Ultraviolet spectroscopy, determination

As the medium is still further diluted, until nitronium ion is not detectable, the second-order rate coefficient decreases by a factor of about 10 for each decrease of 10% in the concentration of the sulphuric acid (figs. 2.1, 2.3, 2.4). The active electrophile under these conditions is not molecular nitric acid because the variation in the rate is not similar to the correspondii chaise in the concentration of this species, determined by ultraviolet spectroscopy or measurements of the vapour pressure. " ... 

These methods are now obsolete in comparison with spectroscopic methods. Werbel has shown that the structures of these isomers are easily determined by NMR (125) (see also Table VI-5). Furthermore. 2-imino-4-thiazoline derivatives are characterized by their stretching C=N vibration at 1580 cm , absent in their 2-aminothiazole isomers, and by the stretching NH vibration that appears in the range of 3250 to 3310 cm for the former and between 3250 to 3340 cm" for the latter (131). Ultraviolet spectroscopy also differentiates these isomers (200). They can be separated by boiling in ethanol the thiazoline isomer is usually far less soluble in this solvent (131),... 

Quantitative determination is also possible by ultraviolet spectroscopy with the intense absorption at 320 nm (94). They may also be characterized electrochemically with a mercury electrode (95),... 

The pK s of some 2-substituted 4-hydroxythiazoles have been determined by ultraviolet spectroscopy (403) and by potentiometry (419). They range between 6.65 and 6-85 pK. units. 

The presence of iminium salts can be detected by chemical means or by spectroscopic methods. The chemical means of detecting iminium salts are reactions with nucleophiles and are the subject of this review. The spectroscopic methods are more useful for rapid identification because with the large number of model compounds available now the spectroscopic methods are fast and reliable. The two methods that are used primarily are infrared and nuclear magnetic resonance spectroscopy. Some attempts have been made to determine the presence of iminium salts by ultraviolet spectroscopy, but these are not definitive as yet (14,25). 

Determining the structure of an organic compound was a difficult and time-consuming process in the 19th and early 20th centuries, but powerful techniques are now available that greatly simplify the problem. In this and the next chapter, we ll look at four such techniques—mass spectrometry (MS), infrared (IR) spectroscopy, ultraviolet spectroscopy (UV), and nuclear magnetic resonance spectroscopy (NMR)—and we U see the kind of information that can be obtained from each. 

Structure Determination in Conjugated Systems Ultraviolet Spectroscopy 501... 

The [PtCl,] formed appears to be stable on drying at temperatures up to 393k. The subsequent chemistry of the [PtCl,] species as determined from ultraviolet spectroscopy is as follows ... 

IV. Current Methods for the Determination of Methylxanthines in the Biological Fluids A. Ultraviolet Spectroscopy... 

Ultraviolet spectroscopy is used in many clinical laboratories due to its ease of operation and availability. A classical method for theophylline determination in plasma is the one of Schack and Waxier.45 The original method had interferences from phenobarbitol and various xanthine derivatives such as theobromine.46 The modification of the method by Jatlow47 eliminated the interferences from barbituates but included the various... 

The methylxanthines can be determined in foods and biological systems by the chromatographic methods of TLC, GC, HPLC, or CE. Ultraviolet spectroscopy following a separation procedure can also be used. More recently, immunoassay methods have been developed. There is no single best method the analyst must balance the features of each assay with the final requirements for data precision and reproducibility. 

Previous authors have taught the principles of solving organic structures from spectra by using a combination of methods NMR, infrared spectroscopy (IR), ultraviolet spectroscopy (UV) and mass spectrometry (MS). However, the information available from UV and MS is limited in its predictive capability, and IR is useful mainly for determining the presence of functional groups, many of which are also visible in carbon-13 NMR spectra. Additional information such as elemental analysis values or molecular weights is also often presented. 

Once the FBA has been identified, ultraviolet absorption spectroscopy affords a rapid and accurate method of quantitative analysis. Care must be taken when interpreting the spectra of stilbene-type compounds, since turns to cis isomerisation is promoted by ultraviolet radiation. Usually, however, a control spectrum of the turns isomer can be obtained before the compound undergoes any analytically significant isomerisation. FBAs are often marketed on the basis of strength comparisons determined by ultraviolet spectroscopy. 

Ultraviolet spectroscopy has been applied to the determination of lead and lead speciation studies [407]. Scaule and Patterson [408] used isotope dilution-mass spectrometry to determine the lead profile in the open North Pacific Ocean. 

Ultraviolet spectroscopy does not lend itself to leucovorin analysis for two reasons. First, because commercial samples are frequently contaminated with uv-absorbing impurities, a reliable molar absorptivity has not been determined for leucovorin. Recently, in this laboratory a value of 3.09 x 10 M-1 cm-1 was derived from thorough analysis of a relatively pure sample. This value is in reasonable agreement with that of Zakrzewski and Sanson.21... 

