Absorbance, optical

The double-stranded structure of DNA can be separated into two component strands (melted) in solution by increasing the temperature or decreasing the salt concentration. Not only do the two stacks of bases puU apart but the bases themselves unstack while still connected in the polymer by the phosphodiester backbone. Concomitant with this denaturation of the DNA molecule is an increase in the optical absorbance of the purine and pyrimidine bases—a phenomenon referred to as hyperchromicity of denaturation. Because of the... 

Ru(bipy)3 formed in this reaction is reduced by the sacrificial electron donor sodium ethylenediaminetetra-acetic acid, EDTA. Cat is the colloidal catalyst. With platinum, the quantum yield of hydrogenation was 9.9 x 10 . The yield for C H hydrogenation was much lower. However, it could substantially be improv l by using a Pt colloid which was covered by palladium This example demonstrates that complex colloidal metal catalysts may have specific actions. Bimetalic alloys of high specific area often can prepared by radiolytic reduction of metal ions 3.44) Reactions of oxidizing radicals with colloidal metals have been investigated less thoroughly. OH radicals react with colloidal platinum to form a thin oxide layer which increases the optical absorbance in the UV and protects the colloid from further radical attack. Complexed halide atoms, such as Cl , Br, and I, also react... 

A variety of formats and options for different types of applications are possible in CE, such as micellar electrokinetic chromatography (MEKC), isotachophoresis (ITP), and capillary gel electrophoresis (CGE). The main applications for CE concern biochemical applications, but CE can also be useful in pesticide methods. The main problem with CE for residue analysis of small molecules has been the low sensitivity of detection in the narrow capillary used in the separation. With the development of extended detection pathlengths and special optics, absorbance detection can give reasonably low detection limits in clean samples. However, complex samples can be very difficult to analyze using capillary electrophoresis/ultraviolet detection (CE/UV). CE with laser-induced fluorescence detection can provide an extraordinarily low LOQ, but the analytes must be fluorescent with excitation peaks at common laser wavelengths for this approach to work. Derivatization of the analytes with appropriate fluorescent labels may be possible, as is done in biochemical applications, but pesticide analysis has not been such an important application to utilize such an approach. 

Absorbance detectors are also commonly used in combination with postcolumn reactors. Here, most issues of detector linearity and detection limit have to do with optimization of the performance of the reactor. In a typical application, organophosphorus compounds with weak optical absorbances have been separated, photolyzed to orthophosphate, and reacted with molybdic acid, with measurement being performed by optical absorbance.58... 

Laser flash photolysis techniques offer the possibility of examining in detail the transient processes responsible for the photostabilizing effect discussed above. The triplet lifetimes are frequently too short, even for this technique however, they can still be estimated using as a probe the quenching by 1-methyl-naphthalene, which leads to the formation of its easily detectable triplet. The optical absorbance due to the 1-methylnaphthalene triplet (Aft) produced as a result of energy transfer is related to the Stern-Volmer slope by equation 5, where N stands for... 

A breakthrough came in 1988 when Hamada et al. demonstrated that a dye layer with a relatively low optical absorbance and high reflectance at the recording wavelength could be interposed between the substrate and reflecting layer of a CD-ROM type structure, and, with appropriate optimization of the optical properties, this would record and reproduce in accordance... 

There are no clear advantages in terms of functional performance between the two classes of dyes, except that phthalocyanines are generally more light stable but tend to be more expensive to synthesize and modify. Phthalocyanine dyes are not suitable for DVD-R media, since the main chromophore cannot readily be modified to produce a sufficiently large hypsochromic shift. Other dyes potentially suitable for DVD-R include metal azo complexes, quinophthalones, and diphenyl-methanes. The cyanine dyes are particularly useful as they can be readily modified to tailor the optical absorbance requirements for all current optical disk recording applications.199... 

There are several unique features about PAC. First, PAC and the related methods are the only experimental techniques currently available, which can measure the heats of reaction of carbenes on the microsecond and faster time scale. This usually allows for an accurate determination of the heats of formation of these reactive intermediates. Second, PAC can monitor the reactions of transients which are optically transparent, i.e. do not have an UV-VIS optical absorbance. Hence, in addition to thermodynamics, PAC can also provide important kinetic information about these invisible species. 

Thin-layer chromatography does not provide quantitative information of the highest precision and accuracy. Linear relationships between the mass of a substance and the logarithm or square-root of the spot area can sometimes be established under very closely controlled conditions. The optical absorbance of a spot determined by reflectance measurements can be similarly related to mass, or the substances can be scraped from the plate and dissolved in a suitable solvent for a spectrometric determination. The main difficulties with area and density measurements lie in defining the boundaries of spots and controlling chromogenic reactions in a reproducible manner. Relative precision can be as good as 1-2% but is more usually 5-10%. 

Figure 7.6 Mirrors (a) an ordinary car driver s mirror reflects the lights of a following car, which can dazzle the driver (b) in an electrochromic mirror, a layer of optically absorbing chemical is electro-generated in front of the reflector layer, thereby decreasing the scope for dazzle. The width of the arrows indicates the relative light intensity...

In fact, because the integrated first-order rate equation (Equation (8.24)) is written in terms of a ratio of concentrations, we do not need actual concentrations in moles per litre, but can employ any physicochemical parameter that is proportional to concentration. Obvious parameters include conductance, optical absorbance, the angle through which a beam of plane-polarized light is rotated (polarimetry), titre from a titration and even mass, e.g. if a gas is evolved. 

The function on the y-axis of the spectrum in Figure 9.4 is the absorbance (as defined on p. 441). Absorbance is also called optical density or optical absorbance in older books these three terms each mean the same thing. We can see from the spectrum that more light is absorbed at 300 nm (in the near infrared) than at 500 nm... 

Melanin is a complicated mixture of optically absorbing materials. chromophores), of which melanin is the most important. Melanin is a complicated mixture of optically absorbing materials, formed as an end product during the photo-assisted metabolism of the amino acid tyrosine. Both are bound covalently to the surrounding proteins within the skin, and other pigmented regions of the body. 

The spectrum of melanin has minimal optical absorbances at wavelengths greater than about 750 nm, suggesting that it does not protect us against the heat from the sun (i.e. infrared light). 

The transmittance T is a useful measure of how many photons are absorbed nevertheless, most spectroscopists prefer to work in terms of the absorbance A. (Absorbance is also called optical absorbance and optical density - each of these three terms means the same thing.)... 

FIGURE 6.25 Schematic diagram showing an OLED with a black cathode, the presence of an optical absorbing layer causes a destructive interference leading to a low reflection. 

Optical Spectra. The optical spectra of the unsymmetrical crown porphyrazines are given in Table XXV. The spectra are typical pz spectra with two intense optical absorbance bands, a Soret band at "k < 400 nm and a Q band at X > 600 nm. The Q band is split for both the metalated and free base species, with the latter exhibiting the greater splitting due to decreased symmetry. 

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