Spectrum characteristic

When samples are heated, they emit infrared radiation with a characteristic spectrum. The IR emission of ceramics, coals, and other complicated solids and thin films can be studied. Also, if conditions make it difficult to use an infrared source... 

Approximately 70 different elements are routinely determined using ICP-OES. Detection limits are typically in the sub-part-per-billion (sub-ppb) to 0.1 part-per-million (ppm) range. ICP-OES is most commonly used for bulk analysis of liquid samples or solids dissolved in liquids. Special sample introduction techniques, such as spark discharge or laser ablation, allow the analysis of surfaces or thin films. Each element emits a characteristic spectrum in the ultraviolet and visible region. The light intensity at one of the characteristic wavelengths is proportional to the concentration of that element in the sample. 

More complicated molecules, with two or more chemical bonds, have more complicated absorption spectra. However, each molecule has such a characteristic spectrum that the spectrum can be used to detect the presence of that particular molecular substance. Figure 14-17, for example, shows the absorptions shown by liquid carbon tetrachloride, CCfi, and by liquid carbon disulfide, CS2. The bottom spectrum is that displayed by liquid CC14 containing a small amount of C. The absorptions of CS2 are evident in the spectrum of the mixture, so the infrared spectrum can be used to detect the impurity and to measure its concentration. 

The lines in the spectrum from any element always occur in the same positions relative to each other. When sufficient amounts of several elements are present in the source of radiation, each emits its characteristic spectrum this is the basis for qualitative analysis by the spectrochemical method. It is not necessary to examine and identify all the lines in the spectrum, because the strongest lines will be present in definite positions, and they serve to identify unequivocally the presence of the corresponding element. As the quantity of the element in the source is reduced, these lines are the last to disappear from the spectrum they have therefore been called the persistent lines or the rates ultimes (R.U. lines), and simplify greatly the qualitative examination of spectra. 

In a similar system, the reaction of the ferric(edta) complex with peroxycarboxylic acids was probed by adding 2,4,6-tri-fe/r-butyl phenol, ArOH.2 This experiment gave rise to the aryloxyl radical, ArO, which persisted for hours and was detected by its characteristic spectrum. It was indeed formed in the reaction mentioned, at a rate that was independent of [ArOH], It was proposed that ArO results from a reactive oxo-iron intermediate, tentatively (edta)FevO. 

A detailed mechanistic study of the interactions of NO and N02 with MbFeIV = 0 (an analog of compound II above) was recently conducted by Herold and Rehmann (141). These authors demonstrated that MbFeIV= 0 reacts rapidly with NO to form an intermediate possessing a characteristics spectrum, consistent with the formation of the ferric nitrito complex MbFem(ONO). This reaction has analogy with the reactions of NO with the CrIV = O complex described above (112). 

Electronic spectra of metalloproteins find their origins in (i) internal ligand absorption bands, such as n->n electronic transitions in porphyrins (ii) transitions associated entirely with metal orbitals (d-d transitions) (iii) charge-transfer bands between the ligand and the metal, such as the S ->Fe(II) and S ->Cu(II) charge-transfer bands seen in the optical spectra of Fe-S proteins and blue copper proteins, respectively. Figure 6.3a presents the characteristic spectrum of cytochrome c, one of the electron-transport haemoproteins of the mitochondrial... 

The mixture of quinone methides initially formed by combination of the coniferyl radicals in their various mcsomeric forms, i.e. (I), (III), (V), (IX) and others, can be detected by means of their characteristic spectrum with a maximum at about 312 mp (52) the haU-hfe of the mixture in 70 % aqueous dioxan is 1 hour. Those quinone methides that can rearomatize by keto-enol tautomerism, e.g. (IX), or intramolecular additions, e.g. (I) or (III) may become stabilized faster than those of type (V) which rely on addition of a foreign molecule. The quinone methides that rearomatize intramolecularly appear to react exclusively in this way, probably by a concerted mechanism that represents collapse of the activated transition state. 

In a laboratory generator, electrons are accelerated by a potential around 30 kV towards a solid target, where they are stopped by impact. The output contains the line spectmm superimposed upon a continuous spectrum. The line, or characteristic spectrum is characteristic of the element and is used in X-ray fluorescent analysis to identity the type and amount of an element present in a sample. The continuous radiation is also called the Bremsstrahlung, from the... 

David Alter observes that each element has a characteristic spectrum. 

The iodine was removed from the reaction mixtures by vacuum sublimation and subsequently identified by its characteristic spectrum in pyridine solution. A solution of the sublimate from these reaction products gave an absorption maximum at 369 m/jL, while a solution prepared by dissolving elemental iodine in pyridine produced an absorption band of similar shape with a maximum at 370 m. ... 

Although gross outliers should be deleted in advance (see the initial paragraphs in Section 4.4), it is likely to occur that some sample with some particular characteristic (spectrum, noise, etc.) shows a suspicious or different behaviour. Two plots inspecting the space of the spectra and the relationship of the X-space with the Y-space will be useful here. The X-spectral space is searched for by the z-z scores plot". Here the scores of the second, third, fourth, etc.. 

Figure 3. Frequency spectra obtained by the Fourier analysis of the progressions in Fig. 2 Curve (b) shows the characteristic spectrum of an anharmonic progression while curve (a) is obviously perturbed [7],...

CD spectra determined under various conditions can be extremely informative on the structure and stability of coiled coils. Since there is only one type of secondary structure (i.e., the a-helix) present in coiled coils, interpretation of the CD spectrum is straightforward. A stable coiled coil should be 100% a-helical in benign conditions (0.1 M KC1, 50 mM phosphate, pH 7) and show the characteristic spectrum with maximum ellipticity at 208 and 222 nm, and with the ratio around >1.0 (1.0-1.08).149 541 The percentage a-helix... 

An additional product of these studies was the characterization of many reduced species which have never before been produced or investigated. It may be concluded that the discovery of the solvated electron with its characteristic spectrum of absorption is one of the great achievements in chemistry of the last few decades—not only because it demonstrated the existence of a most elementary chemical species, but also because it firmly establishes the role of electron transfer in chemical reactivity. 

If A and A have a characteristic spectrum, usually a UV absorbance, the time course of A + A can be followed, and its maximum or minimum, as in Equation (25), can be measured. The experimentally obtained quantity on the right-hand side of Equation (26) can then be solved numerically to evaluate a. 

The symmetry is lower in pentacoordinated complexes than in octahedral or tetrahedral geometries, and the spectrum is composed of several widely separated bands. High-spin pentacoordinated Co(II) complexes have absorption bands with intensities intermediate between those of tetrahedral and octahedral species. A characteristic spectrum of a complex with trigonal bipyramidal symmetry is given in Fig. 3. Further examples can be found in a recent review by Ciampolini (13) and in references therein. Ciampolini points out that the band around 16 kK is rather insensitive to field strength and that the band around 12 kK is split in compounds with lower symmetry (21). 

---