Ultraviolet/visible spectroscopy selectivity

Several spectroscopic techniques, namely, Ultraviolet-Visible Spectroscopy (UV-Vis), Infrared (IR), Nuclear Magnetic Resonance (NMR), etc., have been used for understanding the mechanism of solvent-extraction processes and identification of extracted species. Berthon et al. reviewed the use of NMR techniques in solvent-extraction studies for monoamides, malonamides, picolinamides, and TBP (116, 117). NMR spectroscopy was used as a tool to identify the structural parameters that control selectivity and efficiency of extraction of metal ions. 13C NMR relaxation-time data were used to determine the distances between the carbon atoms of the monoamide ligands and the actinides centers. The II, 2H, and 13C NMR spectra analysis of the solvent organic phases indicated malonamide dimer formation at low concentrations. However, at higher ligand concentrations, micelle formation was observed. NMR studies were also used to understand nitric acid extraction mechanisms. Before obtaining conformational information from 13C relaxation times, the stoichiometries of the... 

Despite these apparent limitations, fluorescence methods are employed for the determination of a wide variety of compounds. The selectivity of these analyses arises from the choice of both excitation and emission wavelengths, whereas the sensitivity of the analyses arises from the fact that absolute as opposed to relative measurements of light emission are made. This can be compared to ultraviolet-visible spectroscopy, where the ratio of incident to transmitted light is determined. Fluorescence measurements also have the advantage of a wide linear range of analysis. 

There have been relatively little ultraviolet-visible (UV-Vis) spectroscopic data for 1,4-oxazines, but selected data are presented in Table 8. UV spectroscopy is important for photochromic compounds, such as spirooxazines. The UV spectra of 33 spirooxazines in five different solvents are collected in a review <2002RCR893>, and the more recently reported examples of photochromic oxazines 65, 66, 101, and 102 are shown here. It can be seen from Table 8 that both adding methoxy substituents to the oxazine and changing to a more polar solvent give a UV maximum at a higher wavelength. This solvent effect can also be seen in the case of 102, which also has important fluorescence properties, discussed in Section 8.06.12.2. 

In principle, absorption spectroscopy techniques can be used to characterize radicals. The key issues are the sensitivity of the method, the concentrations of radicals that are produced, and the molar absorptivities of the radicals. High-energy electron beams in pulse radiolysis and ultraviolet-visible (UV-vis) light from lasers can produce relatively high radical concentrations in the 1-10 x 10 M range, and UV-vis spectroscopy is possible with sensitive photomultipliers. A compilation of absorption spectra for radicals contains many examples. Infrared (IR) spectroscopy can be used for select cases, such as carbonyl-containing radicals, but it is less useful than UV-vis spectroscopy. Time-resolved absorption spectroscopy is used for direct kinetic smdies. Dynamic ESR spectroscopy also can be employed for kinetic studies, and this was the most important kinetic method available for reactions... 

Valuable spectroscopic studies on the dithiolene chelated to Mo in various enzymes have been enhanced by the knowledge of the structure from X-ray diffraction. Plagued by interference of prosthetic groups—heme, flavin, iron-sulfur clusters—the majority of information has been gleaned from the DMSO reductase system. The spectroscopic tools of X-ray absorption spectroscopy (XAS), electronic ultraviolet/visible (UV/vis) spectroscopy, resonance Raman (RR), MCD, and various electron paramagnetic resonance techniques [EPR, electron spin echo envelope modulation (ESEEM), and electron nuclear double resonance (ENDOR)] have been particularly effective probes of the metal site. Of these, only MCD and RR have detected features attributable to the dithiolene unit. Selected results from a variety of studies are presented below, chosen because their focus is the Mo-dithiolene unit and organized according to method rather than to enzyme or type of active site. 

The terms specificity and selectivity are often interchangeably used. A method is said to be specific if it provides a response for only a single analyte. If the response in question is distinguished from all other responses, the method is said to be selective. The International Conference on Harmonization (ICH)-2 does not differentiate both terms and defines specificity or selectivity as the ability to unambiguously determine the analyte in the presence of other components whose presence is to be expected. This includes typical impurities, decomposition products, and matrix components. The specificity of the method for TLC analyses was proved by identification and purity checks of the analyte spots. This can be done by measuring in situ the ultraviolet and visible spectroscopy (UV-VIS) spectra of... 

Rapid-scanning spectroscopy (RSS) is a method in which a selected portion of the ultraviolet, visible, or near-infrared spectrum is scanned on a time scale ranging from several sec to a few /isec. The applications of this technique to systems in which short-lived transient species exist or large reaction rates are encountered are numerous... 

Klebsiella pneumonia KYCC 4352, co//ATCC10536, and . aureus ATCC 6538 were the three different bacteria with gram-postive and gram-negative characteristics that were selected to determine the antimicrobial activity of nanofibrous webs made from PVP/ CTAB salt. A detailed chemical characterization was performed on the produced nanofibrous webs by carrying out Fourier transform infrared spectroscopy (FTIR], ultraviolet visible (UV-Vis], and electrochemical impedance spectroscopy (EIS] measurements. ... 

The derivatization of analytes is very important in several branches of analytical chemistry. It expands the fields of application of various spectroscopic techniques (ultraviolet-visible (UV-vis), fluorimetry, nuclear magnetic resonance (NMR), and mass spectroscopies), and in several cases increases also the selectivity and sensitivity of these techniques. Derivatization is also an inevitable tool in all chromatographic and electrophoretic techniques. In gas chromatography (GC), the main importance of derivatization is the improvement of the volatility/thermal stability of the analytes, and in all of the discussed separation techniques it has the potential of increasing the selectivity of the separation (including enantiomeric separations) and the sensitivity of the detection. 

