Permeation devices

Air passing through the NO pathway enters the reaction chamber, where the NO present reacts with the ozone. The light produced is measured by the photomultiplier tube and converted to an NO concentration. The NO2 in the air stream in this pathway is unchanged. In the NO pathway, the NO- and N02-laden air enters the converter, where the NO2 is reduced to form NO all of the NO exits the converter as NO and enters the reaction chamber. The NO reacts with O3 and the output signal is the total NO concentration. The NO2 concentration in the original air stream is the difference between NO and NO. Calibration techniques use gas-phase titration of an NO standard with O3 or an NOj permeation device. 

Approach ofMotz et al. [55, 56] In 2006, Motz and coworkers published a dissolution permeation device combining a compendial flow through dissolution... 

Figure 9. Configuration of the DS-IC system A, clean air input B, mass-flow controller C, permeation device chamber D and H, vents E, needle valve-rotameter F, needle valve G, mass-flow meter I, diffusion scrubber Jy scrubber liquid reservoir K, needle valve-rotameter L, suction pump M, injection valve Ny peristaltic pump O, eluent flow F, downstream chromatographic components and Q, sample loop. (Reproduced from reference 96.

FIGURE 16.1 Sontra s SonoPrep skin permeation device + the topical anesthetic procedure tray. (From http //www.sontra.com. With permission.)... 

Instrumental accuracy is further highlighted by Figure 4.3, which shows the PTR-MS determination (in both counts-per-second and ppbv) of the output from a dimethyl sulfide (DMS) permeation device with and without an in-line charcoal filter. The device was gravimetrically found to produce a concentration of 2.60 ppbv... 

Figure 4.3 Measured concentrations and raw counts per second (cps) of a gaseous dimethyl sulfide (DMS) standard produced using a permeation device and calibration oven (Hayward et a ., 2002).

Deposition of reduced sulfur compounds to enclosed surfaces could also be measured by adding low loss (5 ng/min, GC Industries) permeation tubes to the inlet of the chambers. This permeation rate adds an additional sulfur load which varied typically between 2 and S times that already within the enclosure. These studies were limited to surfaces where large temporal changes in natural emissions were not occurring, and ultimately became limited by the reliability of the permeation devices. 

Scaringelli, F. P., O Keffe, A. E., Rosenberg, E., Bell, J. P. Preparation of known concentration of gases and vapors with permeation devices calibrated gravimetrically. 

Calibration is the largest source of error in the measurements. N0a permeation rates were determined by the rate of weight loss. Neighing errors amount to + 2%. The uncertainty in the NO/Oa titration method used to check the weight loss method is about 5%. Additional calibration errors of + 2% are caused by temperature variations of the permeation devices. 

Sulphur gases in permeation devices are the usual means for calibrating the response of a gas chromatographic detector. Liquefied gases are sealed in Teflon tubes, or in glass or stainless steel tubes with Teflon "windows" on the ends. Gases permeate through the Teflon at a constant rate until the enclosed supply is exhausted. The rate of permeation is... 

FIGURE 20.1-2 Early Dow hollow-fiber permeation device, Note that permeant is withdrawn from both ends. 

Alternatively, the use of permeation devices has been adopted in later studies for the generation of more accurate gas mixtures. In addition, sample introduction via injection valves has replaced the syringe injection method. 

Chemical kit, 8, 9 Chemical sample, liquid, 6.1 Chemical sample, solid, 6.1 Chemical sample, toxic, liquid or solid, 6.1 First aid kit, 9 Gas identification set, 2.3 Gas sample, non-pressurised, n.o.s., not refrigerated liquid, 2.1, 2.3 Permeation devices, containing dangerous goods, for calibrating air quality monitoring Samples, explosive other than initiating explosives, 1... 

A gas identification set includes containers of small quantities of various gases of known concentration that are used as standards against which to calibrate analytical equipment. Permeation devices contain small containers of compressed or liquefied gases that, on opening, pass through a gas permeable membrane at a known rate and constant temperature. They are also used for calibration. 

Other calibration techniques include mixing a known volume of gas with nitrogen or air in a fixed volume container and the use of liquid standards, where analytes are purchased or prepared in a liquid matrix, such as methanol, and then diluted into the gas phase. These methods usually involve several dilution steps to generate standards in the ppt range. Permeation devices have also found limited use. 

Fig. 10.64 Permeation device with i, GC 2, test cell p 3, test cell oven and sample transfer oven (Vaha-Nissi et al. 2008)...

The permeation rate is determined by the gravimetric method. The permeation device, after it has been assembled, is weighed and then placed in a temperature-controlled cavity with constant flow of nitrogen or clean air to sweep away the permeated chemical vapor. After a period of time, perhaps days, the tubing is weighed again and the permeation rate is determined by weight loss over elapsed time. Since... 

Therefore, the permeate side gas mixture at equilibrium has a composition equal to that of the original feed gas mixture at f = 0 no separation has taken place in the closed system. This is why gas permeation devices in practice are operated as an open system any gas mixture produced hy permeation is immediately withdrawn from the device. Since the intrinsic membrane transport rates of the two species for unit partial pressure difference between the feed and the permeate are different, the gas mixture produced as a permeate has a composition different from that of the feed. The permeate mixture withdrawn from the device must have a composition different from that of the feed mixture for separation. See Sections 6.3.3.S, G.4.2.2, 7.2.1.1 and 8.1.9 for open systems. Continuous energy input will yield separation in a closed system - see Figures 8.1.4(b) for a system operated at total reflux. 

Figure 6.3.32. Gas permeation device, where the hulk gas flow direction is parallel to the direction of force across the membrane for separating a gas mixture the membrane is selective for species A.

Figure 6.4.10. Gas permeation device where both the feed side and the permeate side are completely mixed.

The analysis of spirally wound gas permeation devices shown in Figure 7.2.1(e) will not follow the traditional countercurrent analysis formalism. See Ptm (1983, 1986). 

Figure 8.1.4(b) illustrates two membrane gas permeation devices coupled together with a compressor at the top. Suppose the whole system is filled with a binary gas mixture and the two sides of the membrane are at two different pressures as shown. No permeate product is being withdrawn, however, so there is... 

The most used analytical technique for SO2 monitoring is based on fluorescence. Excitation of sulfur dioxide molecules by ultraviolet radiation (UV) in the 190-230 nm region gives an emitting fluorescent radiation, whose wavelength and intensity are directly proportional to the concentration of sulfur dioxide. The main interfering compound is represented by polycyclic aromatic hydrocarbons to eliminate their effect, the analyzers are equipped with a permeation device which selectively removes hydrocarbon molecules from the gas sample. ... 

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