Diffusion chamber studies

Four field techniques have been used to estimate the import/export of nutrients between saltmarshes and the adjacent coastal water community budgeting, direct creek studies, flume studies and diffusion chamber studies. The direct tidal creek method is the only one which takes account of the processes in all of the saltmarsh subsystems and hence appears to be the best technique, provided the uncertainties in water discharge are minimised. 

Models of chemical reactions of trace pollutants in groundwater must be based on experimental analysis of the kinetics of possible pollutant interactions with earth materials, much the same as smog chamber studies considered atmospheric photochemistry. Fundamental research could determine the surface chemistry of soil components and processes such as adsorption and desorption, pore diffusion, and biodegradation of contaminants. Hydrodynamic pollutant transport models should be upgraded to take into account chemical reactions at surfaces. 

A diffusion chamber (Figure 1) originally designed for neutralization rate studies by Chu and Hopke (1985) has been modified to be an... 

Either Transwell inserts or side-by-side diffusion chambers can be used for transport studies. Bode et al. have provided an excellent review on this subject [60], Briefly, cells are incubated for 30-60 min with a buffer solution. To initiate the transport study, a transport buffer containing the drug under investigation is added to either the apical or the basal chamber depending on the transport direction of interest. At predetermined time points, the respective receiver chamber is sampled and the withdrawn volume is replaced with the same volume of fresh buffer. The permeability coefficient (Papp) is calculated and the ratio of /apP in the basolateral-to-apical direction versus that in the apical-to-basolateral direction gives the efflux ratio. These sort of transport experiments are well suited to determine if drugs/xenobiotics are substrates of the placental efflux proteins. 

The intention to study transport processes at pulmonary epithelia, however, raised two particular problems (i) the apical side of these epithelia is typically in contact with air rather than with a liquid and (ii) in order to maximize the surface area, the lungs have a complex treelike structure, ending in millions of tiny alveolar bubbles. The total surface area of the human alveolar epithelium is almost half of that of the intestines (100-120 m2), with its macroscopic appearance resembling a sponge, and it is virtually impossible to use such a tissue for transport experiments in a diffusion-chamber setup. 

Alternatively the membrane passage of human airway epithehal ceh lines can be studied in vitro. A number of bronchial epithehal ceh hnes is available, such as the 16HBE14o- and Calu-3 cell hnes. These ceh hnes can be installed in diffusion chambers to measure transport rates [34]. A major disadvantage of the currently used cell hnes is that they provide information about bronchial epithehal transport only. Since bronchial epithelium is very different from alveolar epithehum, the information from these in vitro studies is of limited value for the prediction of the bioavahabihty of pulmonary administered proteins. 

Numerous surface-active molecules have been studied as GI absorption promoters in a wide variety of testing conditions, including model membranes, everted intestinal sacs, tissue cultures, intestinal epithelia in diffusion chambers, intact animals, and humans. The physical properties of a chemical enhancer may be strongly dependent on the interactions with the endogenous GI components such as bile salts, pH, and bacteria. Thus the in vitro experiments on enhancing GI absorption are not necessarily predictive of the behavior of the promoter in animals or humans, and we will mainly focus on summarizing results from in vivo studies. 

Human cadaver skin has also been studied in vitro. Human skin shows a higher threshold of sensitivity than does rat skin. The excised or full-thickness slices are also studied in Fran 2 diffusion chambers to evaluate the diffusion or absorption characteristics of test materials. Changes in the amount of a test material at different times and different depths are... 

To summarise, flume and chamber studies are helpful for investigating the relative importance of surface diffusion, wash off and plant uptake/release from aerial shoots, when used in conjunction with direct tidal creek flux measurements. However, the results from these experiments should not be extrapolated to give a flux value for the marsh as a whole as they do not account for sediment drainage, water column and creek bank processes. 

However, the two-sink model as well as other existing adsorption (sink) models do not seem to be able to describe the strong asymmetry between the adsorption/desorption of VOCs on/from indoor surface materials (the desorption process is much slower than the adsorption process). Diffusion combined with internal adsorption is assumed to be capable of explaining the observed asymmetry. Diffusion mechanisms have been considered to play a role in interactions of VOCs with indoor sinks. Dunn and Chen (1993) proposed and tested three unified, diffusion-limited mathematical models to account for such interactions. The phrase unified relates to the ability of the model to predict both the ad/absorption and desorption phases. This is a very important aspect of modeling test chamber kinetics because in actual applications of chamber studies to indoor air quality (lAQ), we will never be able to predict when we will be in an accumulation or decay phase, so that the same model must apply to both. Development of such models is underway by different research groups. An excellent reference, in which the theoretical bases of most of the recently developed sorption models are reviewed, is the paper by Axley and Lorenzetti (1993). The authors proposed four generic families of models formulated as mass transport modules that can be combined with existing lAQ models. These models include processes such as equilibrium adsorption, boundary layer diffusion, porous adsorbent diffusion transport, and conveetion-diffusion transport. In their paper, the authors present applications of these models and propose criteria for selection of models that are based on the boundary layer/conduction heat transfer problem. 

Small Intestinal Cell Lines. The most notable, and certainly best characterized, in vitro tool for studying absorption and, to a less extent, intestinal metabolism utilizes Caco-2 cells grown in a confluent monolayer on porous membrane filters and, for the experiments, mounted in diffusion chambers. Under these growing conditions they differentiate spontaneously into polarized enterocyte-like cells possessing an apical brush border and tight Junctions between adjacent cells, thereby retaining many characteristics of the intestinal brush border. The permeability of a compound is determined by the rate of its appearance in the basolateral compartment. 

Franz diffusion ceii studies, Everted gut sac technique Diffusion Cells using Tissues Ussing Chamber For evaluatton of intestinal and gut wall permeability To detect transport of electrolytes, drugs across epithelial tissue... 

In summary, skin penetration tests consist of placing the piece of skin in a diffusion chamber (e.g., Franz-cell-type static diffusion chamber or flow-through diffusion cell) with two compartments. The external side of the skin is turned toward the upper compartment in which the test solution is applied. After a defined contact time with the skin surface, the effect of the surfactant on skin permeability properties is determined, often by means of a penetration marker (e.g., tritiated water). Many variations in penetration studies have been described and the reader is referred to other articles for more details [34,35]. 

Most significantly, the overall pressme loss across the cyclone was found to decrease by 40% at a penetration of about 75% relative to 0% penetration of the vortex finder into the diffuser chamber. Compared to the case where the gas exhausted directly out the vortex finder (the dashed line), the pressure loss decreased by about 44%. These are very significant reductions, especially in light of the simplicity of the diffuser geometry employed in this study. These results support Dehne s earlier comments, only they indicate a far larger pres-sme drop reduction. They are also very similar to those reported by Idelchik (1986) for the pressure loss coefficient for linear (irrotational) flow of a jet exiting a pipe and impacting a flat baffle plate. See Fig. 15.1.17. 

Nevertheless, from release (collection) studies we know that enough NT must diffuse (overflow) to the collecting system, be that a fine probe in vivo or the medium of a perfusion chamber in vitro, to be detected. Thus one must assume that either the concentration gradient from the collecting site back to the active synaptic release site is so steep that the NT can only reach an effective concentration at the latter, or it is not unphysiological for a NT to have an effect distal from its site of release. 

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