Selective permeation region

FIOURE 28<27 a) Calibration curve for a size-exclusion column. 03) Chromatogram showing peak A containing all compounds with molecular masses greater than the exclusion limit, peaks B and C consisting of compounds within the selective permeation region, and peak D containing all compounds smaller than the permeation limit. 

This is called the membrane-selectivity-limited region, in which the membrane performance is determined only by the membrane selectivity and is independent of the pressure ratio. There is, of course, an intermediate region between these two limiting cases, in which both the pressure ratio and the membrane selectivity affect the membrane system performance. These three regions are illustrated in Figure 8.13, in which the calculated permeate concentration ( , ) is plotted versus pressure ratio pressure ratio of 1, feed pressure equal to the permeate pressure, no separation is achieved by the membrane. As the difference between the feed and permeate pressure increases,... 

Equation (10) gives the concentration of the faster permeating gas in the permeate stream as a function of the membranes selectivity and the pressure ratio across the membrane. It breaks down into two limiting cases. At high driving forces when the pressure ratio is much higher than the selectivity (0 a) Eq. (10) reduces to Eq. (3). We call this a selectivity controlled region, because the enrichment is now independent of the pressure ratio. When on the other hand the pressure ratio becomes much smaller than the selectivity (0[Pg.71]

This diagram shows the permeate concentration dependent on pressure ratio and selectivity. Two regions can be identified in which one indicates the pressure ratio controlled range and the other the selectivity controlled range. 

In situ perfusion studies assess absorption as lumenal clearance or membrane permeability and provide for isolation of solute transport at the level of the intestinal tissue. Controlled input of drug concentration, perfusion pH, osmolality, composition, and flow rate combined with intestinal region selection allow for separation of aqueous resistance and water transport effects on solute tissue permeation. This system provides for solute sampling from GI lumenal and plasma (mesenteric and systemic) compartments. A sensitive assay can separate metabolic from transport contributions. 

Nitrogen Dioxide The line selected for NOa measurement lies in the 1600 cm-1 region. A commercial permeation tube is used for the calibration source for NOB. The permeation rate is determined by weight loss and also checked by N0/03 titration with NO then serving as the calibration standard. The response time is 3 sec and the MDC is 25 pptv. 

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