Medium-pressure mobile phase selection

Eluent pH is limited to a maximum of 7 to 8 due to the reduced chemical stability of a chromatographic bed in an alkaline medium. The nucleophilic attack of Si-0 bonds by hydroxide ions leads to the erosion of the silica surface as shown by back pressure increases caused by the formation of Si(OH)4. With polystyrene-divinyl-benzene-based stationary phases, pH stability is not an issue and a very wide mobile phase pH range can be used, thereby providing additional selectivity [1]. Several silica-based and polymeric columns claimed to be stable in pH ranges from 1 to 13 are commercially available, however, they are not commonly used. 

Since its introduction in the 1960s, SFC has experienced several ups and downs in its development. Either a gas or a Uquid above its critical temperature and pressure is used as the mobile phase for SFC. In most cases, COj is used because of its favorable critical parameters (i.e., a critical temperature of 31 °C and a critical pressure of 7.3 MPa). Moreover, CO2 is cheap, nontoxic, and nonflammable. A high-pressure pump delivers the mobile phase through either a packed (pSFC) or capillary column (cSFC) to the detector. The mobile phase is maintained under supercritical or subcritical conditions via an electronic controlled variable restrictor that is positioned after detection (pSFC) or via a fixed restrictor positioned before a gas-phase detector (cSFC). The retention characteristics of the analytes are influenced by the properties of the stationary phase and by the polarity, selectivity, and density of the CO2 mobile phase. The density is controlled by variation of the temperature and pressure of the supercritical medium. Furthermore, the elution of very polar compounds under high densities can be achieved with a precolumn addition of polar modifiers such as methanol. Nowadays, pSFC formats use the same injector and column configurations as LC methods. Consequently, pSFC formats are considered to be more useful for routine operation than cSFC. The most remarkable... 

The mobile phase plays different roles in GC, LC, and SFC. Ordinarily, in GC the mobile phase serves but one purpose — zone movement. As we have seen in Chapter 28. in LC the mobile phase provides not only transport of solute molecules but also interactions with solutes that influence selectivity factors (or values). When a molecule dissolves in a supercritical medium, the process resembles volatilization but at a much lower temperature than would normally be used in GC. Thus, at a given temperature, the vapor pressure for a large molecule in a supercritical fluid may be 10 ° times greater than in the absence of the fluid. Because of this, high-molecular-mass compounds, thermally unstable species, polymers, and large biological molecules can be eluted from a column at relatively low temperatures. Interactions between solute molecules and the molecules of a supercritical fluid must occur to account for their solubility in these media. The... 

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