Absorption - the Principle

X-rays are absorbed whenever they travel through matter. As a result, the total transmitted intensity, h, measured after passing the absorber is only a fraction, /,//o, of the incident intensity, /q. For amorphous or polycrystalline material the incremental absorption within a layer of thickness M is constant, and by integration along the complete light path i through the sample the absorption law 

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The purpose of the constant magnetic field is to split the single level in two due to the nuclear Zeeman effect. The alternative electromagnetic field 5(co) facilitates transitions between sublevels. The third coil is to measure absorption. The principle difference between NMR and FRS is as follows on account of its enormous resolution, y resonance allows one to define the superfine structure investigating the transition between different nuclei states (levels) with different /, whereas NMR allows one to measure the transition... 

In the early 1970s, air pollution requirements led to the adoption of the double contact or double absorption process, which provides overall conversions of better than 99.7%. The double absorption process employs the principle of intermediate removal of the reaction product, ie, SO, to obtain favorable equiUbria and kinetics in later stages of the reaction. A few single absorption plants are stiU being built in some areas of the world, or where special circumstances exist, but most industriali2ed nations have emission standards that cannot be achieved without utili2ing double absorption or tad-gas scmbbers. A discussion of sulfuric acid plant air emissions, control measures, and emissions calculations can be found in Reference 98. 

Radioisotope dilution assays are based on the principle of competition between radioactive labeled ( Co) vitamin B 2 and cobalamins extracted from matrices for binding sites on the intrinsic factor (a glycoprotein). Binding is in proportion to the concentration of the radioactive and nonradio active B 2 with the concentration of intrinsic factor as the limiting factor. Free cobalamins are separated from those bound on the intrinsic factor by absorption... 

A necessary prerequisite to understanding the subject of absorption with chemical reaction is the development of a thorough understanding of the principles involved in physical absorption, as discussed earlier in this section and in Section 5. There are a number of excellent references the subject, such as the book by Danckwerts Gas-Liquid Reactions, McGraw-Hill, New York, 1970) and Astarita et al. Gas Treating with Chemical Solvents, Wiley, New York, 1983). 

The primary reference method used for measuring carbon monoxide in the United States is based on nondispersive infrared (NDIR) photometry (1, 2). The principle involved is the preferential absorption of infrared radiation by carbon monoxide. Figure 14-1 is a schematic representation of an NDIR analyzer. The analyzer has a hot filament source of infrared radiation, a chopper, a sample cell, reference cell, and a detector. The reference cell is filled with a non-infrared-absorbing gas, and the sample cell is continuously flushed with ambient air containing an unknown amount of CO. The detector cell is divided into two compartments by a flexible membrane, with each compartment filled with CO. Movement of the membrane causes a change in electrical capacitance in a control circuit whose signal is processed and fed to a recorder. 

EXAFS is part of the field of X-ray absorption spectroscopy (XAS), in which a number of acronyms abound. An X-ray absorption spectrum contains EXAFS data as well as the X-ray absorption near-edge structure, XANES (alternatively called the near-edge X-ray absorption fine structure, NEXAFS). The combination of XANES (NEXAFS) and EXAFS is commonly referred to as X-ray absorption fine structure, or XAFS. In applications of EXAFS to surface science, the acronym SEXAFS, for surface-EXAFS, is used. The principles and analysis of EXAFS and SEXAFS are the same. See the article following this one for a discussion of SEXAFS and NEXAFS. 

Certain highly porous solid materials selectively adsorb certain molecules. Examples are silica gel for separation of aromatics from other hydrocarbons, and activated charcoal for removing liquid components from gases. Adsorption is analogous to absorption, but the principles are different. Layers of adsorbed material, only a few molecules thick, are formed on the extensive interior area of the adsorbent - possibly as large as 50,000 sq. ft./lb of material. 

Physiologically based toxicokinetic models are nowadays used increasingly for toxicological risk assessment. These models are based on human physiology, and thus take into consideration the actual toxicokinetic processes more accurately than the one- or two-compartment models. In these models, all of the relevant information regarding absorption, distribution, biotransformarion, and elimination of a compound is utilized. The principles of physiologically based pharmaco/ toxicokinetic models are depicted in Fig. 5.41a and h. The... 

