Sampling in Raman spectroscopy

Raman spectroscopy gives results similar to those from infrared spectroscopy. This is why Raman spectroscopy is often used together with infrared spectroscopy in order to receive additional information about the sample analyzed. The motions of the molecule involved in the analysis of the sample in Raman spectroscopy are similar to those by infrared spectroscopy. These include rotational and vibrational motions. However, the physical causes of the resulting spectrum are different. 

In Raman spectroscopy the intensity of scattered radiation depends not only on the polarizability and concentration of the analyte molecules, but also on the optical properties of the sample and the adjustment of the instrument. Absolute Raman intensities are not, therefore, inherently a very accurate measure of concentration. These intensities are, of course, useful for quantification under well-defined experimental conditions and for well characterized samples otherwise relative intensities should be used instead. Raman bands of the major component, the solvent, or another component of known concentration can be used as internal standards. For isotropic phases, intensity ratios of Raman bands of the analyte and the reference compound depend linearly on the concentration ratio over a wide concentration range and are, therefore, very well-suited for quantification. Changes of temperature and the refractive index of the sample can, however, influence Raman intensities, and the band positions can be shifted by different solvation at higher concentrations or... 

UV-Vis spectroscopy may also provide valuable information if small molecules are studied. However, the photochemical sensitivity of many sulfur-containing molecules may trigger changes in the composition of the sample during irradiation. For instance, this phenomenon has been observed in Raman spectroscopy using the blue or green hnes of an argon ion laser which sometimes decompose sensitive sulfur samples with formation of Sg [2, 3]. Reliable spectra are obtained with the red hnes of a krypton ion or a He-Ne laser as well as with the infrared photons of a Nd YAG laser. 

Minimal sample preparation is involved in Raman spectroscopy samples as thin as a monolayer can be examined. 

An interesting and powerful new development in Raman spectroscopy of catalysts is the use of a UV laser to excite the sample. This has two major advantages. First, the scattering cross section, which varies with the fourth power of the frequency, is substantially increased. Second, the Raman peaks shift out of the visible region of the spectrum where fluorescence occurs. The reader is referred to Li and Stair for applications of UV Raman spectroscopy on catalysts [40]. 

Figure 9.27 In Raman spectroscopy, light from a laser is shone at a sample. It is monochromated at a frequency of v0. Most of the light is transmitted. Most of the scattered light is scattered elastically, so its frequency remains at v0 this is Rayleigh scattered light. Raman scattered light has a frequency V(SCattered) = v0 — vibration) The sample is generally in solution...

Also the infrared microspectroscopy (IR) is a vibrational spectroscopy, but it presents some differences with respect to Raman spectroscopy and also provides different information. In infrared spectroscopy the sample is radiated with infrared light, whereas in Raman spectroscopy a monochromatic visible or near infrared light is used. In this way, the vibrational energy... 

In relation to sample preparation, Raman spectra can be obtained from pure complexes in the bulk state, seeing that for better performance the careful grinding of samples is required. Contrary to FTIR spectroscopy, where samples are mixed with mineral oil (Nujol) or KBr pellets, in Raman spectroscopy a pure substance is used. For this reason, the Raman spectroscopy is called a nondestructive measurement method. Additionally, analysis can be carried out through many containers such as glass, Pyrex reaction vessels, plastic containers, and so on. 

The Fourier transform Raman spectrometer is constructed around an interferometer (see Figure 4.20) [57], Normally, a continuous wave Nd YAG laser (1064nm) is used for the sample excitation. In relation to the sample arrangement inside the spectrometer, there are two fundamental geometries in which a sample is tested in Raman spectroscopy, that is, the 90° geometry, where the laser beam... 

The origin of Raman spectra is markedly different from that of IR spectra. In Raman spectroscopy, the sample is irradiated by intense laser beams in the UV-visible region (v0), and the scattered light is usually observed in the direction perpendicular to the incident beam (Fig. 1-7 see also Chapter 2,... 

The experimental setup for matrix Raman spectroscopy is essentially the same as that for matrix IR spectroscopy. The major difference lies in optical geometry. Namely, backscattering geometry must be employed in Raman spectroscopy since the matrix gas and sample vapor are deposted on a cold metal (Cu, Al) surface. Figure 3-27 shows the optical arrangement... 

Xie et al. (2001) measured UV-vis spectra of the bulk oxides niobia, molybdena, tungsten trioxide, and vanadia and determined the position of the band edge as a function of temperature. In a separate experiment, they investigated the samples by Raman spectroscopy. To understand the changes in the Raman intensities, they analyzed the intensity in the UV-vis spectra at the wavelength of the incident laser irradiation and at the... 

Developments in Raman spectroscopy and wood sampling techniques have made it possible to carry out in situ studies on the organization of lignin and... 

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