Membrane Teflon

A second form of desolvation chamber relies on diffusion of small vapor molecules through pores in a Teflon membrane in preference to the much larger droplets (molecular agglomerations), which are held back. These devices have proved popular with thermospray and ultrasonic nebulizers, both of which produce large quantities of solvent and droplets in a short space of time. Bundles of heated hollow polyimide or Naflon fibers have been introduced as short, high-surface-area membranes for efficient desolvation. 

The apparatus is sometimes referred to as an oxygen electrode , but it is actually a cell. Although the Teflon membrane is impermeable to water and, therefore, to most substances dissolved in water, dissolved gases can pass through, and gases, such as chlorine, sulphur dioxide and hydrogen sulphide, can affect the electrode. The apparatus can be made readily portable and it is, therefore, of value for use in the field and can be used to monitor the oxygen content of rivers and lakes (see Ref. 53). 

Diaphragm discharge is a liquid phase discharge at ambient temperature and atmospheric pressure [9], The reactor configuration, as shown in Fig. 1 [10], consists of Pyrex tube and a pair of electrodes set in the liquid fuel. The insulated teflon-membrane (diaphragm membrane), with its single pinhole, is placed in the gap of electrodes. 

M.I. Song, F.F. Bier, and F.W. Scheller, A method to detect superoxide radicals using Teflon membrane and superoxide dismutase. Bioelectrochem. Bioenerg. 38, 419—422 (1995). 

Catalase was immobilized with gelatin by means of glutaraldehyde and fixed on a pretreated Teflon membrane served as enzyme electrode to determine hydrogen peroxide [248], The electrode response reached a maximum when 50mM phosphate buffer was used at pH 7.0 and at 35°C. Catalase enzyme electrode response depends linearly on hydrogen peroxide concentration between 1.0 X 10-5 and 3.0 X 10-3 M with response time 30 s. 

Figure 3 Schematic diagram of a solid-phase N02 sensor. The sensor consists of a small cell supporting the polymer-coated, glass substrate behind a glass window in full view of a PMT. The CL reagent is immobilized on the hydrogel substrate. The gel is sandwiched between the glass window and a Teflon PTFE membrane. The purpose of the Teflon membrane is to permit the diffusion of N02 from the airstream into the gel while preventing the loss of water from the hydrogel. Inlet and outlet tubes (PTFE) allow a vacuum pump to sample air (2 L/min) directly across the surface of the chemical sensor. (Adapted with permission from Ref. 12.)...

Generally, a distinction can be made between membrane bioreactors based on cells performing a desired conversion and processes based on enzymes. In ceU-based processes, bacteria, plant and mammalian cells are used for the production of (fine) chemicals, pharmaceuticals and food additives or for the treatment of waste streams. Enzyme-based membrane bioreactors are typically used for the degradation of natural polymeric materials Hke starch, cellulose or proteins or for the resolution of optically active components in the pharmaceutical, agrochemical, food and chemical industry [50, 51]. In general, only ultrafiltration (UF) or microfiltration (MF)-based processes have been reported and little is known on the application of reverse osmosis (RO) or nanofiltration (NF) in membrane bioreactors. Additionally, membrane contactor systems have been developed, based on micro-porous polyolefin or teflon membranes [52-55]. 

A typical extraction manifold is shown in Figure 13.2. The sample is introduced by aspiration or injection into an aqueous carrier that is segmented with an organic solvent and is then transported into a mixing coil where extraction takes place. Phase separation occurs in a membrane phase separator where the organic phase permeates through the Teflon membrane. A portion of one of the phases is led through a flow cell and an on-line detector is used to monitor the analyte content. The back-extraction mode in which the analyte is returned to a suitable aqueous phase is also sometimes used. The fundamentals of liquid liquid extraction for FIA [169,172] and applications of the technique [174 179] have been discussed. Preconcentration factors achieved in FIA (usually 2-5) are considerably smaller than in batch extraction, so FI extraction is used more commonly for the removal of matrix interferences. 

The filter material of choice is a thin teflon membrane since it minimizes artifact formation and maximizes analytical sensitivity by X-ray fluorescence analysis. Although X-ray fluorescence (XRF) may not be the only analytical technique used, it is generally accepted as being the most cost effective analysis for source apportionment. ( 2) Its background and therefore, analytical sensitivity, is dependent on the filters surface density. The analytical sensitivity of XRF for aerosols deposited on a stretched teflon membrane with a density of about 0.3 to 0.4 mg/cm, for example, is about three times greater than an aerosol deposited on a cellulose based filter with a surface density of about 4 mg/cm. This difference can be translated into either more information for the same analytical costs or the same information for a lower analysis cost. 

Figure 2.8 Schematic diagram of the concentration of fruit juice by vapour transport across a porous Teflon membrane.

What pressure must be applied to force water through an initially dry Teflon membrane which has a uniform pore size of 0.5 pm diameter What factors can reduce this pressure Give examples of the industrial and everyday use of this type of porous material. 

Water can just be forced through a Goretex (porous Teflon) membrane at an applied pressure of 1.5 bar. What is the pore size of the mebrane ... 

From Smith and Harrison (1996). Sampler elevation 15 m, situated 300 ni from a high traffic spine roadway. Particle phase collected on Teflon membrane filter (TMF) gas-phase species trapped downstream on PUF plugs. Daily 24-h samples collected during February 1992 mean TSP during that period 60 /j.g m-3. 

Finally, passive samplers have also been developed for ozone, primarily for use in epidemiological studies. For example, Brauer and Brook (1995) describe the application of a passive sampler in which air containing ozone diffuses through a Teflon membrane and reacts with nitrite. The sampler is then extracted and the nitrate product measured using ion chromatography. 

Figure 19.6— The Clark sensor for oxygen determination. The Teflon membrane, permeable to gas, must be very close to the cathode so that the double diffusion process through the membrane and the liquid film will lead to a stable signal within a few seconds.

Electrochemical (amperometric) techniques provide the possibility for in situ, continuous and automated measurements of ozone in the liquid. The membrane electrode usually consists of a gold cathode, a silver anode, an electrolyte (e. g. AgBr, K2S04 or KBr) and a teflon membrane. Several companies offer such electrodes in different configurations. The application range and accuracy differs depending on the kind of electrode. 

Fig. 7.6 Effect of flow velocity on oxygen electrodes of different dimensions. Open circles represent O2 - saturated water and full circles represent areated water. The electrodes are (a) 50pm diameter Pt disks (b) 50pm diameter Pt disks with 25pm Teflon membrane (c) 300pm Pt disks and (d) 10pm Pt disk with 6.25pm Teflon membrane (adapted from Fatt, 1976)...

Halothane can be effectively blocked by a 12 pm thick Teflon membrane (Fig. 9.31). 

Fig. 9.31 Response of fluorescent gas sensor with Teflon membrane, according to (5.49) (adapted from Wolfbeis et al., 1985)...

A schematic diagram of the microbial sensor is illustrated in Figure 1. The sensor consisted of double membranes of which one layer was the bacteria-collagen membrane (thickness 40jam), the other an oxygen permeable Teflon membrane (thickness 27jam), an alkaline electrolyte, a platinum cathode, and a lead anode. 

Bacteria-collagen membrane 2. Teflon membrane 3. Platinum cathode 4. Lead anode 5. Electrolyte (KOH) 6. Ammeter 7. Recorder... 

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