Since many of the uses of antihistamines involve conditions such as rashes, which should be treatable by local application, there is some rationale for developing drugs for topical use. The known side effects of antihistamines could in principle be avoided if the drug were functionalized so as to avoid systemic absorption. The known poor absorption of quaternary salts make such derivatives attractive for nonabsorbable antihistamines for topical use. Thus, reaction of the well-known anti his-taminic drug promethazine (104) with methyl chloride leads... 

In the classical set-up of bulk liquid membranes, the membrane phase is a well-mixed bulk phase instead of an immobilized phase within a pore or film. The principle comprises enantioselective extraction from the feed phase to the carrier phase, and subsequently the carrier releases the enantiomer into the receiving phase. As formation and dissociation of the chiral complex occur at different locations, suitable conditions for absorption and desorption can be established. In order to allow for effective mass transport between the different liquid phases involved, hollow fiber... 

There is limited application for CO trim systems which are widely used on utility and other large water-tube boilers. The principle of operation is for an infrared beam to traverse the flue from emitter to sensor. The absorption of the infrared radiation is proportional to the CO content. 

Now if any irreversible changes occur in the system itself during the execution of the cycle, the principle of dissipation of energy shows that the available energy will be diminished in virtue of these, and since the available energy of the system must be the same after as it was before the execution of the cycle, because the state of the system is unaltered, it follows that some available energy must have been absorbed from outside in connection with the absorption of heat hence (A ) and therefore also... 

Though the element was discovered in 1789 it was not prepd in the pure state until 1914. It may be prepd commercially by the reaction of zirconium chloride with magnesium (the Kroll process) and other methods. The principle ore is zircon, deposits of which are found in the USA, Australia and Brazil. A number of special properties, such as exceptional resistance to corrosion and a low absorption cross section, have led to the use of Zr or alloys contg Zr, in many... 

The principles of and instrumentation for absorption and fluorescence spectroscopy have been discussed in detail in standard texts (S). 

Detection is primarily based on the principle of fluorescence quenching by substances absorbing UV light. It is also possible to detect certain substances whose absorption wavelengths interfere with the uranyl cation [1]. 

In the author s laboratory, an Autoanalyzer turntable is used which rotates, at a speed, so that the 40 specimens can be assayed in a 10 - 15 minute period. Figure 24 shows the principle of atomic absorption. Note that an elongated absorption path is provided for higher sensitivity. 

Figure 24, The basic principle used in atomic absorption. The sample is sprayed into the flame, and the calcium and magnesium emission from the lamp is absorbed. The extent of absorption is measured on the detector arm translated in terms of concentration.

Fig. 5.17 CdS-ZnO coupled semiconductor system (a) interaction between two colloidal particles showing the principle of the charge injection process and (b) light absorption and electron transfer on an electrode surface leading to the generation of photocurrent. (Reproduced from [330])...

In culture, the human colon carcinoma cell hne Caco-2 spontaneously differentiates at confluency into polarized cells with enterocyte-like characteristics. The principle of this approach consists of following the passage of the compound of interest from the apical or lumen-like sides to the basolateral or lymph-hke sides of Caco-2 cells, thus following the absorption of the compound per se. One obhgate step for fat-soluble nutrients such as carotenoids to cross the intestinal barrier is their incorporation into CMs assembled in the enterocytes. Under normal cell culture conditions, Caco-2 cells are unable to form CMs. When supplemented with taurocholate and oleic acid, Caco-2 cells were reported to assemble and secrete CMs. ... 

The force Rayleigh scattering method was developed by Nagashima s group. Thermal diffusivity can be measured in a contact-free manner within a time interval of 1 ms, with a small temperature rise of 0.1 K and with a small volinne of about 10 mm The sample needs to be colored by an admixture of a dye for suitable absorption of a heating laser beam. The principle is schematically shown in Fig. 32. Two beams of equal intensity divided by means of abeam splitter cross in the sample to create... 

As a generalization, to be orally well absorbed a compound must be soluble in the contents of the gastrointestinal lumen [4]. Solubility in aqueous buffer is commonly used as a simpHfying surrogate for intestinal content solubility. There are rare exceptions to the principle that to be absorbed a compound must be soluble. SoHd particles, e.g. starch, can be absorbed. Absorption of very small quantities of even biologically very large compounds can occur via lymphoid tissue, e.g. orally active vaccines. Very hpophihc basic compounds, e.g. certain antimalarials, can be absorbed via the intestinal lymphatics and dehvered directly to the heart... 

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