Transport, studies

Monitoring Well Design for Contaminant Transport Studies... 

Individual polyethers exhibit varying specificities for cations. Some polyethers have found appHcation as components in ion-selective electrodes for use in clinical medicine or in laboratory studies involving transport studies or measurement of transmembrane electrical potential (4). The methyl ester of monensin [28636-21 -7] i2ls been incorporated into a membrane sHde assembly used for the assay of semm sodium (see Biosensors) (5). Studies directed toward the design of a lithium selective electrode resulted in the synthesis of a derivative of monensin lactone that is highly specific for lithium (6). 

EPA, 1996, is the third edition of EML/DVIES on CD-ROM for distributing exposure models, documentation, and the IMES about many computer models used for exposure assessment and other fate and transport studies as developed by the EPA s Office of Research and Development (ORD). 

Nakamura, Takagi and Ueno have also utilized 4 -nitrobenzo-l 5-crown-5 as a starting material Their goal was the formation of a colored crown which could be utilized in transport studies. They have prepared 4 -picrylaminobenzo-l 5-crown-5 for this purpose in the following way. 4 -Nitrobenzo-l 5-crown-5 was hydrogenated and then picryl chloride was added. Nucleophilic aromatic substitution apparently ensued (deep red color) and the product was th n isolated by standard techniques as a yellow solid (mp 155°, max 395 nm) in 72% yield as shown in Eq. (3.17). 

Compared with other alternative motor fuel options (reformulated gasoline, compressed or liquefied natural gas, ethanol from corn or coal, methanol and electricity), propane has the lowest greenhouse gas emissions except for natural gas. According to a 1998 study by the Institute of Transportation Studies, greenhouse emissions from propane vehicles arc 21.8 percent less than from gasoline or diesel. 

Center for Transportation Studies Massachusetts Institute of Technology... 

Institute of Transportation Studies, University of California, Davis 1 lybrid Vehicles... 

Lauchli, A., Spurr, A.R. Wittkop, R.W. (1970). Electron probe analysis of freeze substituted, epoxy resin embedded tissue for ion transport studies in plants. Planta, 95, 341-50. 

The differential vulnerability of fine and beaded 5-HT axons, combined with evidence from anterograde transport that fine and beaded fibers arise from the DR and MR nuclei, respectively, led to the proposal that axons from the DR nucleus are selectively vulnerable to the neurotoxic effects of psychotropic amphetamines, while the MR projection is resistant. The prior anterograde transport study (Kosofsky 1985 Kosofsky and Molliver 1987) sampled a relatively small number of neurons in the central portions of the DR and MR nuclei and suggested a predominantly differential origin of the two axon types. In order to determine directly whether the DR and MR projections are differentially sensitive to psychotropic amphetamines,... 

A general review of actinide transport on colloids, and how this may relate to radionuclide transport studies, is provided by Ivanovich (1991). It has been found that colloids can carry a large fraction of U (Dearlove et al. 1991). Due to the greater reactivity. 

W. E., Morf, The Principles of Ion-Selective Electrodes and of Membrane Transport (Studies in Analytical Chemistry, 2), Elsevier, Amsterdam, 1981. 

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

Differentiating between these cases is critical to the selection or development of an experimental method which corresponds to the case of interest. Incorrectly interpreting the data from a transport study by assuming one case when another prevails will lead to erroneous conclusions. Other theoretical aspects which affect the experimental outcome are discussed in connection with each method. 

The side-by-side diffusion cell has also been calibrated for drug delivery mass transport studies using polymeric membranes [12], The mass transport coefficient, D/h, was evaluated with diffusion data for benzoic acid in aqueous solutions of polyethylene glycol 400 at 37°C. By varying the polyethylene glycol 400 content incrementally from 0 to 40%, the kinematic viscosity of the diffusion medium, saturation solubility for benzoic acid, and diffusivity of benzoic acid could be varied. The resulting mass transport coefficients, D/h, were correlated with the Sherwood number (Sh), Reynolds number (Re), and Schmidt number (Sc) according to the relationships... 

The ability of any experimental method to produce accurate and reproducible results and provide the sensitivity needed to discern differences between transport mechanisms depends on minimizing variability intrinsic to the method. However, formal error analysis is rarely undertaken, even for commonly used methods. Fawcett and Caton [45] performed an error analysis of the capillary method for determining diffusion coefficients more than 25 years after the method was introduced. The value of the analysis is that it reveals which factors contribute the greatest variability to the dependent variable of interest. In the case of transport studies, the dependent variable of primary interest is diffusant concentration, C(t), where... 

Side-by-side diffusion cell (membrane method) Diffusion coefficient determination mass transport studies 10-14... 

Diffusion cell for heterogeneous systems (membrane method) Mass transport studies from emulsions 18... 

Diffusion cell for gels (free boundary method) Mass transport studies from water-miscible gels 19... 

Rotating disk method (free boundary method) Diffusion coefficient determination from solids mass transport studies 25-29... 

Precellular solute ionization dictates membrane permeability dependence on mucosal pH. Therefore, lumenal or cellular events that affect mucosal microclimate pH may alter the membrane transport of ionizable solutes. The mucosal microclimate pH is defined by a region in the neighborhood of the mucosal membrane in which pH is lower than in the lumenal fluid. This is the result of proton secretion by the enterocytes, for which outward diffusion is slowed by intestinal mucus. (In fact, mucosal secretion of any ion coupled with mucus-restricted diffusion will provide an ionic microclimate.) Important differences in solute transport between experimental systems may be due to differences in intestinal ions and mucus secretion. It might be anticipated that microclimate pH effects would be less pronounced in epithelial cell culture (devoid of goblet cells) transport studies than in whole intestinal tissue. 

As will be discussed in the following section, a variety of experimental designs are available and have been described to conduct such transport studies. While these variations are all available, the choice of a system and design of the experiment will be dictated by the information desired from the study. However, as illustrated in the applications section, if the variables present in the experimental design are taken into proper consideration, it will be possible to extract mechanistic information which is essentially independent of the system used. In this way it should be possible to compare results from one system or laboratory with those of another. 

Several examples have already been pointed out in which the properties of the solute itself can impact on the results obtained from a transport experiment. Metabolic instability and propensity for nonspecific adsorption are problems which can frequently be encountered and must be considered any time a new solute is to be studied. In addition to these problems, there are several other solute-related factors which must be considered in the design and interpretation of transport studies. 

The transport studies are summarized in Figure 26, in which the Pe values of the compounds are plotted against the apparent log PC values at pH 7.4 and... 

ME Dowty, P Braquet. (1991). Effect of extracellular pH on cytoplasmic pH and mechanism of pH regulation in cultured bovine corneal endothelium Possible importance in drug transport studies. Int J Pharm 68 231-238. 

Limiting-current phenomena may also result from a slow step in the reaction mechanism at the surface of the electrode (V4, p. 233 ff). Electrode reactions of this type are not suitable for mass-transport studies. 

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