Overlapping spectra

This experiment demonstrates the chemical limitations to Beer s law using the NO2-N2O4 equilibrium as an example. Blanco, M. Iturriaga, H. Maspoch, S. et al. A Simple Method for Spectrophotometric Determination of Two-Components with Overlapped Spectra, /. Chem. Educ. 1989, 66, 178-180. 

Blanco and co-workers" reported several examples of the application of multiwavelength linear regression analysis for the simultaneous determination of mixtures containing two components with overlapping spectra. For each of the following, determine the molar concentration of each analyte in the mixture. 

MS is a means of examining a compound, also in the gas phase, so that its stmcture or identity can be deduced from its mass spectrum. MS alone is not good for examining mixtures because the mass spectrum of a mixture is actually a complex of overlapping spectra from the individual components in the mixture. 

Modern instruments capable of obtaining excitation-emission matrices (EEMs) allow use of new data-analysis techniques to resolve overlapped spectra. Resolution techniques such as the ratio method (28) and others (29,30) may provide further differentiation of the components present in the phases separated by solvent extraction. 

The identification of xanthophylls in vivo is a complex task and should be approached gradually with the increasing complexity of the sample. In the case of the antenna xanthophylls, the simplest sample is the isolated LHCII complex. Even here four xanthophylls are present, each having at least three major absorption transitions, 0-0, 0-1, and 0-2 (Figure 7.4). Heterogeneity in the xanthophyll environment and overlap with the chlorophyll absorption add additional complexity to the identification task. No single spectroscopic method seems suitable to resolve the overlapping spectra. However, the combination of two spectroscopic techniques, low-temperature absorption and resonance Raman spectroscopy, has proved to be fruitful (Ruban et al., 2001 Robert et al., 2004). 

Biological fluorescent dyes and FPs, although indispensable tools of modern biomedical research suffer from several drawbacks. Particularly, they are prone to photobleaching, which is a problem for quantitative imaging and FRET-FLIM experiments [36], and they tend to have broad overlapping spectra and generally do not have a wide range of lifetimes. 

There may, however, be specific reasons to study a signal over an extended temperature range. For one, a linear increase in EPR amplitude with the inverse of the temperature (Curie s law) is proof that a spin system is a two-level system, i.e., an S = 1/2 or an effective S = 1/2 system. More importantly, in complex multicenter metalloproteins, overlapping spectra may be deconvoluted by virtue of their Tu value being different if two centers, a and b, have rMa < TMb then at TMb the spectrum of center a is broadened and that of center b is not. It is once more emphasized that these types of studies require determination of (anisotropic) saturation behavior at all relevant temperatures. 

Figure 4 shows the size distribution of these ultrafine particles at low humidities being shifted up in size by increasing SOp concentration. There is an overlap of spectra at 11 ppb and 110 ppb of SOp concentrations. Figure 5 reveals the same results at higher humidities, but the overlapped spectra were separated with a smaller size distribution at 110 ppb in the the presence of additional water vapor. We also obtained two different size distributions at 1.1 ppm in two sets of experiments. Thus, there are still some uncertainties in understanding the size distribution of ultrafine particles. 

The s.a.n.s. experiments were carried out using the D17 camera at the I.L.L., Grenoble. Data were collected at two wavelengths, 0.8 and 1.4 nm at a sample to detector distance of 1.8 m. The overlapping spectra were combined to give a sufficiently wide Q range to enable the data to be numerically inverted to obtain the density distributions. The latex dispersions were prepared at a solids concentration of 4% and polymer solution concentrations between 200 and 300 ppm. 

When the perester [31] was used as a source of succinimidyl radicals in the presence of MNP, the 27-line spectrum of nitroxide [32] was recorded (Chalfont et al., 1970). This nitroxide was the only species detected when a chlorobenzene solution of [31] and MNP was heated and then examined spectroscopically only after it had been allowed to cool. When, instead, a reaction mixture was heated in the spectrometer cavity, overlapping spectra of... 

A fluorescence spectrum may result from overlapping spectra of several fluorescent species (or several forms of a fluorescent species). If each of them is characterized by a single lifetime, it is possible to decompose the overall spectrum into its components. 

Fort he determination of preservatives and sweeteners in soft drinks or fruit juices LC analysis with UV detection is widely used. The sample pretreatment, prior to LC analysis, often consists only of degassing, filtration and dilution of the Uqirid [2]. Sometimes a Uqirid-Uqitid extraction, suitable not only for soft drinks but also for more complex matrices, is appUed [3]. Chemometric methods appUed to overlapped spectra offer the advantage of minimizing or eliminating sample preparation by allowing to simirltaneoirsly determining one or more analytes in relatively complex matrices. 

