Ultraviolet-visible absorbance

Owing to the varied structures of various food dyes, they can often be differentiated from one another by their characteristic ultraviolet/visible absorbance spectra. Using HPLC coupled with a diode array detector (HPLC-DAD) it is possible to collect a compound s absorbance spectrum as it elutes from the HPLC column, which greatly assists in identification. At Reading Scientific Services Ltd (RSSL) this type of detector is routinely used in a range of analyses of such substances as patulin, a mycotoxin found in apple juice, and in the analysis of colours and vitamins, which allows a more certain assignment of a particular peak to a specific compound to be made. 

The first exposure to spectroscopy for most scientists is ultraviolet/ visible absorbance. As virtually every HPLC chromatograph employed in the pharmaceutical industry uses UV absorbance as the detection method, it is no wonder that the most popular hyphenated technique is HPLC-DAD. DAD spectrographs have been coupled to all liquid-based chromatographic systems including HPLC (preparative, analytical, and microbore), capillary electrophoresis (CE) and supercritical fluid chromatography (SFC). There have been several successes with TLC plates,18 but it is more common for developed plates to be scraped and the sample analyzed offline. 

TCE Trichloroethene UV Ultraviolet spectrum UV-vis Ultraviolet visible absorbance spectrum... 

Minor, E.C., Pothen, J., Dalzell, B.J., Abdulla, H and Mopper, K. (2006). Effects of salinity changes on the photodegradation and ultraviolet-visible absorbance of terrestrial dissolved organic matter. Limnol. Oceanogr., 51(5), 2181-2186. 

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

Discussion. Potassium nitrate is an example of an inorganic compound which absorbs mainly in the ultraviolet, and can be employed to obtain experience in the use of a manually operated ultraviolet/visible spectrophotometer. Some of the exercise can also be carried out employing an automatic recording spectrophotometer (see Section 17.16). 

Hapten density is important for both immunization and assay performance, and hence the extent of conjugation or hapten density should be confirmed by established methods. A characteristic ultraviolet (UV) or visible absorbance spectrum that distinguishes the hapten from the carrier protein or use of a radiolabeled hapten can be used to determine the degree of conjugation. If the hapten has a similar A. iax to the protein, the extent of incorporation can still be estimated when the concentration of the protein and the spectral characteristics of the hapten and protein are known. The difference in absorbance between the conjugate and the starting protein is proportional to... 

The ultraviolet-visible spectrophotometer is the most widely used detector for HPLC. The basis of UV-VIS detection is the difference in the absorbance of light by the analyte and the solvent. A number of functional groups absorb... 

An increase in absorbance at 351 nm and a concomitant decrease in absorbance at 380 nm in the ultraviolet visible spectrum of methylcobalamin during the abiotic transfer of the methyl group to Hg2+ are characteristic for the loss of the methyl group and formation of aquocobalamin. In experiments monitored by both analytical techniques, gas chromatographic measurements of methylmercury formation were in good agreement with the spectropho-tometric measurement of aquocobalamin formation from methylcobalamin at 351 nm. Aerobic versus anaerobic reaction conditions had no measurable effect on either the methyl transfer rates, the stability of the reactants, or on the reaction products. 

Explain what types of quantized absorbances are expected in the ultraviolet, visible and infrared spectral regions. 

An ultraviolet-visible light spectrophotometer (Lambda 10, Perkin Elmer, Norwalk, CT) was used to measure the characteristic absorbance of the samples taken from the receptor half-cell. Using a calibration curve derived from known concentrations of the model dmgs, the concentration of each sample taken from the receptor half-cell could be determined. 

Ultraviolet-visible spectroscopy has played only a minor role in the investigation of the thietanes and its counterparts. As expected, the colorless compounds absorb in the UV region below 300 nm. For photolysis experiments, many phenylated and alkylated thietanes and thietane oxides have been measured in methyl cyanide or methanol by Langendries and de Schryver. The spectra exhibit a maximum in the vicinity of 250-265 nm ( 12,000-25,000). 

Most conventional techniques for the determination of biological molecules or other species with similar properties use their ability to absorb ultraviolet or visible light, their fluorescence after excitation with light of the appropriate wavelength, or their electrochemical behaviour. It possible to enhance the detectability of some species by making them react with UV-visible absorbing or fluorescent compounds. Applied to complex matrices, these detection methods are at best only selective, because a wide variety of chromophores will give a response. 

The single most useful and versatile physicochemical detectors in drug residue analysis are probably those based on ultraviolet-visible (UV-Vis) spectrophotometry. These detectors allow a wide selection of detection wavelengths, thus offering high sensitivity for analytes that exhibit absorbance in either the ultraviolet or the visible region of the electromagnetic radiation. 

COLOR CENTERS. Certain crystals, such as the alkali halides, can be colored by the introduction of excess alkali metal into the lattice, or by irradiation with x-rays, energetic electrons, etc. Thus sodium chloride acquires a yellow color and potassium chloride a blue-violet color. The absorption spectra of such crystals have definite absorption bands throughout the ultraviolet, visible and near-infrared regions. The term color center is applied to special electronic configurations in the solid. The simplest and best understood of these color centers is the F center. Color centers are basically lattice defects that absorb light. 

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