UV-visible absorbance spectroscopy

Bennett, G. E. and Johnston, K. P. (1994) UV-visible Absorbance Spectroscopy of Organic Probes in Supercritical Water, J. Phys. Chem 98, 441. 

UV-Visible Absorbance Spectrometry UV-visible absorbance spectroscopy is used to characterize Pd nanoparticles in primarily two ways (i) to monitor the palladium precursor as it is reduced to the zero-valent state, indicating formation of palladium nanoparticles and/or (ii) to monitor the position of the palladium nanoparticles surface plasmon resonance (SPR) peak. 

UV-visible absorbance spectroscopy has often been used to reveal the completion of reduction, as the absorption for Pd(0) and Pd(II) differ significantly. Obare et al. monitored the change in the metal-to-ligand charge transfer (MLCT) band located at 400nm of Pd3(OAc),s as it was reduced from Pd to Pd [48]. The reduction was accompanied by a color change from yellow to black, as shown in the inset of Figure 9.26. 

Infrared spectroelectrochemical methods, particularly those based on Fourier transform infrared (FTIR) spectroscopy can provide structural information that UV-visible absorbance techniques do not. FTIR spectroelectrochemistry has thus been fruitful in the characterization of reactions occurring on electrode surfaces. The technique requires very thin cells to overcome solvent absorption problems. 

Typically, sample detection in electromigration techniques is performed by on-column detection, employing a small part of the capillary as the detection cell where a property of either the analyte, such as UV absorbance, or the solution, such as refractive index or conductivity, is monitored. This section briefly describes the major detection modalities employed in capillary electromigration techniques, which are accomplished using UV-visible absorbance, fluorescence spectroscopy, and electrochemical systems. The hyphenation of capillary electromigration techniques with spectroscopic techniques employed for identification and structural elucidation of the separated compounds is also described. 

Background and principles UV spectroscopic analysis was one of the first applications of spectroscopy in an analytical context. While a powerful technique of yesteryear, this method is now rather restricted in its use in a modem bioanalytical laboratory. However, UV/visible spectroscopy is still used in specific biochemical analyses, such as dyestuffs in forensic applications. Despite these limitations, a bench spectrophotometer that measures UV/visible absorbance is commonly found in both teaching and research laboratories. In order to better understand the underlying principles it is necessary to consider the excitation of electrons, electronic transition and how this is related to energy and absorbance. 

Some organic molecules are able to absorb optical irradiation and pass on the increased energy to other molecules, which then degrade these processes are called photosensitized or secondary photochemical reactions. The absorbing molecules, which are denoted photosensitizers, are not decomposed in the photochemical reactions. Excipients, solvents, or small amounts of impurities may act as photosensitizers that initiate the photochemical reaction in the formulation. The photosensitizers may be present in concentrations not detectable by conventional analytical methods, such as UV-visible absorption spectroscopy or HPLC with UV detection. 

UV-visible spectroscopy UV-visible absorbance microspectrophotometry and microspectrofluorime-try are used as nondestructive methods for analyzing inks directly on paper. They measure, respectively, the absorption and emission spectrum of inks and allow the discrimination of similarly colored inks. Therefore, they are a convenient way to render the comparison of ink colors more objectively than the naked eye. 

Infrared reflectance spectroscopy, like UV-visible reflectance spectroscopy, is based on the specular reflection of the incident light on the substrate surface. It is therefore relevant to give briefly, in this section, the basic equations describing the propagation of an electromagnetic plane wave in an absorbing medium and its reflection at the boundary which separates two contiguous phases of different optical properties. 

Absorption spectroscopy provides a means to study particular details about a monolayer. Transmission spectroscopy is difficult because the film, which is thin, absorbs little. Gaines [1] describes multiple-pass procedures for overcoming this problem. Reflection spectroscopy in the UV-visible range has been reported for lipid monolayers [150,151] and in the IR range for oleic acid [152]. 

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