Qualitative analysis Raman

The use of vibrational Raman spectroscopy in qualitative analysis has increased greatly since the introduction of lasers, which have replaced mercury arcs as monochromatic sources. Although a laser Raman spectrometer is more expensive than a typical infrared spectrometer used for qualitative analysis, it does have the advantage that low- and high-wavenumber vibrations can be observed with equal ease whereas in the infrared a different, far-infrared, spectrometer may be required for observations below about 400 cm. ... 

Determination of surface functional groups, e.g., —OH, —C - C—, and >C = O, and identificadon of adsorbed molecules comes principally from comparison with vibrational spectra (infixed and Raman) of known molecules and compounds. Quick qualitative analysis is possible, e.g., stretching modes involving H appear for v(C—H) at 3000 cm and for v(0—H) at 3400 cm L In addition, the vibrational energy indicates the chemical state of the atoms involved, e.g., v(C=C) " 1500 cmT and v(C=0) " 1800 cm"L Further details concerning the structure of adsorbates... 

Raman spectra may facilitate qualitative analysis, asking what is it and also quantitative analysis - asking how much ... 

The analytical chemist can use Raman and infrared spectra in two ways. At tile purely empirical level, they provide fingerprints of the molecular structure and, as such, permit the qualitative analysis of individual compounds, either by direct comparison of the spectra of the known and unknown materials run consecutively, or by comparison of the spectrum of the unknown compound with catalogs of reference spectra. 

Two infrared absorption methods (i.e., FTIR) and a Raman spectroscopic method were used to quantify polymorphic clopidogrel bisulfate Form-I and Form-II [16,17]. In addition, qualitative analysis of these polymorphs was also conducted using FTIR [16], where each sample was scanned over in the spectral region of 450-4000 cm-1 at a resolution of 4 cm-1. The sampling procedure used KBr pellets, loaded to contain approximately 3% of analyte. It was found that absorption bands associated with C-Fl and C-O bonds were stronger for Form-II relative to Form-I, and that unique absorption bands for Form-I and Form-II were observed at 841 and 1029 cm-1, respectively. These absorption bands were reported to be useful in the quantitative or qualitative analysis of clopidogrel polymorphs. 

The IR spectra of all four crystal modifications were reported by Cady and Smith (1962) and by Holston (1962). The latter did point out some distinguishing features among the polymorphs, but Cady and Smith noted that problems with sample preparation and conversions among forms indicated that the optical properties described by McCrone (1950a) were the basis for the best rapid qualitative and even rough quantitative analysis. Raman spectroscopy, which requires less potentially destructive sample preparation, has been used to distinguish the polymorphs (Goetz and Brill 1979). The low resolution NMR spectra of the four crystal modifications were reported by Landers et al. (1985). The nuclear quadrupole resonance spectrum... 

Raman spectroscopy is by no means a new technique, although it is not as widely known or used by chemists as the related technique of infrared spectroscopy. However, following developments in the instrumentation over the last 20 years or so Raman spectroscopy appears to be having something of a rebirth. Raman, like infrared, may be employed for qualitative analysis, molecular structure determination, functional group identification, comparison of various physical properties such as crystallinity, studies of molecular interaction and determination of thermodynamic properties. 

Of the various analytical methods described, the ISO recommended a thin-layer chromatographic technique [35] for qualitative analysis of saffron, whereas quantitative HPLC methods have been described for the analysis of commercial saffron products [36], Other reports on chemical analysis of saffron constituents including combination of the reported methods, crystal structure analysis, Fourier Transform-Inffa Red (FT-IR) and Raman analysis are worth mentioned [37-40]. Very interesting is also the review article by Rios et al [41] on chemical analysis of saffron. 

Qualitative analysis by Raman spectroscopy is very complementary to IR spectroscopy and in some cases has an advantage over IR spectroscopy. The Raman spectrum is more sensitive to the organic framework or backbone of a molecule than to the functional groups, in contrast to the IR spectrum. IR correlation tables are useful for Raman spectra, because the Raman shift in wavenumbers is equal to the IR absorption in wave-numbers for the same vibration. Raman spectral libraries are available from commercial and government sources, as noted in the bibliography. These are not as extensive as those available for IR, but are growing rapidly. 

The IR or Raman spectrum of a polymer thus usually contains a number of peaks which is of order 3n or less, where n is the number of atoms in the repeat unit. Because the spectrum of a polymer is, to a first approximation, that of its repeat unit, the spectrum is an aid to qualitative analysis, that is, to finding out what kinds of repeat units are present in a sample. Additives and impurities will also have their own characteristic peaks in the spectrum and can therefore be identified. 

The qualitative analysis of inorganic substances in the solid state or in aqueous solution was the object of numerous Raman investigations, especially within the aim of the assignment of main lines to vibrational modes. Much less attention was devoted to the identification and quantitative determination of the content of each species in a mixture. The demand of new techniques and instruments for the real-time and on line monitoring of substances in water requires more reliable quantitative processes. Simultaneously the progress in the development of more compact transportable or portable Raman instruments has made possible the availability of appropriate Raman sensors. ... 

