Spectral additivity

Spectral addition enables all the spectra acquired from a given GC peak to be averaged, with the result that a greater signal-to-noise ratio is obtained and... 

Figure 5, An illustration of the use of the spectral addition technique to simulate the Raman spectrum of the quinol/CH CN clathrate (c) by adding the spectra of 3-quinol (a) and liquid CH CN (b).

The spectral subtraction technique can be applied equally successfully in the analysis of both the infrared and Raman spectra of inclusion compounds, revealing guest molecule bands which would otherwise be obscured by the more intense host lattice bands. The spectral addition technique is also shown to be useful in simulating clathrate spectra. 

The tip angle (vl/)-dependent DEPT subspectra (545, Sgo, and 5i3s) (Sec. II.D.3) are combinations of CH, CH2, and CH3 resonances an example is shown in Fig. 32. The pure contributions can be calculated, using standard spectral addition and subtraction routines ... 

Anderson et al. have presented solid-state NMR analysis of local structural environments in phosphate glasses for educational purposes. " In particular, the solid-state NMR wide-line spectra of a series of sodium phosphate glasses have been considered, which can also be simulated by spectral addition of reference solid-state spectra obtained for pure pyrophosphate and metaphosphate salts. The example chosen introduces the principles of solid-state NMR and allows interpretation of the spectrum in terms of the composition and localised phosphate environment. 

K. S. Kalasinsky and G. R. Lightsey, "Quanitative Analysis of Cellulosic Filler Surface Chemistry Using Infrared Reflectance Spectroscopy and Computer Spectral Addition (In press). F. Lowery and G. R. Lightsey, Un-Published Data, Dept, of Chem. Engr., Miss. State Univ. (1980). 

Key words dye concentration measurement, Beer-Lambert law, real-time monitoring and control, spectral additivity, spectral morphing. 

The dyes obey the super-position (spectral additivity) law. 

When two or more dyes are mixed together, if there is no chemical interaction between them, the sum of the spectra of the individual dyes should equal the spectrum of the mixture. Dyes which have this property obey the law of spectral additivity. A number of investigations with direct and vat dyes in mixtures have shown that this statement is often not valid. Neale and Stringfellow (Neale and Stringfellow, 1943) concluded that for certain direct dyes the spectra of the mixtures in water are not additive. In an aqueous solution, pairs of (fyes interact with each other, possibly through the operation of resonance bonds or residual valence forces similar to those which are responsible for anchoring the dye onto the hydroxyl groups of cellulose. 

Figure 8.8 shows the absorbance spectra of a Direct 83.1, Direct Blue 85 and Direct Red 89 at concentrations of 2.60 g/1, l.OOg/1 and 0.90 g/f respectively. When these three dyes are mixed together, the dye interaction between the Direct Blue 85 and the other two dyes shifts the spectrum compared with a spectrum comprised of the linear sum of the absorbance spectra, as shown in Fig. 8.9. Such shifts due to spectral additivity (or super-position) can cause significant errors in concentration predictions rrsing Eq. 8.2.

Additionally, more sophisticated pulse sequences (the procedure is called spectral editing) enable one to obtain spectra, after addition or subtraction, where only the following are present (see, for example Bouquet, 1986) ... 

When investigating time parameters it was shown, that a storage time in Xe of spectral purity was 10 mcs, while a restoring time was 10 ms with subsequent decrease in the case of addition of small amounts of air (less than 1%). As basic processes influencing time parameters, both dissociative recombination and three-particle adhesion of electrons to oxygen molecules have been considered. 

Wliat does one actually observe in the experunental spectrum, when the levels are characterized by the set of quantum numbers n. Mj ) for the nonnal modes The most obvious spectral observation is simply the set of energies of the levels another important observable quantity is the intensities. The latter depend very sensitively on the type of probe of the molecule used to obtain the spectmm for example, the intensities in absorption spectroscopy are in general far different from those in Raman spectroscopy. From now on we will focus on the energy levels of the spectmm, although the intensities most certainly carry much additional infonnation about the molecule, and are extremely interesting from the point of view of theoretical dynamics. 

In addition to the many applications of SERS, Raman spectroscopy is, in general, a usefiil analytical tool having many applications in surface science. One interesting example is that of carbon surfaces which do not support SERS. Raman spectroscopy of carbon surfaces provides insight into two important aspects. First, Raman spectral features correlate with the electrochemical reactivity of carbon surfaces this allows one to study surface oxidation [155]. Second, Raman spectroscopy can probe species at carbon surfaces which may account for the highly variable behaviour of carbon materials [155]. Another application to surfaces is the use... 

Optical metiiods, in both bulb and beam expermrents, have been employed to detemiine tlie relative populations of individual internal quantum states of products of chemical reactions. Most connnonly, such methods employ a transition to an excited electronic, rather than vibrational, level of tlie molecule. Molecular electronic transitions occur in the visible and ultraviolet, and detection of emission in these spectral regions can be accomplished much more sensitively than in the infrared, where vibrational transitions occur. In addition to their use in the study of collisional reaction dynamics, laser spectroscopic methods have been widely applied for the measurement of temperature and species concentrations in many different kinds of reaction media, including combustion media [31] and atmospheric chemistry [32]. 

As discussed above, the spectrum must be assigned, i.e. the quantum numbers of the upper and lower levels of the spectral lines must be available. In addition to the line positions, intensity infomiation is also required. 

Specinfo has an additional tool for calculating NMR spectra that is based on the data sets of the compounds contained in the database. This leads to quite reliable calculated spectral parameters for the compound classes which are registered in the database. 

Several empirical approaches for NMR spectra prediction are based on the availability of large NMR spectral databases. By using special methods for encoding substructures that correspond to particular parts of the NMR spectrum, the correlation of substructures and partial spectra can be modeled. Substructures can be encoded by using the additive model greatly developed by Pretsch [11] and Clerc [12]. The authors represented skeleton structures and substituents by individual codes and calculation rules. A more general additive model was introduced... 

Accuracy The accuracy of a fluorescence method is generally 1-5% when spectral and chemical interferences are insignificant. Accuracy is limited by the same types of problems affecting other spectroscopic methods. In addition, accuracy is affected by interferences influencing the fluorescent quantum yield. The accuracy of phosphorescence is somewhat greater than that for fluorescence. 

Chiral separations present special problems for vaUdation. Typically, in the absence of spectroscopic confirmation (eg, mass spectral or infrared data), conventional separations are vaUdated by analysing "pure" samples under identical chromatographic conditions. Often, two or more chromatographic stationary phases, which are known to interact with the analyte through different retention mechanisms, are used. If the pure sample and the unknown have identical retention times under each set of conditions, the identity of the unknown is assumed to be the same as the pure sample. However, often the chiral separation that is obtained with one type of column may not be achievable with any other type of chiral stationary phase. In addition, "pure" enantiomers are generally not available. 

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