UV-Vis region

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. 

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

The scope of the present chapter will be exclusively concerned with investigations of diffuse reflectance work performed in the UV/VIS region of the spectrum, and with colors that can be perceived by the human eye. Much work has been conducted in the near-infrared region of the spectrum, but that aspect will be covered elsewhere in this book. 

The quinone diazide 2o was investigated using LFP in solution at room temperature.25 As with la, the carbene lo does not show characteristic absorptions in the UV /vis region and therefore could not be directly monitored during the LFP experiments. By extrapolation of quenching plots to zero quencher concentration the life-time of lo in Freon-113 was determined to 2.5 p,s, slightly longer than that of la with 1.65 ps. 

As discussed above, Li+ can be isolated from a solution containing other cations by complexation with specific macrocyclic ligands and subsequent extraction of the Li+-ligand complex into organic solvents. Complexation of the ligand alters the absorption profile in the UV-vis region and can therefore be followed spectrophotometrically. 

Any extract of soil containing large amounts of organic matter may have multiple absorption bands in the UV-Vis region of the spectrum. A UV-Vis spectrum of a simple aqueous, low-organic-matter soil extract is shown in Figure 14.2. As seen in this spectrum, even a simple soil extract can have... 

The intrinsic sensors are based on the direct recognition of the chemicals by its intrinsic optical activity, such as absorption or fluorescence in the UV/Vis/IR region. In these cases, no extra chemical is needed to generate the analytical signal. The detection can be a traditional spectrometer or coupled with fiber optics in those regions. Sensors have been developed for the detection of CO, C02 NOx, S02, H2S, NH3, non-saturated hydrocarbons, as well as solvent vapors in air using IR or NIR absorptions, or for the detection of indicator concentrations in the UV/ Vis region and fluorophores such as quinine, fluorescein, etc. 

The use of ultraviolet (UV) spectroscopy for on-line analysis is a relatively recent development. Previously, on-line analysis in the UV-visible (UV-vis) region of the electromagnetic spectrum was limited to visible light applications such as color measurement, or chemical concentration measurements made with filter photometers. Three advances of the past two decades have propelled UV spectroscopy into the realm of on-line measurement and opened up a variety of new applications for both on-line UV and visible spectroscopy. These advances are high-quality UV-grade optical fiber, sensitive and affordable array detectors, and chemometrics. 

Diffused reflectance spectra in the UV-VIS region were examined to characterize the structure of vanadium species supported on the AIPO -S surface. The value of the election-charge-transfer energy was reported to be strongly influenced by the number of ligands of the central vanadium ions and give information on the symmetry of the vanadium ions in the clusters [21,22]. 

The availability of solid-state detectors (such as the charge-coupled detector, CCD) makes it possible to acquire simultaneously significant portions of the spectra or even the entire rich spectra obtained by ICP-OES in the UV-Vis region, thus providing a large amount of data. The commercial availability of ICP-OES instruments with these multichannel detectors has significantly renewed interest in this technique. However, some limitations, such as the degradation of the spectral resolution compared with photomultiplier-based dispersive systems, still remain. 

NMR transitions correspond to wavelengths in the radio wave region of the spectrum, vibrational transitions correspond to wavelengths in the IR region, and electronic transitions to the UV-Vis region,... 

Detailed studies of the solvent dependencies of the charge-transfer transitions in the UV/Vis region of [Os(NH3)5(Mepz)]3+ and a variety of... 

Since all of the chalcogenometalates have strong and characteristic absorption bands in the UV-vis region, the reactions in which they are formed and decomposed can be readily followed by spectrophotometric methods. When hydrogen sulfide is passed into an aqueous solution of an oxometalate the electronic spectrum changes, the bands of all the species M04 S (n = 1-4 M = V, Mo, W, Re) appearing in succession (cf Figure 3 of Chapter 16.1). 

Chalcogenometalate ions are distinguished by characteristic high-intensity absorption bands in the UV-vis region but they can also be traced by IR and Raman spectroscopy. Total assignments and normal coordinate analyses are available for many of the species (also with recourse to isotope shifts, as in the case of MoOjSj-, 92/100MoOS2 or 92,100MoS4-). 

Electronic transitions in the UV-Vis region are commonly used to detect compounds eluting from a chromatographic system. Suppose you are looking for a particular compound in the eluent. It would help if you knew what its strongest electronic transitions were. Computational chemistry provides a way to estimate transitions in UV-Vis spectra of compounds even before they are isolated and characterized. 

Figure 16.11. DMPO-OH adduct EPR signal for following samples (a) Ti02, (b) soil HA, (c) peat HA, (d) aquatic HA, (e) peat FA, and (f) aquatic FA. All HS were purchased from IHSS and used at 50mg liter 1 of initial concentration. DMPO concentration at 20 mmol liter1 and 3 min of irradiation at 880mWcm 2 of light intensity in the UV-vis region. Reprinted...

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