Phase separation technique

Phase Separation. Microporous polymer systems consisting of essentially spherical, intercoimected voids, with a narrow range of pore and ceU-size distribution have been produced from a variety of thermoplastic resins by the phase-separation technique (127). If a polyolefin or polystyrene is insoluble in a solvent at low temperature but soluble at high temperatures, the solvent can be used to prepare a microporous polymer. When the solutions, containing 10—70% polymer, are cooled to ambient temperatures, the polymer separates as a second phase. The remaining nonsolvent can then be extracted from the solid material with common organic solvents. These microporous polymers may be useful in microfiltrations or as controlled-release carriers for a variety of chemicals. 

Truly porous, synthetic ion exchangers are also available. These materials retain their porosity even after removal of the solvent and have measurable surface areas and pore size. The term macroreticular is commonly used for resins prepared from a phase separation technique, where the polymer matrix is prepared with the addition of a hq-uid that is a good solvent for the monomers, but in which the polymer is insoluble. Matrices prepared in this way usually have the appearance of a conglomerate of gel-type microspheres held together to... 

Human osteoblast-like MG63 cells were cultured on the macroporous chitosan scaffolds reinforced with hydroxyapatite or calcium phosphate invert glass were fabricated using a thermally induced phase separation technique. 

Despite its widespread application [31,32], the kinetic resolution has two major drawbacks (i) the maximum theoretical yield is 50% owing to the consumption of only one enantiomer, (ii) the separation of the product and the remaining starting material may be laborious. The separation is usually carried out by chromatography, which is inefficient on a large scale, and several alternative methods have been developed (Figure 6.2). For example, when a cyclic anhydride is the acyl donor in an esterification reaction, the water-soluble monoester monoacid is separable by extraction with an aqueous alkaline solution [33,34]. Also, fiuorous phase separation techniques have been combined with enzymatic kinetic resolutions [35]. To overcome the 50% yield limitation, one of the enantiomers may, in some cases, be racemized and resubmitted to the resolution procedure. 

The strategy of using two phases, one of which is an aqueous phase, has now been extended to fluorous . systems where perfluorinated solvents are used which are immiscible with many organic reactants nonaqueous ionic liquids have also been considered. Thus, toluene and fluorosolvents form two phases at room temperature but are soluble at 64 °C, and therefore,. solvent separation becomes easy (Klement et ai, 1997). For hydrogenation and oxo reactions, however, these systems are unlikely to compete with two-phase systems involving an aqueous pha.se. Recent work of Richier et al. (2000) refers to high rates of hydrogenation of alkenes with fluoro versions of Wilkinson s catalyst. De Wolf et al. (1999) have discussed the application and potential of fluorous phase separation techniques for soluble catalysts. 

ESI-MS is the most successful method of coupling a condensed phase separation technique to a mass spectrometer. Because the input to ESI is a liquid, electrospray serves as an interface between the mass spectrometer and liquid chromatographic techniques, including SEC and CE (capillary electrophoresis). In LC-MS the flow-rate should lie in the range recommended for the HPLC pump and the mass spectrometer (typically 0.001 -l.OmLmin-1). Recent advances in (nano)electrospray technology include the development of the use of very low solvent flow-rates (30 to 1000nLmin-1) [130,131],... 

Principles and Characteristics The main reasons for hyphenating MS to CE are the almost universal nature of the detector, its sensitivity and the structural information obtainable, including assessment of peak purity and identity. As CE is a liquid-phase separation technique, coupling to the mass spectrometer can be achieved by means of (modified) LC-MS interfaces. Because of the low flow-rates applied in CE, i.e. typically below lOOnLmin-1, a special coupling device is required to couple CE and the LC-MS interface. Three such devices have been developed, namely a... 

TABLE 5.1 Classification and Examples of Two-Dimensional Liquid-Phase Separation Techniques... 

Fig. 5. (a) Schematic of the scaffold design showing the inner and outer scaffolds, (b and c) Inner scaffolds seeded with NSCs. (Scale bars 200 pm and 50 tm, respectively.) The outer section of the scaffold was created by means of a solid-liquid phase separation technique that produced long, axially oriented pores for axonal guidance as well as radial pores to allow fluid transport and inhibit the ingrowth of scar tissue (d scale bar, 100 pm), (e) Schematic of surgical insertion of the implant into the spinal cord. [Reproduced with permission from Teng et al. (2002).]... 

We have suggested that separation strategies be planned with the following goal in mind the target product(s) in a final reaction mixture should partition into a phase that is different from all the other components of the mixture.181 When this goal is met, reactions can be purified simply by workup, which involves simple phase separation techniques such as evaporation, extraction, and filtration. 

Fig. 7. Various strategies of the Chemically Induced Phase Separation technique to generate different types of morphologies...

With electrical applications in mind, the chemically induced phase separation technique has been extended to prepare macroporous cyanurate networks with pore sizes in the micrometer range, thus demonstrating the general appli-... 

