Absorption of UV-vis radiation

Environmental Applications Methods for the analysis of waters and wastewaters relying on the absorption of UV/Vis radiation are among some of the most frequently employed analytical methods. Many of these methods are outlined in Table 10.6, and a few are described later in more detail. 

Detectors Most of the detectors used in HPLC also find use in capillary electrophoresis. Among the more common detectors are those based on the absorption of UV/Vis radiation, fluorescence, conductivity, amperometry, and mass spectrometry. Whenever possible, detection is done on-column before the solutes elute from the capillary tube and additional band broadening occurs. 

Both emission and absorption spectra are affected in a complex way by variations in atomisation temperature. The means of excitation contributes to the complexity of the spectra. Thermal excitation by flames (1500-3000 K) only results in a limited number of lines and simple spectra. Higher temperatures increase the total atom population of the flame, and thus the sensitivity. With certain elements, however, the increase in atom population is more than offset by the loss of atoms as a result of ionisation. Temperature also determines the relative number of excited and unexcited atoms in a source. The number of unexcited atoms in a typical flame exceeds the number of excited ones by a factor of 103 to 1010 or more. At higher temperatures (up to 10 000 K), in plasmas and electrical discharges, more complex spectra result, owing to the excitation to more and higher levels, and contributions of ionised species. On the other hand, atomic absorption and atomic fluorescence spectrometry, which require excitation by absorption of UV/VIS radiation, mainly involve resonance transitions, and result in very simple spectra. 

Absorption of UV/VIS radiation in the solid state is different from UV/VIS absorption in the liquid or gaseous phase with respect to photophysical processes taking place in the crystal lattice and to the metallic, semiconductor (SC) or insulator properties of the absorbing solid (Bottcher, 1991). In crystals, multiple atomic or molecular orbitals are combined to form broad energy bands, i.e. a valence band (vb) fully occupied by electrons and a conduction band (cb) unoccupied or only partly occupied by electrons. Conduction bands and valence bands have different energetic positions relative to one another depending on the specific substrate. In a SC cluster, electronic transitions between the valence band and the conduction... 

Absorption of UV /VIS radiation Atomic absorption spectrometry Quantitative elemental analysis... 

The absorption of UV-Vis radiation is caused by transfer of energy from the radiation beam to electrons that can be excited to higher energy orbitals. 

Absorption of UV/VIS radiation UV/VIS molecular absorption spectroscopy, spectrophotometry Qualitative and quantitative determinations of aromatic and unsaturated organic compounds, inciuding naturai products direct and indirect quantitative determination of inorganic ions, organic molecules, and biochemicals... 

Figure 5.27 (a) The only two types of transitions possible with the absorption of UV/VIS radiation. If a compound lacks 77 electrons, no absorption in that electromagnetic region is possible. (b) UV/VIS spectrum of acetone, showing examples of each. 

Photoscience covers a broad spectrum of interdisciplinary and interrelated subjects and it may be subdivided into photomedicine, photobiology, photochemistry and photophysics (Fig. 3-1). Photochemistry, in general, studies the reactions that occur through electronically excited states of molecules. Specifically, photochemistry studies the change of substance quality and characteristics by the influence of UV/VIS radiation. The mechanistic interpretation of the formation of photoproducts and their characterization and identification are typical domains of photochemistry. This research concept is strictly based on photophysics, which investigates the primary event of photon absorption by a molecule, the properties of electronically excited states and their deactivation mechanisms, such as for example fluorescence, phosphorescence and energy or electron transfer reactions, and non-... 

In photochemistry and photochemical technology, conditions with Aio 2 are usually designated as total absorbance, or in the past as total absorption (Braun et al., 1991) of UV/VIS radiation. Thus, for Aio 2 the transmittance T is less than 0.01. 

In catalyzed photolysis either the catalyst molecule (Fig. 5-11, situation B) or the substrate molecule (Fig. 5-11, situation C), or both, are in an electronically excited state during the catalytic step. The electronically excited catalyst molecule is produced via photon absorption by a nominal catalyst (Fig. 5-11, situation B). The reaction of substrate to product is catalytic with, respect to the concentration of the electronically excited catalyst species. It is non-catalytic in photons and therefore, continuous irradiation is required to maintain the catalytic cycle. The quantum yield of product formation Product is equal to or less than unity. Titanium dioxide photocatalysis is the most widely applied example of this type, with Ti02 representing the nominal catalyst that must be electronically excited by photon absorption with formation of the electron hole pair Ti02 (hvb + cb), being the active catalytic species (cf Fig. 3-17 and Fig. 5-9, reaction 1). The oxidation of substrates by the combination of UV/VIS radiation and an appropriate photocatalyst is often called photocatalytic oxidation (PCO). 

Absorption. A ground state molecule (So) may absorb a photon of UV-vis radiation, thus becoming an excited singlet state. The most commonly seen transitions are Sq Si or So S2, but So to higher excited singlet state transitions are also possible. 

Three factors influence the probability of absorption or emission of UV-vis radiation. One is based on symmetry considerations and the quantum mechanical formulation of transition moment integrals. If the initial and final... 

Electronic transitions are said to be vertical, meaning that absorption and emission of UV-vis radiation occurs with no movement of the nuclei, within the limits of the Bom-Oppenheimer approximation. Since most molecules are in the 0th vibrational level of the ground electronic state, the Franck-Condon term measures the probability of the transition from the 0th vibrational level of the ground state to each vibrational level of the excited state. A transition from the 0th vibrational level of So to the 0th vibrational level of Si is marked... 

In this chapter, the main optical techniques used to characterize PLA-based polymers are discussed in four sections (1) absorption and transmission of UV-Vis radiation, (2) index of refraction, (3) specific optical rotation, and (4) infrared and Raman spectroscopy and NMR. 

Section 13 21 Transitions between electronic energy levels involving electromagnetic radiation m the 200-800 nm range form the basis of UV VIS spec troscopy The absorption peaks tend to be broad but are often useful m indicating the presence of particular tt electron systems within a mole cule... 

Infrared instruments using a monochromator for wavelength selection are constructed using double-beam optics similar to that shown in Figure 10.26. Doublebeam optics are preferred over single-beam optics because the sources and detectors for infrared radiation are less stable than that for UV/Vis radiation. In addition, it is easier to correct for the absorption of infrared radiation by atmospheric CO2 and 1420 vapor when using double-beam optics. Resolutions of 1-3 cm are typical for most instruments. 

The determination of an analyte s concentration based on its absorption of ultraviolet or visible radiation is one of the most frequently encountered quantitative analytical methods. One reason for its popularity is that many organic and inorganic compounds have strong absorption bands in the UV/Vis region of the electromagnetic spectrum. In addition, analytes that do not absorb UV/Vis radiation, or that absorb such radiation only weakly, frequently can be chemically coupled to a species that does. For example, nonabsorbing solutions of Pb + can be reacted with dithizone to form the red Pb-dithizonate complex. An additional advantage to UV/Vis absorption is that in most cases it is relatively easy to adjust experimental and instrumental conditions so that Beer s law is obeyed. 

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