In the case of simple molecules, the question of whether a particular vibration is active in the IR spectrum can be answered by considering the forms of the normal modes [49-60, 66]. It can be seen in Fig. 1.4 that the dipole moment changes under all the active vibrations of the H2O molecule. In contrast, the vibrations of homopolar molecules such as H2 and N2 do not produce a dipole moment and thus are inactive in the IR spectrum. When a polyatomic molecule contains a center of symmetry, the vibrations synunetrical about this center are active in the Raman spectrum but inactive in the IR spectrum of this molecule, and vice versa. This result is known as the alternative prohibition rule. In general, the activity of the excitation in the IR spectrum cannot be predicted from such a qualitative analysis, but rather must be determined using group theory [51-54, 62]. 

A simple property of any spectrum can provide a more quantitative picture to the problem of fluorescence in Raman spectroscopy. The signal-to-noise ratio (S/N) describes the quality of the spectrum. A high S/N is desirable for viewing a spectrum for both qualitative analysis and for concentration determination with quantitative analysis. Obviously, the S/N can be increased two ways an increase in signal or a decrease in noise. For example, if the Raman spectrum of cyclohexane is taken from a sample and has been Altered to remove any particulates, the S/N ratio... 

As described at the beginning of this chapter, the types of compounds that absorb UV radiation are those with nonbonded electrons (n electrons) and conjugated double bond systems (it electrons) such as aromatic compounds and conjugated olefins. Unfortunately, such compounds absorb over similar wavelength ranges, and the absorption spectra overlap considerably. As a first step in qualitative analysis, it is necessary to purily the sample to eliminate absorption bands due to impurities. Even when pure, however, the spectra are often broad and frequently without fine structure. For these reasons, UV absorption is much less useful for the qualitative identification of functional groups or particular molecules than analytical methods such as MS, IR, and NMR. UV absorption is rarely used for organic structural elucidation today in modem laboratories because of the ease of use and power of NMR (Chapter 3), IR and Raman (Chapter 4), and MS (Chapters 9 and 10). 

For speetroseopie analysis (e.g. XRF, XRD, IR, Raman, Mossbauer etc.), a plot of the observed intensity versus the eorresponding wavelength or frequeney (or some other related parameter) is ealled the spectrum of that particular analytical method. The spectrum or the data obtained from such experiments contain information about nature of the interactions, atomic and molecular energy levels, chemical bonds, crystallographic information and other related processes. When only the item of interest is identified, it is called qualitative analysis and when the amoimt present is estimated, it is known as quantitative analysis. The effect of heat on a sample is reflected through its variation of thermodynamic properties. Such studies are done by thermal analysis. 

DR Lombardi, C Wang, B Sun, AW Fountain, TJ Vickers, CK Mann, FR Reich, JG Douglas, BA Crawford, FL Kohlasch. Quantitative and qualitative analysis of some inorganic compounds by Raman spectroscopy. Appl Spectrosc 48 875-883, 1994. 

PA Tanner, K-H Leung. Spectral interpretation and qualitative analysis of organophosphorus pesticides using FT-Raman and FT infrared spectroscopy. Appl Spectrosc 50 565-571, 1996. 

Principles and Characteristics Raman spectroscopy is mainly utilised as a qualitative tool, but can also be employed quantitatively [191]. Quantitative analysis by Raman spectroscopy has not kept pace with the rapid growth in the use of Raman spectroscopy for stmctural and qualitative analysis. Quantitative FT-Raman spectroscopy can be made as routine and reliable as absorption spectroscopy [192,193]. 

Several Raman group frequencies, which appear to be better suited for qualitative analysis than infrared, are listed in Table II. Although qualitative analyses are in general done less expensively by the infrared method, it is advisable to consider the Raman method whenever one is working with highly symmetrical molecules. In mixtures, providing no interaction occurs between the components, the Raman spectra ideally will be a superposition of the spectra of the components. In practice, difficulty in qualitative analysis of a mixture occurs wherever complicated spectra are involved, as evidenced by overlapping of Raman lines. 

Group Frequency Shifts in the Raman, which Appear to Be Better Suited for Qualitative Analysis than Infrared U]... 

Groups of atoms or particular kinds of bonds which differ from groups or bonds in other parts of the molecule in their mass or type, respectively, or in their position in the molecule, display, in a series of similar compoimds, absorption bands which occur in approximately the same part of the spectrum. The frequencies of certain absorptions can be designated as characteristic of the appropriate groups or bonds. This fact, observed empirically and also accounted for theoretically, is the basis of qualitative analysis by means of infrared spectra, which makes it possible to identify certain groups of atoms or even whole molecules of unknown structure. Characteristic frequencies of certain bonds are usually listed in monographs or in work in the literature dealing with infrared and Raman spectroscopy. 

Infrared (IR), near-infrared (NIR), and Raman spectroscopy have been successfully used for quantitative and qualitative analysis of polymer films. These methods leverage the fact that vibrational spectra are very sensitive to polymer structure and the strength of chemical bonds of functional groups. NIR spectroscopy has several practical advantages in polymer film analysis, including nondestructive and... 

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