It is important to point out that D. vulgaris hydrogenase contains three multinuclear iron clusters and each cluster may exist in equilibrium between two different oxidation states in each sample. Consequently, the raw Mossbauer spectra are complex, consisting of overlapping spectra originating from different iron sites of these various clusters. For clarity, we present only the deconvoluted spectra of the H cluster. These spectra were prepared by removing the contributions of other iron species from the raw spectra. Details of the analysis are available (Pereira et al. 2001). 

Haaland and coworkers (5) discussed other problems with classical least-squares (CLS) and its performance relative to partial least-squares (PLS) and factor analysis (in the form of principal component regression). One of the disadvantages of CLS is that interferences from overlapping spectra are not handled well, and all the components in a sample must be included for a good analysis. For a material such as coal LTA, this is a significant limitation. 

Despite the lack of inherent selectivity, it is still possible to obtain good quantitative data from online UV/vis monitoring by making use of chemometric techniques to resolve the overlapping spectra. The most common application is in dissolution testing [73, 74], where results that are at least as accurate as those of the reference (and much slower and more costly) HPLC method have been demonstrated. 

A complication is introduced for systems that readily lose protons, or have two different coincident species overlapping in the same m/e region. In this case, deconvolution of the overlapping spectra becomes necessary. Once begun, however, deconvolution must be carried to completion. As shown in Table V, for B3N3H6 serious errors can occur if... 

Coherent Raman detected EPR spectroscopy is a new experimental method which combines optical and EPR transitions. It was applied for the first time to a bioinolecule, azurin. The information obtained allows to test electronic and structural models of the Cu site to be probed.23 In a subsequent report the same group of authors has applied microwave modulated circular dichroism to unravel individual resonance lines in overlapping spectra and to assign them to specific electronic transitions.24... 

First- and second-order derivatives are commonly applied by spectroscopists to tackle constant offsets (e.g. drift in the baseline for which the first derivative is sufficient) or proportional drift and/or scatter effects (which need application of the second derivative). Further, as is known, inflection points (also termed "spectra shoulders ) in the spectral peaks suggest that several species yield overlapped spectra. The place where a first-derivative plot intercepts the abscissa indicates where an inflection point occurs in the original spectra. On the other hand, the second derivative will show a minimum whose position denotes the inflection point on the original peak. 

Allied with the diffraction methods, such as low-energy electron diffraction (LEED) and photoelectron diffraction (PED), which can also be applied in single-crystal research, these advances have led to much better interpretations of the vibrational spectra of chemisorbed hydrocarbons in terms of the structures of the surface species. The new results have in turn led to the possibility of reassessing more reliably earlier interpretations of the infrared or Raman spectra of adsorbed hydrocarbons on the finely divided metal samples (usually oxide supported) that are more closely related to working solid catalysts. Such spectra are more complicated because of the occurrence of a variety of different adsorption sites on the metal particles, with the consequence that the observed pattern of absorption bands frequently arises from overlapping spectra from several different surface species. 

Figure 19-2 Visible spectra of 1.32 mM TI(IV), 1.89 mM V(V), and an unknown mixture containing both ions. All solutions contain 0.5 wt% H202 and 0.01 M H2S04. Absorbance at points shown by dots is listed in Figure 19-3. [From M. Blanco, H. Ilurrlaga, S. Maspoch, and R Tarfn, A Simple Method for Spectrophotometric Determination ot Two Components with Overlapped Spectra," J. Chem. Ed. 1989,66, 178.]...

Visible spectrum of Mn04, Cr2Of, and an unknown mixture containing both ions. [From M. Blanco, H. Iturriaga, S. Maspoch, and R Tarln, A Simple Method for Spectrophotometrlc Determination of Two-Components with Overlapped Spectra,"... 

As the removal of a proton from thymine results in the establishment of a tautomeric equilibrium between its two monoanionic forms,3425 the emission spectrum of singly ionized thymine may consist of overlapping spectra of both monoanions. In fact, Gill344 has observed some inconsistencies between the absorption and the fluorescence excitation spectra of thymine in 0.01 N NaOH at room temperature. These inconsistencies were of the same kind as those found later by Berens and Wierzchowski,345 who suggested that at room temperature only the thymine monoanion tautomer (34) fluoresced, while at 77°K emissions of both monoanionic species contributed to the observed luminescence spectrum. 

Chemical interferences may be produced by overlapping spectra of different elements or as the result of x-ray absorption or enhancement. Either effect is common in soil contaminated with heavy metals. Chemical interferences may be substantially reduced through a mathematical correction, but they cannot be completely eliminated. Other factors that affect the accuracy of XRF analysis are the instrument settings and the operator technique, especially in in situ measurements. A correlation of XRF results with laboratory analysis by other analytic techniques should be always established in the early stages of the project implementation and confirmed, if changes in the nature of soil samples have been observed. 

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