Ultraviolet spectroscopy chromophores

The most convenient and sensitive method for detecting the presence of an isoindole is by means of ultraviolet spectroscopy. The isoindole chromophore shows a characteristic series of bands in the range 220-390 m/x, with the band at longest wavelength usually well separated from the remainder of the spectrum and of high intensity... 

The ultraviolet spectroscopy of epoxides has received relatively little attention in the literature, since their ability to delocalize ir-ciectrons in a chromophore, although detectable, is even smaller than that of the cyclopropane ring.1"1 A representative selection of Amu values is shown in Table 3, For additional references from the held of steroid chemistry an excellent review by Dorfman468 should be consulted. 

The bulk composition of the SAN copolymer can be determined by ultraviolet spectroscopy. Absorbances consistent with conjugated systems such as Figures 13.1 and 13.2 have been observed. Studies usually compare the UV spectra of model systems with the actual absorbances seen in SAN copolymers. The models represent chemically reasonable species based upon the starting monomers, known reactivity ratios, and oxidation and rearrangement chemistry [2]. The data are self-consistent with these criteria however, identification of all chromophores responsible for color formation in SAN copolymers is still work in progress. One source of species with extended conjugation is the cyclization of acrylonitrile triads to form heteroaromatic structures (Figure... 

Ultraviolet spectroscopy is only of use if your compound has a characteristic chromophore. There is little point trying to measure weak bands which will provide no information. However, it is of considerable value in several areas of research for example, natural product isolation, heteroaromatic chemistry, porphyrin and related chemistry, and in the study of dyestuffs. The amount of material required is usually very small (fractions of a mg) since the extinction coefficients are usually large. The sample must be as pure as possible and is dissolved in the solvent of choice (usually spectroscopically pure ethanol). The concentration must be known accurately before extinction coefficients can be calculated, and will vary depending upon the type of chromophore. An estimate of the concentration to used can be made if the extinction coefficients of compounds similar to that being studied are available. If this data is not available make up a solution accurately and dilute it (accurately ) until a reasonable spectrum is obtained. 

The presence of two monochromators, and the fact that not all molecules with a chromophore fluoresce, means that fluorimetry is more specific than ordinary ultraviolet spectroscopy. This allows drugs that fluoresce to be assayed in the presence of other compounds that would interfere in an ultraviolet assay. 

If the compound contains an ultraviolet (UV) chromophore that changes with the extent of ionization, then a method involving UV spectroscopy can be used. This method involves measuring the UV spectrum of the compound as a function of pH. Mathematical analysis of... 

This approach— the use of model compounds— is one of the best ways to put the technique of ultraviolet spectroscopy to work. By comparing the UV spectrum of an unknown substance with that of a similar but less highly substituted compound, you can determine whether or not they contain the same chromophore. Many of the books listed in the references at the end of this chapter contain large collections of spectra of suitable model compounds, and with their help you can estabHsh the general structure of the part of the molecule that contains the K electrons. You can then utilize infrared or NMR spectroscopy to determine the detailed structure. 

Chromophore (Section 13 21) The structural unit of a mole cule principally responsible for absorption of radiation of a particular frequency a term usually applied to ultraviolet visible spectroscopy... 

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

J.A. Schmidt, Ultraviolet/visible spectroscopy of chromophores in mechanical pnlps, in Encyclopedia of Analytical Chemistry, John Wiley Sons Ltd, Chichester, 8388-8406 (2000). 

These devices are based on the anisotropic absorption of light. Usually molecular crystals exhibit this property and tourmaline is the classical example for this. For practical purposes, however, micro crystals are oriented in polymer sheets. Polymers containing chromophors become after stretching dichroic polarizers. The devices produced in this manner are called polawids. They have found a broad application in many technologies. Their application in spectroscopy is limited to the near ultraviolet and to the visible and near infrared range of the spectrum. In vibrational spectroscopy polaroids are employed as analyzers only for Raman spectroscopy. 

In this section we have not endeavoured to give a comprehensive review of methods which can be used for reaction monitoring, but we have described the most universal and commonly used modern techniques. Various other monitoring methods can be devised for specific reactions. For instance ultraviolet (uv) spectroscopy can be used if one strong chromophore is being converted to another, but the disadvantage of spectroscopic methods is that they do not indicate how many products are being produced. Nmr spectroscopy can also be employed and this is mentioned further in Chapter 12. 

Pharmaceutical analysts often have no experience in direct polarographic or voltammetric methods, but almost all will have used high-performance liquid chromatography (HPLC) for the determination of drugs and/or metabolites in biological matrices or drugs and/ or their degradation products in pharmaceutical formulations. Spectroscopy (ultraviolet and fluorescence) is the most common detection method in HPLC, but for molecules that do not possess a suitable chromophore or when increased sensitivity or specificity is required, electrochemical detection offers a suitable alternative. ELCD is applicable to any molecular species capable... 

Ultraviolet spectrophotometry is considered a valuable tool as an aid for confirming the identification of pesticide residues. A correlation between the UV spectrum and the structure of several pesticides is discussed. Knowledge of such correlation may provide clues about the general type of chromophore present and may help the analyst to design analytical procedures. The transparency of many groups in the near UV imposes a limitation on interpretations of the absorption bands in this region. However, when taken in conjunction with the information obtained by IR, NMR, and mass spectroscopy, UV spectra may lead to structural proposals of value to the pesticide analyst. A discussion of the methods that have been utilized for the analysis of pesticides on the submicrogram level is also presented. 

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