FT-Raman instrument

The charge-coupled device was first used in Raman spectroscopic applications in the late 1980s [45, 46], followed rapidly by the introduction of holographic notch filters [47]. This combination, coupled with the visibility of FT-Raman instruments introduced at around the same time, helped drive the growth of Raman outside the academic lab. Although the throughput advantage of... 

These developments in Raman instrumentation brought commercial Raman instruments to the present state of the art of Raman measurements. Now, Raman spectra can also be obtained by Fourier transform (FT) spectroscopy. FT-Raman instruments are being sold by all Fourier transform infrared (FT-IR) instrument makers, either as interfaced units to the FT-IR spectrometer or as dedicated FT-Raman instruments. 

As stated earlier, FT-Raman instruments employ a CW Nd YAG laser with an excitation at 1,064 nm (9,395 cm ). The use of such a near IR laser suffers from a 16-fold reduction in signal as compared with a visible laser lasing at 514.5 nm because the cross section of Raman scattering follows the the v4 relationship. The maximum power of the laser is as high as 10 W, although less power ( 1W) generally is used. 

An important aspect of FT-Raman instrumentation is the necessity for optical filtering. The first task is to eliminate the stray light caused by the laser excitation because it will saturate the detector and electronics. The filtering must be capable of reducing the Rayleigh line, which is 106 stronger than the Stokes-shifted lines in the Raman spectrum. In order to be sufficiently... 

FT-Raman instruments are calibrated with an internal laser, which is used to provide the exact location of the movable mirror in the interferometer. Thus the intensity of the interferogram is known as a function of the mirror location (distance in cm), and this is converted through a fast Fourier transform to reciprocal distance or wavenumber (cm-1) in the spectral domain. 

Figure 2-15 Schematic of FT -Raman instrumentation fitted with a bifurcated fiber optic interface. (Reproduced with permission from Ref. 26.)...

Iindustrial applications of the Raman effect have garnered intense interest since the introduction of FT-Raman instrumentation. Concurrent with FT-Raman instrumentation developments have been the fiber optics improvements and the advent of new detectors. These three factors have syner-gized and have led to the present interest in Raman spectroscopy. This has brought the Raman effect from the laboratory and into the plant, where in-situ measurements are now possible in a number of industrial environments. 

The results presented here illustrate that dye spectra may be obtained quickly and yield useful information. Small percentages of dye (1-2%) provide discernible bands. Computer subtraction can be used to remove the excess acrylic polymer bands. Here, also, the use of a micro FT-Raman instrument would be advantageous in lowering the sample size to be investigated. 

Near-infrared surface-enhanced Raman spectroscopy Some of the major irritants in Raman measurements are sample fluorescence and photochemistry. However, with the help of Fourier transform (FT) Raman instruments, near-infrared (near-IR) Raman spectroscopy has become an excellent technique for eliminating sample fluorescence and photochemistry in Raman measurements. As demonstrated recently, the range of near-IR Raman techniques can be extended to include near-IR SERS. Near-IR SERS reduces the magnitude of the fluorescence problem because near-IR excitation eliminates most sources of luminescence. Potential applications of near-IR SERS are in environmental monitoring and ultrasensitive detection of highly luminescent molecules [11]. 

In addition to reduced fluorescence, FT-Raman also provides excellent frequency precision and many other benefits common to FTIR instrumentation (6). In many cases, FT-Raman instruments are modified FTIRs, and several FTIR vendors added Raman accessories to their product line. FT-Raman was responsible for a surge of interest in analytical Raman spectroscopy during the period 1986 to 1990, and steady growth since. 

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. 

P. Hendra, C. Jones, and G. Warnes, FT-Raman Instrumentation and Chemical Application, Ellis Harwood, New York, 1991. 

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. 

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. 

The components for an FTIR instrument and an FT-Raman instrument are shown, respectively, in Figures 2.10 and 2.11. 

The most common detectors in Raman instruments are PDAs and CCDs but for FT-Raman, single channel detectors are used, e.g. InGaAs. An extra requirement for the FT-Raman instrument is a notch or edge filter it is included to reject scattered laser light at the strong Rayleigh line, which could otherwise obscure the FT-Raman spectrum. 

Similar to the infrared instruments, there are two general types of Raman instruments the dispersive and FT Raman spectrometers. The FT Raman instrument incorporates an interferometer that provides similar advantages to those given by the interferometer of an FTIR spectrometer (for further details, see Ref. [3]), although modem dispersive instmments with CCD detectors can measure a range of frequencies simultaneously. 

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