In a large portion of routine and discovery-oriented analyses, mass spectrometry (MS) is used as a qualitative technique. The obtained qualitative data enable detection and structural elucidation of molecules present in the analyzed samples. However, modern chemistry and biochemistry heavily rely on quantitative information. In biochemistry it is often sufficient to conduct quantification of analytes in biofluids every few hours, days, or even weeks. In the real-time monitoring of highly dynamic samples, it is necessary to collect data points at higher frequencies. When it comes to selection of techniques for quantitative analyses, especially in the monitoring of dynamic samples, MS has not generally been favored. In fact, the performance of MS in quantitative analysis is worse than that of optical spectroscopies - especially, ultraviolet-visible (UV-Vis) absorption and fluorescence spectroscopy. 

Ultraviolet Visible (UV vis) Spectroscopy. The UV absorption spectra of both the SBDC photoiniferter and methacrylic acid were measured with a Cary 1 UV—vis spectrophotometer (Varian, Germany). The obtained spectra were taken as a reference for the selection of the spectral UV range in order to ensure effective initiation while avoiding polymerization of the monomer in solution. 

In absorption spectroscopy a beam of electromagnetic radiation passes through a sample. Much of the radiation is transmitted without a loss in intensity. At selected frequencies, however, the radiation s intensity is attenuated. This process of attenuation is called absorption. Two general requirements must be met if an analyte is to absorb electromagnetic radiation. The first requirement is that there must be a mechanism by which the radiation s electric field or magnetic field interacts with the analyte. For ultraviolet and visible radiation, this interaction involves the electronic energy of valence electrons. A chemical bond s vibrational energy is altered by the absorbance of infrared radiation. A more detailed treatment of this interaction, and its importance in deter-... 

In the literature, fluorescence spectroscopy in OFD has been limited to the use of ultraviolet (UV) or visible dyes as molecular probes.(1) The most common fluorescent dye used in OFD is fluorescein and its derivatives/21 23) Fluorescein possesses a good fluorescence quantum yield and is commercially available with an isothiocyanate functionality for linking to the polymeric support/24-26 Additionally, selective laser excitation can be performed because the absorbance maximum of fluorescein coincides with the 499-nm laser line emitted from an argon laser. Unfortunately, argon lasers are costly and bulky, thus limiting the practicality of their use. Similar difficulties exist with other popular commercial dyes. 

Sources. The ultimate source for spectroscopic studies is one that is intense and monochromatic but tunable, so that no dispersion device is needed. Microwave sonrces such as klystrons and Gnnn diodes meet these requirements for rotational spectroscopy, and lasers can be similarly nsed for selected regions in the infrared and for much of the visible-ultraviolet regions. In the 500 to 4000 cm infrared region, solid-state diode and F-center lasers allow scans over 50 to 300 cm regions at very high resolution (<0.001 cm ), but these sources are still quite expensive and nontrival to operate. This is less trne... 

Infrared spectroscopy is a less satisfactory tool for quantitative analyses than its ultraviolet and visible counterparts because of lower sensitivity and frequent deviations from Beer s law. Additionally, infrared absorbance measurements are considerably less precise. Nevertheless, in instances where modest precision is adequate, the unique nature of infrared spectra provides a degree of selectivity in a quantitative measurement that may offset these undesirable characteristics. ... 

In the columns identifying the experimental method, MW stands for any method studying the pure rotational spectrum of a molecule except for rotational Raman spectroscopy marked by the rot. Raman entry. FUR stands for Fourier transform infhired spectroscopy, IR laser for any infiured laser system (diode laser, difference frequency laser or other). LIF indicates laser induced fluorescence usually in the visible or ultraviolet region of the spectrum, joint marks a few selected cases where spectroscopic and diffraction data were used to determine the molecular structure. A method enclosed in parentheses means that the structure has been derived from data that were collected by this method in earlier publications. The type of structure determined is shown by the symbols identifying the various methods discussed in section II. V/ refers to determinations using the Kraitchman/Chutjian expressions or least squares methods fitting only isotopic differences of principal or planar moments (with or without first... 

Absorption spectroscopy based on ultraviolet and visible radiation is one of the must useful tools available to the scientist for quantitative analysis. Important characteristics of spectrophotomctric and photometric methtxls include (1) wide applicability to both organic and inorganic systems. (2) typical detection limits of 10 to 10 M (in some cases, certain modifications can lead to lower limits of detection), (. ) moderate to high selectivity. (4) good accuracy (typically, relative uncertainties arc I % to. V /o. although with special precautions, errors can be reduced to a few tenths of a percent), and (5) case and convenience of data acquisition. 

On the other hand, spectroscopy in the ultraviolet or visible is a very fast, simple, and photometrically exact method. Because of the broad bands in this wavelength region of electronic spectroscopy, however, the signals obtained give a summation over the concentrations of all the reactants in the solution. Therefore requirement (3) cannot be fulfilled except in special cases. On the other hand, on-line analysis can be easily achieved. The lack in selectivity is overcome by two approaches ... 

The selection rules help to predict the probability of a transition but are not always strictly followed. If the transition obeys the rules it is allowed, otherwise it is forbidden. A molecule can become excited in a variety of ways, corresponding to absorption in different regions of the spectrum. Thus certain properties of the radiation that emerges from the sample are measured. The fraction of the incident radiation absorbed or dissipated by the sample is measured in optical (ultraviolet and visible) absorption spectroscopy and some modes of nuclear magnetic resonance spectrometry (NMR). Because the relative positions of the energy levels depend characteristically on the molecular structure, absorption spectra provide subtle tools for structural investigation. 

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