Diffusion of reagents through

The process described is related to the unique effects carbon dioxide and moisture have on coal. Carbon dioxide readily and extensively penetrates the coal structure O). In fact, this has led to the advocacy of employing carbon dioxide to measure the internal area of coals. It is likely the CO2 diffuses into the coal along the lines of mineral inclusion as has been seen for other gases ( ). This is important since the diffusion of reagents through solid coal may be the rate-limiting step in many reactions. This diffusion has also been shown to cause a dimensional expansion of the structure. 

The assembly of such core-shell particles, both in two and three dimensions, further enables tailored complex structures to be created with new properties. The formation of even more complex structures can be accomplished by making use of the semipermeable nature of the silica shell. The slow diffusion of reagents through the shell allows selective core etching and chemical conversion to be effected. The kinetics of such reactions are controlled by the porosity and the thickness of the shell, so that the cores can be made resistant to almost any chemical reagent. 

Utilization of whole cells and tissues in biosensor has increasingly been used. Enzyme stability, availability of different enzymes and reaction systems, and characteristics of cell surface are the advantages of using cells and tissues in biosensor designs. Multi-step enzyme reactions in cells also provide mechanisms to amplify the reactions that result in an increase in the detectability of the analytes. The presence of cofactors such as NAD, NADP, and metals in the cells allows the cofactor-dependent reactions to occur in the absence of reagents. (34, 50, 69). However, the diffusion of analytes through cell wall or membrane imposes constraint to this type of biosensors and results in a longer response time compared to the enzyme biosensors. 

Lesser reactivity of insoluble polymer-bound reagents and catalysts is normal, because diffusion of reactants through the polymer network or through the pores of a macroporous catalyst becomes partially rate-limiting for fast reactions. This diffusional limitation of catalyst activity has been analyzed in detail for phase transfer catalysis 1201 and for transition metal catalysis in chapter 4 by Eterdt... 

Fig. 2.4p shows three types of post-column reactor. In the open tubular reactor, after the solutes have been separated on the column, reagent is pumped into the column effluent via a suitable mixing tee. The reactor, which may be a coil of stainless steel or ptfe tube, provides the desired holdup time for the reaction. Finally, the combined streams are passed through the detector. This type of reactor is commonly used in cases where the derivatisation reaction is fairly fast. For slower reactions, segmented stream tubular reactors can be used. With this type, gas bubbles are introduced into the stream at fixed time intervals. The object of this is to reduce axial diffusion of solute zones, and thus to reduce extra-column dispersion. For intermediate reactions, packed bed reactors have been used, in which the reactor may be a column packed with small glass beads. 

There are two possible configurations for this type of flow-through sensor integrating gas diffusion, reaction and detection that differ in whether the reagent is dissolved in the acceptor solution or immobilized on a sensing microzone located near the diffusion membrane. The descriptions below are based on such a difference. 

An experimental set-up such as that depicted in Fig. 5.12.C was also used by Nieman s group for the determination of free and total cholesterol in serum [37]. A solution of the enzyme (cholesterol oxidase) at pH 7 was forced through a microporous membrane. The pH gradient across the membrane (from 7 to 9) facilitated the enzymatic degradation of the analyte and subsequent diffusion of the hydrogen peroxide formed to the PMT in the reagent/sample stream in order to react with luminol in the presence of horseradish peroxidase as the catalyst at pH 9 —the copper chelate commonly used as catalyst for this purpose requires pH 11. The system per-... 

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