Cawse PA.The determination of nitrate in soil solutions by ultraviolet spectroscopy. Analyst 1967 92 311-315. 

Hennig [40] has applied ultraviolet spectroscopy to the determination of aromatic constituents of residual fuel oil in hexane extracts of marine sediment samples. Examination of the ultraviolet spectra of samples of an oil pollutant from a beach and crude oil, at various concentrations, revealed strong absorption maxima at approximately 228nm and 256nm. The ratio of the peak heights at these wavelengths is constant for a particular oil, and is independent of concentration. These permit quantitative analysis of sediment samples many months after an oil spill. 

Infrared spectroscopy is used for the determination of benzene in motor and/or aviation gasoline (ASTM D4053), while ultraviolet spectroscopy is employed for the evaluation of mineral oils (ASTM D2269) and for determining the naphthalene content of aviation turbine fuels (ASTM D1840). 

Diels-Alder dimer or its reaction with styrene is the rate-determining step in initiation is not completely established. The dependence of Rp on [M] is closer to third-order than second-order, indicating that Eq. 3-63b is the slow step. The Diels-Alder dimer has not heen isolated, but ultraviolet spectroscopy of the reaction system is entirely compatible with its presence. There are indications that the photopolymerization of neat styrene proceeds by a similar mechanism. 

The water and ammonia content of anhydrous N2Hj are determined by a gas chromatography (Refs 37 38) method, and the analysis of the aniline in the mixture by ultraviolet spectroscopy. The total N2Hj content can be then determined by difference. Other methods are given at the end of section on Hydrazine Chemistry... 

Some plasticizer mixes require pretreatment, such as saponification, but in most instances chromatographic separations can be accomplished with the mix. In addition to the usual identification of substances by organochemical analysis, other methods now being used include color tests, physical tests (determinations of boiling point and refractive index), and infrared and ultraviolet spectroscopy. 

Anthracene was introduced into a film by sorption from a methanol solution the concentration of anthracene in the film could be controlled by variation of the concentration in the solution and the concentration could be determined by ultraviolet spectroscopy. After irradiation hydroperoxides are formed which are thermally decomposed in the presence of a vinyl monomer leading to the formation of a graft copolymer. The films were recovered and washed thoroughly with water to remove any homopolymer. The amount of graft copolymer which was produced was determined by a comparison of the mass of starting material with the mass of the final film. ... 

Tryon, C.R., Edwards, P.A., and Chisholm, M.G. 1988. Determination of the phenolic content of some French-American hybrid white wines using ultraviolet spectroscopy. Am. J. Enol. Vitic. 39 5-10. 

Infrared, visible and near ultraviolet spectroscopy are particularly important when used in conjunction with polarising devices in determining the mean orientation of molecules or particular parts of molecules. 

Commercial cylindrical quartz cells can be adapted for gas-phase work as illustrated in Fig. 9.18. Such a cell finds use in the near infrared for the determination of overtone vibrational frequencies, and also in visible and ultraviolet spectroscopy. A much less expensive cell which is adequate for most gases may be constructed from Pyrex along the lines of the cell shown in Fig. 9.18. Quartz windows may then be attached by epoxy resin. A cell which is filled from a conventional vacuum line will generally contain mercury vapor which absorbs at 2537 A. Once the origin of this absorption is recognized, it causes little difficulty because of its narrow bandwidth. 

Ultraviolet spectroscopy has great utility in the characterization of expls and related materials, and serves as a primary analytical tool for the quantitative determination of reactant composition and purity. Additionally, it can provide the principal method of monitoring expl kinetics and reaction mechanisms, since the high temps characteristic of expins are effective in creating electronic excitations... 

Because of the interaction between the double bonds, systems containing conjugated double bonds tend to be more stable than similar systems with isolated double bonds. In this chapter, we consider the unique properties of conjugated systems, the theoretical reasons for this extra stability, and some of the characteristic reactions of molecules containing conjugated double bonds. We also study ultraviolet spectroscopy, a tool for determining the structures of conjugated systems. 

The elucidation and confirmation of structure should include physical and chemical information derived from applicable analyses, such as (a) elemental analysis (b) functional group analysis using spectroscopic methods (i.e., mass spectrometry, nuclear magnetic resonance) (c) molecular weight determinations (d) degradation studies (e) complex formation determinations (f) chromatographic studies methods using HPLC, GC, TLC, GLC (h) infrared spectroscopy (j) ultraviolet spectroscopy (k) stereochemistry and (1) others, such as optical rotatory dispersion (ORD) or X-ray diffraction. 

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