. The wide range of applications of FT Raman spectroscopy is discussed by Almond et al. (1990). Specific examples of its use in metal speciation are the observation of the Co-C stretch at 500 cm-1 in methylcobalamin and the shift to lower frequency of the corrin vibrations when cyanide is replaced by the heavier adenosyl in going from cyanocobalamin to adenosylcobalamin (Nie et al., 1990). 

Conventional Raman spectroscopy cannot be applied directly to aqueous extracts of sediments and soils, although it is occasionally used to provide information on organic solvent extracts of such samples. Fourier transform Raman spectroscopy, on the other hand, can be directly applied to water samples. The technique complements infrared spectroscopy in that some functional groups, eg unsaturation, give a much stronger response in the infrared. Several manufacturers (Perkin-Elmer, Digilab, Broker) now supply Fourier transform infrared spectrometers. 

Wilson, A. S., Edwards, H. G. M., Farwell, D. W., and Janaway, R. C. (1999). Fourier transform Raman spectroscopy Evaluation as a non-destructive technique for studying the degradation of human hair from archaeological and forensic environments. J. Raman Spectrosc. 30, 367-373. 

The instrumentation for conventional Raman spectroscopy will be discussed in this chapter. Special techniques of Raman spectroscopy will be described in Chapter 3. Most Raman spectroscopic investigations have been performed on dispersive instruments. However, Fourier transform (FT) techniques have become increasing important as a means of reducing interference from fluorescence. Both dispersive and FT-Raman spectroscopy will be discussed in this chapter. 

Raman spectroscopy has made remarkable progress in recent years. The synergism that has taken place with the advent of new detectors, Fourier-transform Raman and fiber optics has stimulated renewed interest in the technique. Its use in academia and especially in industry has grown rapidly. 

Infrared spectroscopy is an important technique for studying acidity. Acidic OH groups can be studied directly. Probe molecules such as pyridine may be used to study both Bronsted and Lewis acidity since two forms of adsorbed probes are easily distinguished by their infrared spectra. Quantitative infrared spectroscopy may be performed by measuring the spectrum of acidic OH or probes adsorbed on thin, self-supporting wafers of the acidic solid. Other spectroscopic methods which may provide information in specific cases include Fourier Transform Raman spectroscopy, electron spin resonance spectroscopy, ultraviolet spectroscopy, and nuclear magnetic resonance spectroscopy. 

Once deposition is complete and the initial reaction product is trapped in an inert gas matrix, characterization is carried out spectroscopically. Several spectroscopic techniques have been used the most common is infrared spectroscopy, either dispersive or Fourier transform. Raman spectroscopic studies have been carried out as well, but low signal levels have made this approach difficult. When the trapped intermediate is a free radical, electron spin resonance techniques are valuable as well. Finally, a number of researchers are employing electronic spectroscopy, when the species of interest has an absorption in the visible or ultraviolet tegion. 

The Raman spectra of all isotopomers of hydrogen were investigated by Edwards et al. (1979, 1986) and Veirs and Rosenblatt (1987). Using the technique of Fourier-Transform Raman spectroscopy in the visible Jennings et al. (1986, 1987) determined the molecular constants of H2 and D2 with high accuracy. Density shifts of the g-branch lines of H2 were measured by Rahn and Rosasco (1990) at temperatures from 295 K to 1000 K. 

A number of recently developed techniques, such as time resolved resonance Raman, surface enhanced Raman spectroscopy, and near IR Fourier transform Raman, are described in Section 4.7.2.2. 

Schrader B (1994) Sampling techniques, chapt II. In Chase DB, Rabolt JF (eds) Fourier Transform Raman Spectroscopy. Academic Press, Boston Schrader B, Ansmann A (1975) Angew Chem 87 345... 

If sticky labels are attached directly to foodstuffs such as fruit and vegetables, then some of the adhesive may remain on the foodstuff when the label is removed and the food is eaten. It is not uncommon to be able to detect by eye and by touch the presence of a small sticky label residue on fruit. The presence of adhesive residues on the food surface has been investigated by Fourier transform-Raman spectroscopy although at the low levels present on the foodstuff after removal of the label this technique could not be used to provide quantitative data. 

Developments in Raman spectroscopy, with applications for colorants, have included resonance Raman, surface enhanced Raman spectroscopy (SERS), surface enhanced resonance Raman spectroscopy (SERRS) and near-infrared Fourier transform Raman spectroscopy (NIR-FT-Raman), with the latter technique discussed in the next section. 

In 1986, a Raman instrument based on NIR excitation (1064 nm) and a Michaelson interferometer became available [16]. This development revolutionized Raman spectroscopy. In addition to the advantages of throughput and multiplex inherent to Fourier Transform (FT) techniques, this instrument overcame the obstacle of fluorescence. Fluorescence was eliminated by excitation at a NIR wavelength where electronic transitions in most samples are absent. Availability of such NIR FT-Raman instruments was particularly useful in the studies of lignin. 

More research is needed with both spectroscopic tools, especially with the recent advances in the field of Fourier transform Raman spectroscopy and the tremendously improved sensitivity and stability of the IR interferometers. In addition, the availability of new algorithms such as the ratio method (32), factor analysis (20, 33), and recently developed nonlinear techniques (34), coupled with an easy access to fast computers, will advance spectral analysis tremendously and make it more precise and reliable. 

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. 

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