Spectrophotometry visible spectrum

Spectrophotometry Visible spectrum Nonsubjective comparison of color and aid in identifying some pigments 24,44... 

For a compound to be analyzed by spectrophotometry, it must absorb light, and this absorption should be distinguishable from that due to other substances in the sample. Because most compounds absorb ultraviolet radiation, measurements in this region of the spectrum tend to be inconclusive, and analysis is usually restricted to the visible spectrum. If there are no interfering species, however, ultraviolet absorbance is satisfactory. Proteins are normally assayed in the ultraviolet region at 280 nm because the aromatic groups present in virtually every protein have an absorbance maximum at 280 nm. 

It has not been possible to measure the acidity of the CF3S03H-SbFs system by spectrophotometry because of its very strong absorption in the UV-visible spectrum. In comparison with the strength of the related perfluoroalkanesulfonic acids, its acidity should be very close or slightly higher (by 1 H0 unit) than the acidity of the perfluoroethane- and perfluorobutanesulfonic acid-SbF5 mixtures, which was... 

An important aspect of the aqueous chemistry of viologen cation radicals is their tendency to dimerize [16, 17]. The extent of the dimerization reaction can be easily estimated by visible spectrophotometry of the cation radical solutions. Figure 4 shows the visible spectrum recorded after reduction of a ImM solution of... 

Visible spectrophotometry has also been applied similarly to the analysis of copper phthalocyanine green (PG7). This pigment is even less soluble than the non-chlorinated phthalocyanine blue. For quantitative analysis, extreme care is needed in order to ensure that the sample is in solution. Sulfuric acid is the preferred solvent except in the case of polymeric matrices. Figure 21-3 shows the Visible spectrum of PG 7 at a concentration of 140 pg/L in DMSO (A ax is at 727.5nm). 

Chance B 1951 Rapid and sensitive spectrophotometry. I. The accelerated and stopped-flow methods for the measurement of the reaction kinetics and spectra of unstable compounds in the visible region of the spectrum Rev. Sci. Instrum 22 619-27... 

Spectrophotometry proper is mainly concerned with the following regions of the spectrum ultraviolet, 185-400 nm visible 400-760 nm and infrared, 0.76-15 /tm. Colorimetry is concerned with the visible region of the spectrum. In this chapter attention will be confined largely to the visible and near ultraviolet region of the spectrum. 

Of course, not all dissolved ions produce colored solutions, and therefore not all ions in solution can be quantified by colorimetry. Noncolored solutions can sometimes, however, be converted to colored solutions by introducing chromophore species which complex with (i.e., attach themselves to) the target ion to produce a colored solution, which may then be measured by UV/visible colorimetry. An important archaeological example of this is the determination of phosphorus in solution (which is colorless) by com-plexation with a molybdenum compound, which gives a blue solution (see below). The term colorimetry applies strictly only to analytical techniques which use the visible region of the spectrum, whereas spectrophotometry may be applied over a wider range of the electromagnetic spectrum. 

Although its precise structure has not yet been settled, the hydrated electron may be visualized as an excess electron surrounded by a small number of oriented water molecules and behaving in some ways like a singly charged anion of about the same size as the iodide ion. Its intense absorption band in the visible region of the spectrum makes it a simple matter to measure its reaction rate constants using pulse radiolysis combined with kinetic spectrophotometry. Rate constants for several hundred different reactions have been obtained in this way, making kinetically one of the most studied chemical entities. 

Developments in traditional forms of spectrophotometry, as well as new methods, could find greater use in ocean measurements. Spectroscopy based on absorption of visible light may have reached a limit in its traditional form, however. Spectrophotometry will remain in wide use due to its ease, low cost, and great versatility. In many ways it remains the first choice for analysis, but its low sensitivity makes it useful for a rather limited spectrum of analytes. 

PHOTOMETRIC ANALYSIS. Chemical analysis by means of absorption or emission of radiation, primarily in the near UV, visible, and infrared portions of the electromagnetic spectrum. It includes such techniques as spectrophotometry, spectrochemical analysis, Raman spectroscopy, colorimetry, and fluorescence measurements. 

Frequently industrial hygiene analyses require the identification of unknown sample components. One of the most widely employed methods for this purpose is coupled gas chromatography/ mass spectrometry (GC/MS). With respect to interface with mass spectrometry, HPLC presently suffers a disadvantage in comparison to GC because instrumentation for routine application of HPLC/MS techniques is not available in many analytical chemistry laboratories (3). It is, however, anticipated that HPLC/MS systems will be more readily available in the future ( 5, 6, 1, 8). HPLC will then become an even more powerful analytical tool for use in occupational health chemistry. It is also important to note that conventional HPLC is presently adaptable to effective compound identification procedures other than direct mass spectrometry interface. These include relatively simple procedures for the recovery of sample components from column eluate as well as stop-flow techniques. Following recovery, a separated sample component may be subjected to, for example, direct probe mass spectrometry infra-red (IR), ultraviolet (UV), and visible spectrophotometry and fluorescence spectroscopy. The stopped flow technique may be used to obtain a fluorescence or a UV absorbance spectrum of a particular component as it elutes from the column. Such spectra can frequently be used to determine specific properties of the component for assistance in compound identification (9). 

UV/visible spectrophotometry utilizes radiation in the UV region of the electromagnetic spectrum to induce electronic transitions. 

The amount of substrate transformed into products during an enzyme-catalyzed reaction can be measured with any appropriate analytical method, such as spectrophotometry, fluorometry, or chemiluminescence. For example, if an enzyme reaction is accompanied by a change in the absorbance characteristics of some component of the assay system, in either the visible or ultraviolet spectrum, it can be photometrically observed while it is proceeding. Self-indicating reactions of this type are particularly valuable as... 

The absorption of electromagnetic radiation of wavelengths between 200 and 800 nm by molecules which have n electrons or atoms possessing unshared electron pairs can be employed for both qualitative and quantitative analysis as such, it is known as spectrophotometry. As a wide variety of pharmaceutical substances absorb radiation in the near-ultraviolet (200-380 nm) and visible (380-800 nm) regions of the electromagnetic spectrum, the technique is widely employed in pharmaceutical analysis. 

UV-visible spectrophotometry enables giving indications about the pollution maturation level in contaminated soils. Stabilised leachates from old contaminated soils are characterised by a monotonous decreasing spectrum (soil A), while younger ones show a specific spectrum where additional compounds are responsible for visible accidents (soil B). 

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