UV-Vis absorbance

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. 

Solutes that do not absorb UV/Vis radiation or undergo fluorescence can be detected by other detectors. Table 12.8 provides a list of detectors used in capillary electrophoresis along with some of their important characteristics. 

FAAS Flame atomic absorption spectroscopy the flame atomizes metals in solutions. Once in the gas phase, the atoms absorb UV-vis light, exciting electrons to higher energy levels. The amount of light absorbed is used to determine the metal concentration. 

The quantum yield 0 of a photophysical or photochemical event is a quantitative measure of the overall efficiency of this process (Braun et al., 1991). It is a unitless constant, which usually ranges from zero to one. However, some authors express 0 in units of mol einstein, which in fact is unit-less, because an einstein is defined as one mol of photons. Quantum yields greater than one indicate photo-induced chain reactions, which may involve radical species or photo-generated catalysis. Commonly used definitions of 0 are collected in Tab. 3-7. These definitions describe quantum yields of photophysical events and of photochemical reactions with regard to the reactant diminution or to the formation of the photoproduct Quantum yields may be dependent on the wavelength of the absorbed UV/VIS radiation, but many photochemical systems exist that have a constant quantum yield 0 over a defined wavelength range. Such chemical systems can be... 

Most neat plastic materials used for packaging of pharmaceutical products absorb UV-VIS radiation below 280 nm. Therefore, photostabilization by plastic packaging is only possible by the addition of colorants, UV absorbers, or pigments to the plastic mass. 

Table 5.2 Organic Functional Groups that can Absorb UV/VIS Radiation...

Now that we know how UV/Vis spectra are formed, we need to consider the types of substances that will absorb UV/Vis radiation and how we can identify them. It is true that, because we see coloured substances, they are absorbing or transmitting in the UV/Vis regions of the electromagnetic... 

The molecule A-H has an acidic proton as well as a r system that may absorb UV-vis radiation and remain intact after the deprotonation reaction. The... 

Irradiation of the a,6-unsaturated ketone 92 produced the dienol 93, which tautomerized to the 6,y-unsaturated ketone 94 (equation 12.69). The notable aspect of this reaction is that it converts a more stable, conjugated enone to a less stable, nonconjugated enone. The reactant, being more conjugated, absorbs UV-vis radiation at longer wavelength than does the product, so the reaction can be driven by radiation that the reactant absorbs but that the product does not absorb efficiently. 

In the previous section, we noted that a compound containing a conjugated it system will absorb UV-Vis light. The region of the molecule responsible for the absorption (the conjugated tt system) is called the chromophore, while the groups attached to the chromophore are called auxochromes. 

Discoloration is a serious commercial problem in polymers [14-15]. The two contributors are (a) impurities with non-homogeneity presents in poljnner from the manufacturing which absorb UV-VIS light [16] and (b) sacrificed or depleted consumption of stabilizers arises during life time... 

The analysis and identification of polymers with chromophores absorbing UV/Vis photons (e.g., carbonyl and aromatic groups). 

---