Characteristic wavelength

Different light-absorbing groups, called chromophores, absorb characteristic wavelengths, opening the possibility of qualitative analysis based on the location of an absorption peak. 

If there is no band overlap in a spectrum, the absorbance at a characteristic wavelength is proportional to the concentration of chromophores present. This is the basis of quantitative analysis using spectra. With band overlap, things are more complicated but still possible. 

Many sources of energy are used to excite samples to emit characteristic wavelengths for chemical identification and assay (91,92). Very high temperature sources can be employed but are not necessary. AH materials can be vaporized and excited with temperatures of only a few electron volts. The introduction of samples to be analyzed into high temperature or high density plasmas and thek uniform excitation often are problematic. 

In Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), a gaseous, solid (as fine particles), or liquid (as an aerosol) sample is directed into the center of a gaseous plasma. The sample is vaporized, atomized, and partially ionized in the plasma. Atoms and ions are excited and emit light at characteristic wavelengths in the ultraviolet or visible region of the spectrum. The emission line intensities are proportional to the concentration of each element in the sample. A grating spectrometer is used for either simultaneous or sequential multielement analysis. The concentration of each element is determined from measured intensities via calibration with standards. 

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. 

Assuming that A << /o and that /o varies appreciably only over distances x L, it is easy to show that A//o —XjL, where A is the mean free path length i.e. /o is a good approximation if the characteristic wavelengths of p, T and u are all much greater than the mean free path. The exact solution / can then be expanded in powers of the factor X/L. This systematic expansion is called the CAia.pma.n-Enskog expansion, and is the subject of the next section. 

D. gigas AOR was the first Mo-pterin-containing protein whose 3D structure was solved. From D. desulfuricans, a homologous AOR (MOD) was purified, characterized, and crystallized. Both proteins are homodimers with-100 kDa subunits and contain one Mo-pterin site (MCD-cofactor) and two [2Fe-2S] clusters. Flavin moieties are not found. The visible absorption spectrum of the proteins (absorption wavelengths, extinction coefficients, and optical ratios at characteristic wavelengths) are similar to those observed for the deflavo-forms of... 

Electron microscopy is a rather straightforward technique to determine the size and shape of supported particles [S. Amelinckx, D. van Dyck, J. van Landuyt and G. van Tendeloo, Handbook of Microscopy (1997), VCH, Weinheim]. Electrons have characteristic wavelengths of less than 1 A, and come close to monitoring atomic detail. Figure 4.13 summarizes what happens when a primary electron beam of energy between 100 and 400 keV hits a sample ... 

The spectral pattern associated with principal component 1 is presented in Fig. 8. It provided the characteristic wavelengths which were the most discriminant to separate the... 

The spectrometer is set to the appropriate Bragg angle 0 of the requisite characteristic wavelength, and only these X-rays will reach the detector and be counted. The detector employed is the gas proportional counter, which can operate at much faster count rates than the EDS crystal detector. 

Second, the technology has mediocre reproducibility. Software is available to morph images so that spots can be lined up such software is expensive, difficult to use, and not always accurate in its alignment. To overcome this problem and to simplify quantitative comparisons between samples, Unlu et al. (1997) developed differential gel electrophoresis (DIGE), where two samples are each labeled with different fluorescent tags, pooled, separated on the same gel, and scanned at characteristic wavelengths to resolve the components. This technology has been commercialized by Amersham. 

Several structure sizes caused by microphase separation occurring in the induction period as well as by crystallization were determined as a function of annealing time in order to determine how crystallization proceeds [9,18]. The characteristic wavelength A = 27r/Qm was obtained from the peak positions Qm of SAXS while the average size of the dense domains, probably having a liquid crystalline nematic structure as will be explained later, was estimated as follows. The dense domain size >i for the early stage of SD was calculated from the spatial density correlation function y(r) defined by Debye and Buche[50]... 

The results for the glass crystallization of PET annealed at 80 °C as before are shown in Fig. 8. In the early stage of spinodal decomposition up to 20 min, the characteristic wavelength A remains constant at a value of 15 nm, which agrees with the theoretical expectation that only the amplitude of density fluctuations increases whilst keeping a constant characteristic wavelength. In the late stage from 20 to 100 min it increases up to 21 nm just before crystallization. Such a time dependence of A in nm can be represented by... 

Fig. 8 Various structure parameters appearing in the crystallization process of PET at 80 °C A, characteristic wavelength of SD D, dense domain size L, long period /p, persistence length Dc, lamellar stem length [19]...

Fig. 9 Spinodal decomposition model for glass crystallization at a low temperature just above Tg. The indicated structure parameters mean (A), the average characteristic wavelength (D)y the average size of the dense domain (L), the average long period after crystallization. The numerical values are for PET crystallized at 80 °C...

This problem is very important, but it is extremely difficult to make SAXS measurements at higher temperatures because the induction period becomes too short to observe the time evolution of SAXS intensities. For example, as was seen in Sect. 2.2, the induction period was only 100 s when the PET glass was crystallized even at 115 °C, 40 K higher than Tg, where a detailed analysis of the SAXS data was impossible. Of course, as the crystallization temperature approaches the melting temperature, the induction period is expected to become longer. However, as will be shown below, no characteristic peaks of SD could be detected in SAXS curves either. This is probably because the crystallization temperature was not in the unstable state, or the characteristic wavelength was much larger compared with the lower resolution limit of... 

Fig.28 Characteristic wavelength A as a function of quenching or jumping-up temperature Tx [16]. ( ) quenched from the molten state, (A) jumped up from the glassy state. The dotted line represents a fitting curve with Eq. 14 for the melt quenching...

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