Stationary phases porous polymer

The porous polymer stationary phases which for many years have been available in packed gas chromatography columns has only recently become available as a coated capillary [24]. These cross-linked porous polymer columns are produced by copolymerizing styrene and divinylbenzene. The pore size and surface are varied by altering the amount of divinylbenzene added to the polymer. These PLOT capillary columns exhibit the same separative characteristics as Poropak Q packed columns. 

Many polymers can be separated using toluene, trichlorobenzene (140°C), tetrahy-drofuran (30-45°C), dimethylformamide (85-145°C), hexafluoroisopropanol and water as a mobile phase, sometimes containing additives to minimize sorptive and ionic interactions with the stationary phase. Some typical examples are summarized in Table 4.11 [21,503,508]. Several common porous polymer stationary phases, although compatible with a number of solvents, swell to different extents in each solvent, and may not allow solvent exchange without deterioration of colunrn properties. Higher temperatures are also quite common in SEC to maintain sample solubihty and to minimize mobile phase viscosity. 

Figure 7.7. System constants of the solvation parameter model for retention on a porous polymer stationary phase with a binary mixture of carbon dioxide and 1,1,1,2-tetrafluoroethane as the mobile phase. Column 25 cm X 4.6 mm I.D. Jordi-Gel RP-C18 with a 5 pm average particle diameter. The total fluid flow rate was 1.0 ml/min, backpressure 200 bar and ternperamre I25°C.

The separation of ethylene oxide and acetaldehyde is shown in Rg. 7-39. The two compounds are well separated from the C3-hydrocarbons and are just baseline separated. The eluting peaks are symmetrical which is a typical characteristic of porous polymer stationary phases. Ethylene oxide and acetaldehyde can be quantified if the concentrations of the compounds do not differ by more them a factor of 10. At higher concentrations there will be overlap due to... 

Non-silica-based RP-HPLC stationary phases have also been developed and their separation capacity has been compared with those of silica-based ones. The porous structure of crosslinked polymer gels may be responsible for the markedly different selectivity and retention characteristics. Up till now, the mode of separation on polymer stationary phases is not entirely understood at the molecular level. It has been established that the size-exclusion effect may influence the retention of analyses on polymer gels. 

To achieve a detailed classification of the possible interaction of sohd porous non-polar polymeric phases with different functional groups of solute molecules, Gastello and D Amato used the following polarity reference substances ethanol, 2-butanone, nitromethane, benzene, pyridine, w-butanol, 2-pentanone, and 1-nitropropane. The first five represent the test substances proposed by Rohrschneider, while the last three were recommended by McReynolds. The retention indices of these substances enable evaluation of the polarity of any sohd porous polymeric stationary phase. In their studies Porapak Q, as the least polar commercially available porous polymer, was used as the reference stationary phase. 

Stationary phase (pacidng) Porous, microparticulate silica, chemically-modified silica (bonded phases) or styrene/divinyl benzene co-polymers Stationary phase (pacidng) Porous, microparticulate silica, chemically-modified silica (bonded phases) or styrene/divinyl benzene co-polymers... 

Two classes of micron-sized stationary phases have been encountered in this section silica particles and cross-linked polymer resin beads. Both materials are porous, with pore sizes ranging from approximately 50 to 4000 A for silica particles and from 50 to 1,000,000 A for divinylbenzene cross-linked polystyrene resins. In size-exclusion chromatography, also called molecular-exclusion or gel-permeation chromatography, separation is based on the solute s ability to enter into the pores of the column packing. Smaller solutes spend proportionally more time within the pores and, consequently, take longer to elute from the column. 

G. Gastello and G. D Amato, Gomparison of the polarity of porous polymer-bead stationary phases with that of some liquid phases , J. Chromatogr. 366 51-57 (1986). 

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

The stationary phase consists of porous polymer resin particles. The components to be separated can enter the pores of these particles and be slowed from progressing through this stationary phase as a result. Thus, the separation depends on the sizes of the pores relative to the sizes of the molecules to be separated. Small particles are slowed to a greater extent than larger particles, some of which may not enter the pores at all, and thus the separation occurs. The mobile phase for this type can also only be a liquid, and it too is discussed further in Chapter 13. The separation mechanism is depicted in Figure 11.11. 

The production of conventional stationary phases in the form of porous polymer particle is based on suspension polymerization. Namely, the polymerization is allowed to proceed in a solvent under vigorous stirring that assures obtaining particles of the desired diameter. Since the particle size is typically in the range of a few micrometers, no problems with heat transfer are encountered. In contrast, the preparation of monoliths requires a so-called bulk polymerization. A polymer mixture consisting of monomers and porogenic solvent is mixed with an initiator. As the temperature is increased, the initiator decomposes and oligomer nu-... 

In actual practice, the inert gels of dextran (I)-a polyglucose or other types of polymers, for instance agarose and polyacrylamides, wherein the macromolecules invariably are cross-linked to afford a reasonably porous 3D-structure, served as the stationary phases in size-exclusion chromatography. 

In gas-solid chromatography (GSC) the stationary phase is a solid adsorbent, such as silica or alumina. The associated virtues associated therewith, namely, cheapness and longevity, are insufficiently appreciated. The disadvantages, surface heterogeneity and irreproducibility, may be overcome by surface modification or coating with small amounts of liquid to reduce heterogeneity and improve reproducibility 4,15). Porous polymers, for example polystyrene and divinyl benzene, are also available. Molecular sieves, discussed in Chapter 17, are used mainly to separate permanent gases. 

The average molecular mass of synthesized polymers can be measured by SEC. This system has been called gel-permeation liquid chromatography, using porous polystyrene gel as the stationary phase material. Polymers are usually dissolved in THF or chloroform for separation by SEC. The molecular density... 

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