Contributions to Absorbance

In general, the total optical signal change, 5S, produced by a perturbation will contain several contributions as, for instance, demonstrated for electric-field-induced absorbance changes in ribosomal RNA. The concentration shifts of the components Bj in a composite interacting system are summarized in a chemical term Density (volume) changes and 

In isotropic systems and at isotropic perturbations such as temperature and pressure changes, the term 55 only reflects density (volume) changes. 

In solutions and suspensions of anisotropic molecules directing external forces such as a hydrodynamic flow or electric field forces may cause signal changes 55 originating from molecule rotations. In particular optically and electrically anisotropic (dipolar or polarizable) macromolecules exhibit major electric dichroism and electric birefringence. 

There are numerous reviews on dichroism and birefringence as well as on the use of optical signals to indicate concentration changes. Less frequent, however, are accounts where it is emphasized that both chemical and orientational changes may contribute to the measured optical signals. 

We now recall the basic absorbance equation for a multicomponent system. Per centimeter light path 

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S. N. Dudd, M. Regert and R. P. Evershed, Assessing microbial contributions to absorbed acyl lipids during laboratory degradations of fats and oils and pure triacylglycerols absorbed in ceramic potsherds. Organic Geochemistry, 29, 1345 1354 (1998). 

Table I. Percentage mole fractions of non-planar conformers and their contributions to absorbance at 300 nm for TIN and TINS...

All the bases in DNA contribute to the absorbance at 260 nm, whereas only a few side chains of proteins contribute to absorbance at 280 nm these are mostly Trp, Tyr, and disulfide-bonded Cys residues. 

If the 4S-LM is used to produce desalinated water for a plantation in the desert or a similar place, it would indirectly contribute to absorbing some carbon dioxide. 

Edelhoch (104) stressed that contaminants, cofactors and turbidity contribute to absorbance at 280-290 nm. A good method for checking for the presence of absorbing impurities is to examine the absorbance of the protein in neutral 6M guanidine hydrochloride at 315 nm, where tyrosine and tryptophan do not absorb. 

Note that in this special case, the heat absorbed directly measures a state fiinction. One still has to consider how this constant-volume heat is measured, perhaps by an electric heater , but then is this not really work Conventionally, however, if work is restricted to pressure-volume work, any remaining contribution to the energy transfers can be called heat . 

In the Hiickel or extended Hiiekel methods no explicit reference is made to electron-electron interactions although such contributions are absorbed into the V potential, and... 

Selectivity Selectivity is rarely a problem in molecular absorption spectrophotometry. In many cases it is possible to find a wavelength at which only the analyte absorbs or to use chemical reactions in a manner such that the analyte is the only species that absorbs at the chosen wavelength. When two or more species contribute to the measured absorbance, a multicomponent analysis is still possible, as shown in Example 10.6. 

The hnearity between M and makes the concept of absorbance so usehil that measurements made by sampling methods other than transmission are usually converted to a scale proportional to absorbance. The linearity between M and i is maintained only if the resolution of the spectrometer is adequate to eliminate contributions from wavelengths not absorbed by the species being measured. In addition, the apparent value of a is very dependent on resolution because a is 2l strong function of wavelength (30,31). 

Ozone, which occurs in the stratosphere (15—50 km) in concentrations of 1—10 ppm, is formed by the action of solar radiation on molecular oxygen. It absorbs biologically damaging ultraviolet radiation (200—300 nm), prevents the radiation from reaching the surface of the earth, and contributes to thermal equiHbrium on earth. 

Separate sample blanking requires an additional analytical channel, and is therefore wasteflil of both reagents and hardware. An alternative approach that is used on several automated systems, eg, Du Pont ACA, BM-Hitachi 704, Technicon RA-1000, is that of bichromatic analysis (5) where absorbance measurements are taken at two, rather than one, wavelength. When the spectral curves for the interference material and the chromogen of the species measured differ sufficiently, this can be an effective technique for reducing blank contributions to assay error. Bichromatic analysis is effective for blanks of both the first and second type. 

Carbon Dioxide The contribution to the emissivity of a gas containing CO9 depends on gas temperature Tc, on the CO9 partial pressure-beam length product p L and, to a much lesser extent, on the total pressure P. Constants for use in evaluating at a total pressure of 101.3 kPa (1 atm) are given in Table 5-8 (more on this later). The gas absorptivity Ot equals the emissivity when the absorbing gas and the emitter are at the same temperature. When the emitter surface temperature is Ti, Ot is (Tc/Ti)° times , evaluated using Table 5-8 at T instead of Tc and at p LTi/Tc instead of Line broadening, due to... 

Reactive control is also possible through synchronous condensers. As they rotate, the rotor stores kinetic energy which tends to absorb sudden Huctuations in the supply system, such as sudden loadings. They are. however, sluggish in operation and very expensive compared to thyristor controls. Their rotating masses add inertia, contribute to the transient oscillations and add to the fault level of the system. All these factors render them less suitable for such applications. Their application is therefore gradually disappearing. 

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