Gas-diffusion Membranes

The membranes used in FI gas-diffusion separation systems may be classified according to the mechanism of mass transfer of gaseous components across them. Microporous or heterogeneous membranes, usually made of PTFE or polypropylene, allow penetra- 

The required properties of membranes used in FI gas-diffusion applications may include the following  

PTFE membranes 0.01-0.08 mm thick and 0.1-0.45 pm pore size come close to the above requirements except for its relatively low mechanical strength. However, with some mechanical support (cf. Sec. 5.2.2) the lifetime may be prolonged. Polypropylene membranes were reported to have better mechanical properties [14], and higher water entrance pressures [15] than PTFE membranes and may be a good substitute for the latter. 

The lifetime of the membrane will also depend on the sample species. With relatively clean water samples a membrane could last for months under continuous operation, whereas the membrane may have to be changed everyday if the sample contains appreciable amounts of suspended materials. 

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Button cells consist of cathode and anode cans (used as the terminals), powdered zinc anode, containing gelled electrolyte and the corrosion inhibitor, separator with electrolyte, thin (0.5 mm) carbon cathode with catalyst and PTFE, waterproof gas-permeable (teflon) layer and air distribution layer for the even air assess over the cathode surface. Parameters of battery depend on the air transfer rate, which is determined by quantity and diameters of air access holes or porosity of the gas-diffusion membrane. Air-zinc batteries at low rate (J=0,002-0,01C at the idle drain and J= 0,02-0,04C at the peak continuous current) have flat discharge curves (typical curve is shown by Figure 1). 

Figure 5.7 shows a typical application of gas-diffusion membranes isolation of the circulating sample from a voltammetric or potentiometric electrode for the electrochemical determination of gaseous species. The ion-selective electrode depicted in this Figure includes a polymer membrane containing nonactin that is used for the potentiometric determination of ammonia produced in biocatalytic reactions. Interferences from alkali metal ions are overcome by covering the nonactin membrane with an outer hydro-... 

This type of sensor is schematically depicted in Fig. 5.4.B. The sensing microzone is crossed by two streams rather than one. One such stream acts as the donor and contains the aspirated or injected sample, which is conditioned in order to provide a gaseous reaction product. The other stream acts as the acceptor for the volatile species transferred across the gas-diffusion membrane that isolates the two streams. 

In 1985, Ruzicka and Hansen established the principles behind flow injection optosensing [13-15], which has subsequently been used for making reaction-rate measurements [16], pH measurements by means of immobilized indicators [17,18], enzyme assays [19], solid-phase analyte preconcentration by sorbent extraction [20] and even anion determinations by catalysed reduction of a solid phase [21] —all these applications are discussed in Chapters 3 and 4. Incorporation of a gas-diffusion membrane in this type of sensor results in substantially improved sensitivity (through preconcentration) and selectivity (through removal of non-volatile interferents). The first model sensor of this type was developed for the determination of ammonium [13] and later refined by Hansen et al. [22,23] for successful application to clinical samples. 

Gas-diffusion flow injection analysis is capable of detecting very low concentrations of chlorine dioxide in water (i.e., detection limit is 5 ppb). A chemiluminescence flow-through detector cell is used to measure the concentration chlorine dioxide as a function of chemiluminescence intensity. A gas diffusion membrane separates the donor stream from the detecting stream and removes ionic interferences from iron and manganese compounds, as well as from other oxychlorinated compounds, such as chlorate and chlorite (Hollowell et al. 1986 Saksa and Smart 1985). 

Although some inorganic membranes such as porous glass and dense palladium membranes have been commercially available for some time, the recent escalated commercial activities of inorganic membranes can be attributed to the availability of large-scale ceramic membranes of consistent quality. As indicated in Chapter 2, commercialization of alumina and zirconia membranes mostly has been the technical and marketing extensions of the development activities in gas diffusion membranes for the nuclear industry. 

The membranes used for analytical pervaporation are hydrophobic membranes of the types usually employed in ultrafiltration and gas-diffusion processes. In practice, PTFE is the most frequently used membrane material, followed by hydrophobic polyvinylidene-fluoride (PVDF). Ultrafiltration membranes are very thin, which, in combination with the large surface area of both the donor and acceptor chamber, leads to their easy bending. This results in changes in the ffux of the permeating component through an altered membrane area and hence in changes in the efficiency of the process. As a result, membranes must be replaced fairly often. Because of their thickness, gas-diffusion membranes are not so easily bent, so the same membrane can be used over long periods. The pore size of the... 

The use of gas-diffusion membranes instead of ultrafiltration membranes can also improve the precision as the former are not easily bent this avoids changes in permeate flux resulting from both irreproducible membrane areas and increased volumes of the upper chamber [153,171]. 

From 1988, several commercial adaptations of the MAGIC [71] were described (Ch. 4.8). The PBI most closely resembles the MAGIC. It contains a more user-friendly and robust momentum separator and the cross-flow pneumatic nebulizer is replaced by a concetrfric pneumatic nebulizer [82-83]. In the thermabeam interface a TSP nebtrlizer is trsed [84]. The universal interface features the use of a TSP nebulizer and a cormtercrrrrent gas-diffusion membrane separator between the... 

In subsequent years (1988), the MAGIC system was commerciahzed, first by Hewlett-Packard (nowadays Agilent Technologies), and subsequently by other instrument manufacturers. Four commercial versions of the system have been available (1) the particle-beam interface, featuring an adjustable concentric pneumatic nebulizer, (2) the thermabeam interface with a combined pneumatic-TSP nebulizer, (3) the universal interface, in which TSP nebulization and an additional gas diffusion membrane is applied, and (4) the capillary-EI interface, which resulted from systematic modifications to existing PBI systems by Cappiello [83]. The first system was most widely used, and is discussed in more detail below. For some years, PBI was widely used for environmental analysis, especially in the US. 

