Permeation tube technique

Generation of test atmospheres of organic vapours by the syringe injection technique Generation of test atmospheres of organic vapours by the permeation tube method On-site validation of sampling methods... 

A clever means of dynamic generation of standards at the part-per-million level involves permeation through a polymer. In 1966 O Keeffe and Ortman (34) described this technique primarily for air pollution standards. A condensable gas or vapor is sealed as a liquid in a Teflon tube under its saturation vapor pressure as shown in Figure 4.14. After an initial equilibration period the vapor permeates through the tube wall at a constant rate. This rate is determined by weight loss over a period of time. Temperature must be controlled to within .0.1°C to maintain 1% accuracy. In use the tube is thermostatted in a chamber that permits a diluent gas to fully flush the chamber. The concentration is then determined by the same equation used for diffusion tubes. However, since the rate is generally much less in permeation tubes it is usually reported in ng/min. 

Permeation tubes are not refilled, have a limited life and cannot be turned off. However their life can be prolonged during periods of non-use by storing them in a refrigerator to reduce the permeation rate. Not many solutes are practical for use in permeation tubes. However, when the technique can be used it is generally preferred as a means of standard preparation. 

CI2 Permeation tube No suitable technique for continuous measurements... 

The limbs of the annealed thin Pd-Ag foil are then welded to form the permeator tube. Several joining techniques have been considered ... 

Annual Book ofASTM Standards, Vol. 11.03, Atmospheric Analysis, D3609, Standard Practice for Calibration Techniques Using Permeation Tubes, American Society for Testing and Materials, Philadelphia, PA, 2000. 

Finally, we note that diffusion tubes can be used as an alternative to permeation tubes. These involve the use of a liquid reservoir which is connected to the external environment by a capillary. Vapour will diffuse along this tube at a fixed rate at a fixed temperature. Consequently, a diffusion tube can then be used in a similar manner to a permeation tube. However, diffusion tubes require more operator skill and are suitable for fewer compounds than permeation tubes. Permeation tubes are therefore used far more widely than diffusion tubes in dynamic calibration techniques. 

Three methods of calibration are permitted. These are the Standard Sample Method (see 11.2), the Permeation Tube Method (see 11.3) and Blend Preparation Techniques (see 11.4). Obtain a calibration standard according to one of these methods, which are described below. Then follow the... 

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. 

Fukishi and Hiiro [222] determined sulfide in seawater by this technique. The method is based on the generation of hydrogen sulfide by the addition of sulfuric acid to the water sample. The gas permeated through a microporous polytetrafluoroethylene (PTFE) tube, and was collected in a sodium hydroxide solution. The carbon dioxide in the permeate was removed by adding a barium cation-exchange resin to the sodium hydroxide solution. Injection into the... 

Although most univalent ions are permeable in the gramicidin channel, the channel has some conductance properties that suggest that the permeation process may be more complicated than ion motion down a narrow tube . Channel block by divalent cations is mechanistically complicated (14). Also, when Tl+ ion is present as the sole permeant ion, it is an excellent permeant ion that retains linear behavior to very large transmembrane potentials (15). However, when this ion is present as the minority cation in variable mole fraction solutions of Tl+ and Na+, Tl+ ion severely limits set channel currents (16). Such anomalous behavior can be elucidated if the local velocity of a specific ion (e.g., Tl+ ion) can be determined experimentally. In fact, the laser Doppler scattering technique produces a detectable scatter only with the Tl+ so that ion motions within the gramicidin channels can be differentiated. 

Raghuraman and Wiencek [11] developed a hybrid technique where an emulsion is fed into a hollow fiber contactor on the tube side. Since the solid membrane support is hydrophobic, the continuous phase of the water-in-oil emulsion easily wets the pores of the tube wall and permeates to the shell side. On the shell side of the hollow fiber, the aqueous feed phase is exposed and held at an elevated pressure that prevents the permeating liquid membrane phase from exiting the pores. Thus, extraction occurs on the shell side, and stripping on the tube side of the hollow fiber membrane module. This methodology is closely related to SLMs, but the key difference is the presence of the emulsion on the tube side, which allows for long-term stability because the membrane liquid is continuously replenished to make up for any loss by solubility. 

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