Specific absorption

Laser Photochemistry. Photochemical appHcations of lasers generally employ tunable lasers which can be tuned to a specific absorption resonance of an atom or molecule (see Photochemical technology). Examples include the tunable dye laser in the ultraviolet, visible, and near-infrared portions of the spectmm the titanium-doped sapphire, Tfsapphire, laser in the visible and near infrared optical parametric oscillators in the visible and infrared and Line-tunable carbon dioxide lasers, which can be tuned with a wavelength-selective element to any of a large number of closely spaced lines in the infrared near 10 ]lni. 

The dB/d/is limited to 6 T/s out of concern that larger values could cause nerve stimulation. The r-f exposure is limited to a specific absorption rate (SAR) of 0.4 W/kg for the whole body, 0.32 W/kg averaged over the head, and less than 8.0 W/kg spatial peakia any one gram of tissue. These numbers are designed to limit the temperature rise to less than 1°C and localized temperature of no greater than 38°C head, 39°C tmnk, and 40°C ia the extremities. 

Infrared spectroscopy can also be used incisively to identify the six main varieties of asbestos fibers. Specific absorption bands in the infrared spectmm can be associated with the asbestos fibers, first in the 3600 3700 cm range (specific hydroxyl bands) and, second, in the ranges 600—800 and 900 1200 cm (specific absorption bands for various siUcate minerals (10)). 

The specific absorption rate = i (Cj,B ) is a func tion of h and may be computed by combining the rate equation... 

Accordingto Eq. (14-74), when /cl is very large and the ratio ki /mkc is much larger than unity, Xi —x = ykc/k and the specific absorption rate is defined by the equation... 

At the other extreme, when the ratio ki /mkc is much smaller than unity, the interfacial concentration of reactant A may be approximated by the equihbrium relation Xi = y/m, and the specific absorption rate expression is... 

Figure 15-33. Time dependence of specific absorption bands. MDMO-PPV (1506 cm1) (downward full triangles ), 1 3 mixture of MDMO-PPV/C, at the MDMO-PPV band at 1506 cm-1 (downward open triangles).

The specific absorption (or extinction) coefficient Es (sometimes termed absorbancy index) may be defined as the absorption per unit thickness (path length) and unit concentration. 

Specific absorption coefficient 650 Specific gravities solutions of acids, (T) 829 of selected reagents (T), 829 Spectral half bandwidth 663 Spectrofluorimetry 731 Spectrograph adjustment of, 771 commercial instruments, 761, 775, 776 Spectrographic analysis see Emission spectrographic analysis... 

Zeolites form a unique class of oxides, consisting of microporous, crystalline aluminosilicates that can either be found in nature or synthesized artificially [J.M. Thomas, R.G. Bell and C.R.A. Catlow in Handbook of Heterogeneous Catalysis (Ed. G. Ertl, H. Knbzinger and J. Weitkamp) (1997), Vol. 1, p. 206, VCH, Weinheim.]. The zeolite framework is very open and contains channels and cages where cations, water and adsorbed molecules may reside and react. The specific absorption properties of zeolites are used in detergents, toothpaste, and desiccants, whereas their acidity makes them attractive catalysts. 

To purify, concentrate, and recover different pigments (flavonoids or anthocya-nins), various ion-changing resins were used. Recent screenings of 13 commercial resins [acrylic or styrene-divinylbenzene (SDVB)] for the purification and specific absorption of anthocyanins - used ethanol, methanol, and water mixtures as eluents at pH 3.5. DDVB resins (EXA-118 and EXA-90) were found most suitable using a mixture of methanol and water (1 1) for elution. The other routinely used resins like XAD-7 showed low efficiency. 

In 1941, Mackinney ° published the first specific absorption coefficients for chlorophyll a and chlorophyll b in 80% acetone, quickly followed by other reports citing different solvents. Chlorophylls form aggregates in various organic solvent-water mixtures that may interfere strongly with the absorbance maximum wavelength and the shapes of spectra. 

Applications Applications of UV/VIS spectrophotometry can be found in the areas of extraction monitoring and control, migration and blooming, polymer impregnation, in-polymer analysis, polymer melts, polymer-bound additives, purity determinations, colour body analysis and microscopy. Most samples measured with UV/VIS spectroscopy are in solution. However, in comparison to IR spectroscopy additive analysis in the UV/VIS range plays only a minor role as only a limited class of compounds exhibits specific absorption bands in the UV range with an intensity proportional to the additive concentration. Characteristic UV absorption bands of various common polymer additives are given in Scheirs [24],... 

Interroute extrapolation. The IEUBK Model includes an exposure module that simulates age-specific lead exposures via inhalation, and ingestion of lead in diet, dust, lead-based paint, soil, and water. The total exposure from each route is defined as the total lead uptake ( pg/day) over a 1-month period. Other routes of exposure may be simulated by the IEUBK Model pending available information from which to characterize both the exposure and media-specific absorption variables. Values for variables in the biokinetic component of the IEUBK Model are independent of the route of exposure. 

Once specific absorption features are assigned, kinetic studies can be performed via tuning the probe laser to a frequency absorbed by the fragment whose reaction kinetics are of interest. Ideally, it is also desirable to measure the rate of formation of the reaction product and to verify that these two rates correlate with each other. This has been done for the Fe(C0)x system with added CO where the reaction can be schematically depicted as... 

More interesting is the use of nonpolar solvents (e. g. xylene, toluene, carbon tetrachloride, hydrocarbons), because these are transparent to MW and absorb them only weakly. They therefore enable specific absorption by the reactants. If these reactants are polar, energy transfer occurs from the reactants to the solvent and the results might be different under the action of MW and A. This effect seems to be clearly dependent on the reaction and is, therefore, the subject of controversy. In xylene under reflux, for example, no MW-specific effects were observed for the Diels-Alder reaction [5] whereas important specific effects were described for aryldiazepinone synthesis [33]. 

It must be stressed that a liquid component can be substituted with an efficient absorber of microwave irradiation together with a low-melting component. The use of most typical PTC solvents (nonpolar aromatic or aliphatic hydrocarbons, or highly chlorinated hydrocarbons) is most interesting for microwave activation, because such solvents are transparent or absorb microwaves only weakly. They can, therefore, enable specific absorption of microwave irradiation by the reagents, and the results or product distributions might be different under microwave and conventional conditions [7]. 

The radiofrequency pulses involved in MRI cause thermal heating of the tissues, and are thus subject to FDA limits on the amount of RF power that is transmitted to a subject during a medical scan. The RF power unit is specified as the specific absorption rate (SAR) and is measured in watts per kilogram of body tissue (W/kg tissue). Powers that exceed this level put the subject at risk of tissue damage incurred as a result of the tissue s inability to remove the heat through blood flow. 

Again, care has to be taken for the non-ideal (or real) behavior of the measurement system. Applications are limited by non-specific absorption of molecules on the surface, mass transfer effects (under conditions of laminar flow a 1-5-pm layer between sensor surface and volume flow is not whirled and has to be passed by passive diffusion) or limited access for the immobilized molecules [158-160]. 

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