Micro-diffusion method

A micro-diffusion method has been devised by Winnick101 for the determination of acetaldehyde. Procedures for the assay of glycerose were developed by Jayme, Satre and Maris.167,168 They dehydrated glycerose to pyruvic aldehyde by distillation from mineral acid, and determined the pyruvic aldehyde as its phenylosazone. 

Free ammonia and amide ammonia were determined by modifying the micro-diffusion method of Conway (6) the diffusates were assayed by means of Nessler reagent. In the measurement of free ammonia, the samples were diluted three times with saturated K2C03 solution in the outer compartment of the diffusion cell recovery of ammonia in the acid compartment (0.1N H2S04) is complete in three hours. For total ammonia (free plus amide) the sample was made 2N in NaOH the amide... 

In flow-injection analysis, volatile analytes or analyte compounds may be separated from interferents in an ill-defined sample stream and transplanted into a liquid or gaseous acceptor stream with well-defined composition. Reaction conditions for effecting the gas-liquid separation and detection of the separated species may be optimized independently, often greatly enhancing the selectivity of the determinations. The gas-liquid separations are effected through on-line separators incorporated in the FI manifolds. The effects of the separation process are often equivalent to batch distillation or isothermal distillation procedures, such as the Conway micro-diffusion method [1], developed some forty years ago, which are much less efficient and consume much more sample and reagent. 

More recently [34] it was discovered that the C02 treatment is much more efficient if a semi-continuos apparatus (called a micro-bubble method) is employed instead of the batch apparatus the contact surface between the gas and the medium where the microbes live is much larger, as the liquid appears in the form of micro-bubbles created by the turbulence of the gas-flow. In this way, the diffusion to the liquid phase is enhanced as well as the material... 

The study of reactivity by QM calculations concerns only a small part of a catalytic event, as phenomena such as macro and micro diffusion of the reactants and products outside and inside the zeolite micropores cannot be investigated. On the other hand, QM methods are the only theoretical methods available that provide information on the reactivity. 

In addition to the vapor diffusion method described previously, other techniques such as the batch and micro-batch methods, bulk and micro dialysis, free interface diffusion, liquid bridge, and concentration dialysis have also been developed to produce crystals for x-ray diffraction analysis (see McPherson, 1982 and McPherson, 1999). 

While the carbon dioxide produced by decarboxylation of histidine can be measured by the standard Warburg manometer technique, the utility of the method is severely limited by the very small degree to which decarboxylation occurs with most mammalian histidine decarboxylase preparations even at high substrate concentrations. In practice, only bacterial histidine decarboxylases have proved sufficiently active to be measured conveniently by the manometric method . Small amounts of carbon dioxide can, however, be determined by the sensitive micro-diffusion technique of Conway , and this has been used successfully for measuring the activity of mammalian histidine decarboxylases . 

The diffusivity of O in high-purity polycrystalline material was determined, by using 18o as a tracer, in specimens which had been prepared by annealing powder at 1400C. The diffusion process was monitored by means of reaction gas analysis or by using micro-balance methods. It was found that, at temperatures ranging from 800 to 1 lOOC, the O tracer diffusion coefficient could be described by ... 

The volatile material to be estimated is placed in the outer annular compartment, the absorbent being in the inner compartment, where it is titrated subsequently without removal from the unit. The apparatus may be employed for the micro-estimation of ammonia, urea, chloride, bromide and carbonate, and for the detection of many volatile substances, such as alcohol, aldehydes and acetone. For details of the apparatus and its applications consult Conway, E. J. (1939), Micro-Diffusion Analysis. The various micro-methods available for the clinical estimation of urea are critically discussed by Lee and Widdowson Biochem. J., 1937, 31, 2035). The error arising from formation of ammonia by an enzyme-substrate system in jack bean urease extracts has been studied by Howell (1939), who reports that it can be eliminated by the use of extracts prepared five to twenty hotu-s before use, and by keeping the pH value of the digests between 6 and 6-6 by means of a citrate buffer. 

Four characteristics of this method deserve mention. First, with careful technique it is accurate to better than 0.3%. The caveat is careful technique it is unusually easy to fool yourself with this equipment, obtaining reproducible inaccurate results. Second, the small size of the diffusion cell dictates careful chemical analysis of very small volumes of solution. In practice, this suggests using either radioactive tracers or some other micro-analytical method. Third, the power series in Eq. 5.6-14 converges rapidly. If you use reasonably long experiments, you can base your analysis on the first term in the series. Finally, for radioactive tracers this method may give an intradiffusion coefficient, not a binary coefficient (cf. Section 7.5). 

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

Molecular diffusion ( ) has been used in various ways in micro analysis. In Figure 27, is seen the Conway diffusion dish, in which the substance to be tested is placed on the outside, and the reagent is placed in the central cup. The cup is covered, and after a time, the substance being analyzed diffuses into the central cup where it produces an effect which can then be interpreted in various ways. In the Figure, carbon monoxide is being determined. This same method is very useful for alcohol determination, where dichromate oxidizes the alcohol after it diffuses into the dichromate from the blood. 

Table 1 summarizes several of the experimental methods discussed in this chapter. A need exists for new or revised methods for transport experimentation, particularly for therapeutic proteins or peptides in polymeric systems. An important criterion for the new or revised methods includes in situ sampling using micro techniques which simultaneously sample, separate, and analyze the sample. For example, capillary zone electrophoresis provides a micro technique with high separation resolution and the potential to measure the mobilities and diffusion coefficients of the diffusant in the presence of a polymer. Combining the separation and analytical components adds considerable power and versatility to the method. In addition, up-to-date separation instrumentation is computer-driven, so that methods development is optimized, data are acquired according to a predetermined program, and data analysis is facilitated. 

Equation (9.15) was written for macro-pore diffusion. Recognize that the diffusion of sorbates in the zeoHte crystals has a similar or even identical form. The substitution of an appropriate diffusion model can be made at either the macropore, the micro-pore or at both scales. The analytical solutions that can be derived can become so complex that they yield Httle understanding of the underlying phenomena. In a seminal work that sought to bridge the gap between tractabUity and clarity, the work of Haynes and Sarma [10] stands out They took the approach of formulating the equations of continuity for the column, the macro-pores of the sorbent and the specific sorption sites in the sorbent. Each formulation was a pde with its appropriate initial and boundary conditions. They used the method of moments to derive the contributions of the three distinct mass transfer mechanisms to the overall mass transfer coefficient. The method of moments employs the solutions to all relevant pde s by use of a Laplace transform. While the solutions in Laplace domain are actually easy to obtain, those same solutions cannot be readily inverted to obtain a complete description of the system. The moments of the solutions in the Laplace domain can however be derived with relative ease. 

Application of this method within the Cap d Ours segment of the Glenwood rhyolite has identified distinct distributions of alteration associated with a low-temperature hydrothermal system. The ability of this method to detect diffuse alteration will make micro-XRF analyses a powerful tool in exploration. 

Lawrence and Sanderson proposed another micro-method for measuring chymosin and other proteolytic enzymes. Measurement of concentration was based on the rate of radial diffusion of the enzyme through a thin layer of caseinate-agar gel. The limit of diffusion was marked by a zone of precipitated casein (Emstrom and Wong 1974). Holmes et al (1977) developed a microdiffusion assay for residual proteolytic enzymes in curd and whey that is more sensitive than the method of Lawrence and Sanderson or the clotting-time assay of Reyes (1971). 

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