Trapped materials porous polymers

Various sample enrichment techniques are used to isolate volatile organic compounds from mammalian secretions and excretions. The dynamic headspace stripping of volatiles from collected material with purified inert gas and trapping of the volatile compounds on a porous polymer as described by Novotny [3], have been adapted by other workers to concentrate volatiles from various mammalian secretions [4-6]. It is risky to use activated charcoal as an adsorbent in the traps that are used in these methods because of the selective adsorption of compounds with different polarities and molecular sizes on different types of activated charcoal. Due to the high catalytic activity of activated charcoal, thermal conversion can occur if thermal desorption is used to recover the trapped material from such a trap. 

Whereas, most of the varius porous polymers differ from each other only in selectivity, there are two major exceptions. Chromosorb 103 was developed specifically for analysis of amines and therefore is not suitable for acidic compounds. Tenax, because of its excellent thermal stability, can be used at much higher temperatures than the other porous polymers. Because of the minimal bleed from this material it has found use as a trapping medium for concentration of trace components (13) in air. These compounds are then desorbed from the Tenax and subsequently analyzed, permitting detection at much lower levels than by direct analysis of the air. 

Adsorbent choice. The choice of adsorbent material depends on the volatile compounds in the food. Of the synthetic porous polymers, the most widely used and best overall adsorbent is Tenax TA (poly-2,6-diphenyl-p-phenylene oxide) 60 to 80 mesh. While Tenax does not show an adsorption capacity for all volatiles, especially very small polar compounds such as acetaldehyde, it has good thermal stability and desorption capabilities. It also traps little water and generates very few artifacts. Table G1.2.2 shows a few limitations and advantages of various adsorbents, all of which can be purchased from chromatography suppliers. If very small volatiles are the goal, various Carbosieves could be used, or traps containing several adsorbents in series. Traps with mixed adsorbents should be desorbed immediately, before transfer between phases occurs. 

Tenax-GC is a porous polymer of 2,6-diphenyl-/>-phenylene oxide. This material is used both as a chromatographic phase and as a trap for volatile substances prior to analysis. 

Trapping of liquid in the rough surface of the electrode adds virtual mass and may also cause an additional mass loading artifact (Theisen et al., 2004). This effect can be particularly severe when porous materials such as conducting polymers are deposited at the QCM electrode. 

Three general types of solid sorbents are mainly used for trapping VOCs in air inorganic sorbents like silica gels or molecular sieves, carbon-based porous materials and porous organic polymers. 

Many porous organic polymers are derived from the stationary phase used to pack GC columns. Tenax is one such example. This is a macroporous polymer obtained from diphenyl p-phenylene oxide (DPPO). Generally, this polymer is hydrophobic and does not retain water. However, it exhibits some ability to adsorb polar compounds. As a result of its low surface area (30 m /g), its adsorption capacity is limited and very volatile compounds are not trapped. Therefore, it is an appropriate material for trapping heavier compounds with more than four carbon atoms. Co-precipitated graphitized carbon black and Tenax (in the proportion 23 % to 77 %) was introduced on the market as Tenax GR. This adsorbent combines the advantages of both materials and is approximately twice as effective as Tenax TA [50]. 

Organic or inorganic entities as well as polymer particles can also be used as template agents in the preparation of porous ceramic membranes following either the polymeric or the colloidal sol-gel route. The strategy to control microstructure in porous material is illustrated in Fig. 7.13. The template agents are trapped during matrix formation and eliminated in a second step with the aim to define the pore size in the final material. 

The purpose of this paper is to discuss the third area, viz. the enzyme support. Various carriers that have been used over the years for immobilizing enzymes can be classified into three categories. The first is hard particulate substances such as porous glass/ceramics and polymers. The second category is polymers in membranous form, such as reconstituted collagen or ultrafiltration membranes, where the enzyme is trapped behind or within the membrane barrier. The third category is cellulose-derived materials in the form of fibers or beads. Almost all these materials are used either in the form of packed beds or as membranes. In any case, the diffusional resistances are major restrictions to their use as efficient enzyme supports. We will discuss and demonstrate a new type of microporous carrier that can be used very efficiently as an immobilized enzyme support. 

Small amounts of synthetic zeolites (based on metal aluminosilicates) can be added to the system to adsorb unwanted organic odour-producing molecules and/or to remove the moisture that contributes to odours (by trapping them in their highly porous crystal structures). This technique is successfully used in extruded polyolefin pipes, injection and extrusion blow-moulded containers, barrier packaging materials, extrusion coatings, and sealant polymers. 

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