The unique part of the Universal Interface is the membrane separator or gas diffusion cell which allows the solvent vapor to be efficiently removed with essentially no loss of sample contained in the aerosol particles. In this device the aerosol is transported through a central channel bounded on the sides by a gas diffusion membrane or filter medium which is in contact with a countercurrent flow of a sweep gas. For El mass spectrometry helium appears to be most useful for both the carrier and sweep gas. The properties of the... 

M. van Son, R. C. Schothorst, and G. den Boef, Determination of Total Ammoniacal Nitrogen in Water by Flow Injection Analysis and a Gas Diffusion Membrane. Anal. Chim. Acta, 153 (1983) 271. 

A. Trojanek and S. Bruckenstein, Flow-Injection Analysis of Volatile, Electroinactive Organic Compounds at a Platinum Gas Diffusion Membrane Electrode by Use of a Redox Mediator. Anal. Chem., 58 (1986) 981. 

Fig. l A typical sandwich-type membrane gas-diffusion separator, a, side view A, B, plastic blocks with F. threaded fittings and G, engraved grooves M, microporous gas-diffusion membrane, b, top view showing position and configuration of a straight channel groove, c, a simplified schematic presentation of the sandwich-type gas-diffusion separator D, donor stream A, acceptor stream M, membrane. 

Sullivan et al.(4S] used an FI gas-diffusion spectrophotometric system to determine sulfite in food products including fruits, vegetables, shrimps and wines. Sulphur dioxide is separated from the sample using a gas-diffusion membrane separator, and determined by decolorization of Malachite Green. The method shows good selectivity and sensitivity, and sulfite may be determined with a detection limit of 0.1 mg 1 in the food extracts, corresponding to 1-10 mg kg in the food product, and with a precision of 1-2% r.s.d.. 

Silicone modified polypropylene gas-diffusion membranes with different thickness were prepared by casting a mixture of silicone E43 and toluene (l f w/w> onto a polypropylene membrane and dried overnight. Different dilution rates of the separated ethanol were achieved by using membranes with different thickness of 20- /im and undiluted samples may be analyzed within a wide range of ethanol concentration from... 

The FI manifold and operational parameters are shown in Fig. 8.1. A 10-s loading period with 30-s injection is recommended. 100 p sample is injected into a water carrier after being filled into the sample loop, and merged downstream with the sodium hydroxide to generate ammonia which is transferred through the gas diffusion membrane into the acceptor stream. The absorbance change of the acceptor is recorded and peak heights taken for NH4-N concentration evaluation. 

The dual-phase (DP) membrane used in analytical separation usually consists of a polymer, or in some cases a ceramic solid-phase support impregnated with a fluid (i.e., gaseous or liquid phase). If the fluid is air the DP membrane is known as a gas-diffusion membrane. DP membranes incorporating a liquid phase can be considered in a broader sense as liquid membranes. The liquid phase in a liquid DP membrane can be identical to the feed and/or receiver solution (e.g., dialysis membranes, membrane-assisted LLE (MALLE)) or it can form a third immiscible liquid phase in the membrane separation system (e.g., supported and polymer liquid membranes). Membranes incorporating a liquid phase immiscible with the feed and receiver solutions are usually referred to as liquid membranes. This narrower definition of liquid membranes, currently accepted in the literature, will be used in subsequent discussions. The... 

Gas-diffusion membranes Hydrophobic porous polymer membranes with air filling the membrane pores have been used successfully in the online separation of volatile and semivolatile analytes between two miscible liquid streams in flow injection analysis (FIA) systems. The corresponding technique is frequently referred to as gas-diffusion EIA. The mass transfer of an analyte across a gas-diffusion membrane is controlled by the membrane pore size and the solubility of the analyte in the feed and receiver solutions. The latter can be manipulated by appropriately modifying the chemical composition of the two solutions. In this way it is possible to enhance both the evaporation of the analyte from the feed solution into the membrane pores and its subsequent absorption into the receiver solution. 

Passive mass transfer In the case of passive mass transfer, the membrane liquid phase consists of an organic solvent or a mixture of organic solvents. The transfer of the analyte across both membrane/solu-tion interfaces is governed by its partition coefficient. Figure 2A illustrates schematically the passive transport of an organic acid across a liquid membrane involving suitable protolytic reactions in both the feed and receiver solutions. If the volume of the receiver solution is smaller than the volume of the feed solution the analyte of interest can be concentrated as well. The mechanism of passive liquid membrane separation is analogous to that involved in separation based on solid SP and gas-diffusion membranes. However, unlike these membranes, liquid membranes... 

To increase the utility and selectivity of gas-sensing electrodes, another membrane (e.g., pig intestine or collagen serving as a support) with an immobilized enzyme can be placed over a hydrophobic gas-diffusion membrane. A sample is usually injected into a small mixing chamber with the electrode. The immobilized enzyme is then exposed to an analyte and the enzymatic reaction produces a pH-changing gas, which diffuses both back to the sample and towards the pH sensor (through the enzyme support and the gas-permeable membrane). 

The operational life of zinc/air battery is most dependent on the control of gas transmission into and out of the cell. It is evident that water vapor transmission is the key factor in extending the service life for these batteries. A key area for research and development relating to zinc/air batteries is focused on membrane technology. A selectively permeable gas diffusion membrane, which allows air diffusion into the cell but excludes or greatly reduces water vapor transmission, will greatly broaden the range of application for zinc/air batteries. Papers and patent disclosures over the last few years have discussed a series of new materials under study. " °... 

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