Raman spectra contents

The interfacial zone is by definition the region between the crystallite basal surface and the beginning of isotropy. Due to the conformationally diffuse nature of this region, quantitative contents of the interphase are most often determined by indirect measures. For example, they have been computed as a balance from one of the sum of the fractional contents of pure crystalline and amorphous regions. The analysis of the internal modes region of the Raman spectrum of polyethylene, as detailed in the previous section of this chapter, was used to quantify the content of the interphase region (ab). 

A factor analysis of the Raman spectra of a set of linear polyethylenes identified the existence of a third component in addition to the pure crystalline and pure amorphous components [78]. The characteristics of the Raman spectrum of the interphase were very similar to that of the crystalline spectrum indicating that the interphase retains a significant degree of order. Using the Raman method, the content of interphase in linear polyethylenes was found to increase with molecular weight [74—76,78]. For molecular weights below... 

Recently Eliasson and Matousek [26, 62] demonstrated that SORS can provide a chemical signature of the internal content of opaque plastic containers. This is demonstrated in Fig. 3.11 for aspirin tablets held inside an opaque (white) plastic pharmaceutical bottle (1.3mm thick). The conventional Raman signal is overwhelmed by the Raman component originating from the container wall and is consequently ineffective in determining the contents of the bottle. In contrast, the SORS method using a scaled subtraction of two SORS spectra measured at different spatial offsets yields a clean Raman spectrum of the tablets inside the bottle. SORS has also been used in the detection of counterfeit anti-malarial tablets by Ricci et al. [63] the chemical specificity of Raman spectroscopy readily distinguished between genuine and fake tablets and identified the content of the counterfeit tablets. 

Fig. 3.11. Non-invasive Raman spectra of aspirin tablets measured through a white, diffusely scattering 1.3 mm thick plastic container. Conventional Raman and the scaled and subtracted SORS data are shown together with the reference Raman spectra of aspirin and the plastic container. The conventional Raman spectrum is dominated by the Raman signal of the container masking the Raman signal of the aspirin contents. The acquisition time was Is and the laser beam power 250mW (N.A. Macleod, P. Matousek, Emerging non-invasive Raman methods in process control and forensic applications, Pharm. Res. 25, 2205 (2008). Copyright (2008) Springer)...

In the Raman experiments, an excitation wavelength of 785 nm (intensity 1.8 105 W/cm2) was used. The sample, i.e. a drop of Au nanoparticle suspension with soluble pollen content was placed under a (60x) water immersion objective. Raman spectra were recorded with 1 s acquisition time. The control preparations (pollen supernatant with water) did not yield any spectral features. A spectrum of rye pollen supernatant with Au nanoparticles is shown in Fig. 4.9, together with a normal Raman spectrum of a rye pollen grain. The difference in spectral information that can be obtained by both approaches is evident from a comparison of these two spectra. Although an estimate of an enhancement factor is not possible from this experiment, it is clear that... 

Raman spectroscopy is a scattering, not an absorption technique as FTIR. Thus, the ratio method cannot be used to determine the amount of light scattered unless an internal standard method is adopted. The internal standard method requires adding a known amount of a known component to each unknown sample. This known component should be chemically stable, not interact with other components in the sample and also have a unique peak. Plotting the Raman intensity of known component peaks versus known concentration in the sample, the proportional factor of Raman intensity to concentration can be identified as the slope of the plot. For the same experimental conditions, this proportional factor is used to determine the concentration of an unknown component from its unique peak. Determining relative contents of Si and Ge in Si—Ge thin films (Figure 9.38 and Figure 9.39) is an example of quantitative analysis of a Raman spectrum. 

For multiplex measurements, when compared to fluorescence, SERRS also offers significant advantages. In multiplex measurements fluorescence has the disadvantage that the electronic spectra produced are broad (typically 50 to lOOnm full width at half maximum) and therefore overlap so that the technique is limited to the simultaneous measurement of around four dye labels [69, 77]. In contrast, SERRS uses the vibrational Raman spectrum of the label as a spectroscopic molecular fingerprint As a result the information content of the spectra is much higher and, because the vibrational bands are much narrower (about 1 nm full width at half maximum), spectral overlap is much less of a problem. Thus using SERRS it is possible to readily identify the components of a mixture without extensive separation procedures [78] and it has been estimated in the literature that simultaneous measurement with up to 30 SE(R)RS labels should be possible [79]. 

The analysis of the amide I band to obtain the estimation of protein secondary structure content in terms of percentage helix, j3 strand, and reverse turn that was developed by Williams has proved very successful and has now been used by numerous workers.In this method the amide I region is analyzed as a linear combination of the spectra of the reference proteins whose structures are known. As noted above the Raman spectra of globular proteins in the crystal and in solution are almost identical, reflecting the compact nature of the macromolecules. Thus one may use the fraction of each type of secondary structure determined in the crystalline state by the X-ray diffraction studies for proteins in solution. If there are n reference proteins with the Raman spectrum of each of them represented as normalized intensity measurements at p different wave numbers, then this information is related by the following matrix equation ... 

The Raman spectrum provides like a fingerprint of a substance since each line is associated to a particular vibrational mode of the molecule. Thus the line position is related to the vibration frequency, the linewidth depends on any source of order or disorder in the structure whereas the intensity is in principle (see eq2) proportional to the concentration of species. Therefore RS is a convenient technique to detect the different substances in a mixture, and via an appropriate calibration, to determine the content of each species. 

The analysis of a complex mixture requires, initially, the knowledge of the specific signatures of any pure substance and their appropriate calibration. We take as an example given in Fig. 2, the Raman spectrum recorded in ammonium sulfate diluted in water with various contents given in mol/1 or molarity (M). 

The limonene content was estimated by FT-Raman spectroscopy. FT-Raman spectra of limonene and cyclohexane are presented in Fig. 2. The FT-Raman spectrum of R(+)limonene showed characteristic peaks at 1678 cm (vc=c of cyclohexene) and 1645 cm (vcc of vinyl) [10,11]. In FT-IR Raman spectrum of cyclohexane there is no absorption in 1600-1700 cm region. Thus, the limonene amount from cyclohexane solutions can be determined by monitoring the intensities of the 1678 or 1645 cm peaks. For calibration, FT-Raman spectra of different ccmcentrations of limonene in cyclohexane were recorded (see Fig.3). In our study, the intensity of 1645 cm was correlated to limonene content in cyclohexane. There was a linear relationship. The calibration curve had the following equation y = 12.2577x+<).00955, R=0.98 (Fig. 4). Content (%) of R(+)Iimonene in EC microcapsules was measured using the above equation. According to the proposed method, the EC microcapsules contained 9.8 5.5% limonene. 

The IR and Raman spectra of this example ionomer are provided in Reference Spectrum 5. In addition to the spectral features of polyethylene, characteristic bands of COOH (C=0 stretch at 1698 cm ), COONa (COO stretch at 1558 cm ) or COO(Zn)i/2 (COO stretch at 1590 cm ), and C —O stretch at 1262 cm are observed in the infrared spectrum. However, given the low acid content, and the weak Raman scattering cross section of the C=0 group, the Raman spectrum does not provide characteristic C=0-related spectral features. As a result, the Raman spectrum is almost identical to that of polyethylene, except that the line widths are slightly broader. 

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