Stationary phase Fluorinated phases

The high thermal and chemical stability of fluorocarbons, combined with their very weak intermolecular interactions, makes them ideal stationary phases for the separation of a wide variety of organic compounds, including both hydrocarbons and fluorine-containing molecules Fluonnated stationary phases include per-fluoroalkanes, fluorocarbon surfactants, poly(chlorotrifluoroethylene), polyfper-fluoroalkyl) ethers, and other functionalized perfluoro compounds The applications of fluonnated compounds as stationary phases in gas-liquid chroma... 

The second group of recently developed ionic liquids is often referred to as task specific ionic liquids in the literature [15]. These ionic liquids are designed and optimised for the best performance in high-value-added applications. Functionalised [16], fluorinated [17], deuterated [18] and chiral ionic liquids [19] are expected to play a future role as special solvents for sophisticated synthetic applications, analytical tools (stationary or mobile phases for chromatography, matrixes for MS etc.), sensors and special electrolytes. 

The preparative-scale separation of enantiomers on chiral stationary phases (CSPs) by GC cannot match the overwhelming success achieved in the realm of liquid chromatography (LC) (Francotte, 1994, 1996 and 2001). Modern commercial instrumentation for preparative-scale GC is not readily available. In contrast to LC, separation factors a in enantioselective GC are usually small (a = 1.01 - 1.20). This is beneficial for fast analytical separations but detrimental to preparative-scale separations. Only in rare instances are large chiral separation factors (a > 1.5) observed in enantioselective GC. Only in one instance, a separation factor as high as a = 10 was detected in enantioselective GC for a chiral fluorinated diether and a modified 7-cyclodextrin (Schurig and Schmidt, 2003) (vide supra). 

High-performance liquid chromatography (HPLC) is a well-established separation technique it is able to solve numerous analytical problems and there is the possibility of acting on the mobile phases with appropriate additives to improve the quality of the peak. Of course, any additive must be compatible with the MS detector nonvolatile buffer or eluent additives cannot be used strong acids such as trifhioroacetic acid (TFA) may cause significant signal suppression in positive ionization. Different stationary phases are used as an alternative to the classical C18 Phenyl, HILIC, fluorinated, etc. 

Fluorinated IPRs may be harmful for a PGC stationary phase since they may oxidize it, thus reducing its stability at low pH [24]. PGC stationary phases were used in... 

FIGURE 144 (A) Achiral- and (B) chiral-method screen strategies. (A) Platform 1 (HPLC-1) uses two different mobile phases, one is acidic (mobile phase 1) and another is neutral-to-basic (mobile phase 2) to screen on two columns, a classic reverse-phase C-18 and a special polar group-embedded C-18 (AQ). Platform-2 (HPLC-2) uses one mobile phase (pH is usually acidic) to screen four different columns of wide range of polarity (PFP fluorinated, and Phen phenyl-hexyl phases). Platform-3 (SFC) uses three different mobile phases and five different columns (PYD 2-ethylpyridine, BENZ benzamide). Platform-4 (CE) uses two different mobile phases. (B) AD, OJ, OD, AS are different polysaccharide-based chiral stationary phases. 

To make effective use of fluorous biphasic systems, the fluorous phase may also be a stationary phase. Fluorous compounds or compounds carrying fluorous ponytails have high affinity for fluorous reversed-phase silica gel [Ic, 13] which has been modified by means of a fluorous silane [14]. This effect has been used to achieve convenient isolation and purification of a variety of compounds with high fluorine content, first by simple solid-phase extraction (SPE) [15] and later by chromatography with a mobile phase based on a fluorophilicity gradient [16]. 

An original study was done with a perfluoiinated stationary phase whose properties were compared to a classical CIS bonded phase [27, 28]. An important decrease of the methylene selectivity for a homologue series of alkylphenones was obtained with the fluorinated phase with both an SDS... 

Some representative examples of packed column stationary phases belonging to categories (l)-(3) are summarized in Table 4. Hydrocarbons are used as low-selectivity liquid phases for separations of (mainly) low-polarity compounds by differences in their vapor pressure. Highly fluorinated liquid phases... 

Separations are usually carried out in conventional octyl- and octadecylsiloxane (C8 and C18) columns. A significant number of surfactant molecules may be adsorbed on these stationary phases, giving rise to a structure similar to an open micelle. Consequently, column properties change radically, although the subjacent stationary phase (the bonded moiety) still plays a role in the interaction with solutes. Cyano-propylsiloxane-bonded columns are useful for some specific applications. Efficiencies are improved with fluorinated-bonded phases. More recently, ultrawide pore and monolithic octadecylsiloxane columns have been shown to enhance the eluting power of micellar mobile phases. 

Methanol, water. This is a two-step separation. Fluorous molecules are selectively retained on the stationary phase while non-fluorous molecules are not. The fhiorous component is eluted with a fluorophilic solvent, without the need for buffers or fluorinated solvents. 

Among the different solvent combinations, it appears that mixtures based on fluorinated alcohols give the best results in terms of stability of the polymer and compatibility with the stationary phase (conventional silica or PS-DVB columns can both be used). As with pure fluoroalcohols, a salt should be added to suppress polyelectrolyte effects. Toluene, with 20 vol% HFIP, has been used for the SEC fractionation of PA-12 (Fig. 1). However, based on thermodynamic excess properties, this solvent combination should be less efficient in terms of solvation power than mixtures of HFIP with chloroalkanes. 

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