Solvent-modified thermosets display enhanced toughness due to the incorporation of a second phase material. A brittle-tough transition has been observed which cannot be attributed to changes in the interparticle distance. The chemically induced phase separation technique offers new routes and strategies to prepare such materials and enter new areas of applications. Hence, engineered porosity is demonstrated as a research concept developed into a toolbox for material scientists. 

The first approaches to the coupling of liquid-phase separation techniques with mass spectrometry were designed for HPLC needs, starting in the 1970s with since-forgotten techniques such as direct liquid introduction (DLI) and moving belt. In the 1980s, techniques such as thermospray, continuous-flow-fast atom bombardment (CF-FAB), and particle beam arose. 

Chitosan is a polymer produced from hydrolysis of natural chitin. Chitosan is not readily soluble in aqueous solutions, but can be solubilized and is thus considered with other water soluble polymers. In the hydrophobic form, chitosan has been treated in a similar manner to other hydrophobic polymers with microparticles produced by emulsion and phase separation techniques. Microparticles can be taken up by the gastrointestinal lining in a manner similar to that discussed for other hydrophobic microparticles. 

Whereas the optical method indicates attainment of saturation coverage by the dye in its /-state, results obtained with the phase-separation technique show that, after reaching apparent saturation, further adsorption occurs as the dye concentration in solution is increased. The horizontal step observed with the phase separation measurement is in approximate agreement with the maximum surface concentration of dye in its /-state as determined by the optical method. Hence, it is concluded that, contrary to earlier suppositions (70), only the first layer of this dye is adsorbed in its /-state subsequent dye layers must be adsorbed in different states. 

Sorption coefficients quantitatively describe the extent to which an organic chemical is distributed at equilibrium between an environmental solid (i.e., soil, sediment, suspended sediment, wastewater solids) and the aqueous phase it is in contact with. Sorption coefficients depend on (1) the variety of interactions occurring between the solute and the solid and aqueous phases and (2) the effects of environmental and/or experimental variables such as organic matter quantity and type, clay mineral content and type, clay to organic matter ratio, particle size distribution and surface area of the sorbent, pH, ionic strength, suspended particulates or colloidal material, temperature, dissolved organic matter (DOM) concentration, solute and solid concentrations, and phase separation technique. 

Many factors potentially can affect the distribution of an organic chemical between an aqueous and solid phase. These include environmental variables, such as temperature, ionic strength, dissolved organic matter concentration, and the presence of colloidal material, and surfactants and cosolvents. In addition, factors related specifically to the experimental determination of sorption coefficients, such as sorbent and solid concentrations, equilibration time, and phase separation technique, can also be important. A brief discussion of several of the more important factors affecting sorption coefficients follows. 

The most important type of microporous membrane is formed by one of the phase separation techniques discussed in the next section about half of the isotropic microporous membrane used is made in this way. The remaining types are made by various proprietary techniques, the more important of which are described below. 

Although such polymer/LC composite films are usually prepared by emulsification and phase separation techniques, the resulting composites have a variety of morphological characteristics arising from differences in conditions and compositions during sample preparation. The polymer/LC composite films are mainly classified into four types ... 

Compared with conventional particle-packed SPE cartridges, FIT-SPE provides an increased surface area for the extraction medium and a reduced pressure drop during extraction and desorption. Also, the undesirable plugging effect from insoluble materials in real samples can be very much diminished. Utilization of FIT-SPE has been discussed in a few review articles concerning the on-line coupling of miniaturized SPE to microcolumn liquid-phase separation techniques.24,25... 

The use of mass spectrometry (MS) as a detection system is inevitable in the evolution of any separation method, especially CE where the liquid flow rate ( 1 ml/min) is compatible with conventional mass spectrometers. The combination of a high-efficiency liquid-phase separation technique, such as capillary electrophoresis, with MS detection provides a powerful system for the analysis of complex mixtures. Analyte sensitivity and the mass spectrum obtained depend on the electrospray ionization (ESI) voltage, ion-focusing parameters, and buffer composition. In general, the greatest sensitivity is obtained by employing conditions that facilitate desolvation and minimize cluster formation.47 Three ways of interfacing for CE-MS... 

Notwithstanding the reduced reaction times and improved yields, the need to use column chromatography to purify the target 1,4-DHP encumbers the application of the procedure for the fast preparation of screening libraries. To address the purification issue, various phase-separation techniques could be employed, such as solid-phase organic synthesis (SPOS). 

In continuos flow FAB (CFFAB) [7,24,47] the analyte-matrix mixture is delivered continuously to the probe tip through a fused silica capillary which terminates at the probe tip. This configuration provides a means of coupling liquid phase separation techniques with FAB-MS. Addition of the matrix to the analyte solution is accomplished by one of two methods. (1) The matrix is added at concentrations of 5-10% to the mobile phase, and the column effluent is directly fluxed into the CFFAB ion source or (2) column effluent and matrix solution are delivered independently to the probe tip by a coaxial arrangement of two concentric fused silica capillaries. 

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