Stationary phases polysiloxanes

Gyclodextrins. As indicated previously, the native cyclodextrins, which are thermally stable, have been used extensively in Hquid chromatographic chiral separations, but their utihty in gc appHcations was hampered because their highly crystallinity and insolubiUty in most organic solvents made them difficult to formulate into a gc stationary phase. However, some functionali2ed cyclodextrins form viscous oils suitable for gc stationary-phase coatings and have been used either neat or diluted in a polysiloxane polymer as chiral stationary phases for gc (119). Some of the derivati2ed cyclodextrins which have been adapted to gc phases are 3-0-acetyl-2,6-di-0-pentyl, 3-0-butyryl-2,6-di-0-pentyl,... 

In summary, silica gel can be an excellent stationary phase for use in exclusion chromatography in the separation of high molecular weight, weakly polar or polarizable polymers. It cannot be used for separating mixtures that require an aqueous mobile phase or operate at a pH outside the range of 4-8. Examples of the type of materials that can be separated by exclusion chromatography using silica gel are the polystyrenes, polynuclear aromatics, polysiloxanes and similar polymeric mixtures that are soluble and stable in solvents such as tetrahydrofuran. 

Studies of octylsilane (OS) phases, deactivated by end-capping, have shown that such stationary phases lead to a discrimination between compounds according to their H-bond donor capacity, as the stationary phase presents strong accessible H-bond acceptor groups (-Si-O-Si-) [22, 23]. For OS phases with a uniform matrix of cross-Hnked polysiloxane alkyl groups, relatively low correlations between log few and log Poet were found. 

A number of ketones, pharmaceutical compounds, alcohols and hydroxy acids have also been resolved on this phase [724,765-767]. A chiral polysiloxane phase with tartramide substituents has been used for the separation of enantiomers capable of hydrogen bonding interactions with the stationary phase, such as enantiomers containing carboxylic, hydroxyl and amine functional groups [768]. 

Stationary phase Chemically bonded silica, alumina, polymeric resins Polysiloxanes... 

Analysis of reaction products - Liquid reaction products were analyzed by GC using a capillary column CP-Sil-8CB (WCOT Fused Silica - stationary phase 5% phenyl-methyl-polysiloxane, length 50 m, ID 0.32 mm, OD 0.45 mm film... 

The same is true for the chiral polysiloxanes described here. Their use as stationary phases in gas chromatography allows the calculation of the differences in enthalpy and entropy for the formation of the diaste-reomeric association complexes between chiral receptor and two enantiomers from relative retention time over a wide temperature range. Only the minute amounts of the polysiloxanes required for coating of a glas capillary are necessary for such determinations. From these numbers further conclusions are drawn on the stereochemical and environmental properties required for designing systems of high enantio-selectivity in condensed liquid systems. 

The new polysiloxanes are excellently suited as stationary phases for the gas chromatographic separation of the optical antipodes of different compounds classes over a temperature range from 70° to 240° C. 

FIGURE 9.20 Effect of mobile phase composition on shape selectivity with a polymeric octadecyl-polysiloxane stationary phase, column using (a) SRM 869a (b) triphenylene/o-terphenyl (c) chrysene/benzo[a]anthracene with column outlet pressure 20.0 MPa and flow rate 1 mL/min at pump head. (Reprinted from J. W. Coym, 1. G Dorsey,... 

The presence of these 12 stereoisomers was experimentally confirmed by the partial resolution of 12 peaks in HPLC chromatography using an achiral stationary phase (octadecyl polysiloxane, ODS) and different mobile phase compositions (CH3CN/THF 55 45-58 42) and temperatures (288-258 K). Figure 7 shows one of these chromatograms in which 10 partially resolved peaks are clearly distinguished. ... 

Our initial attempts to separate the Cj and Dj isomers of 5 (G = 1) used octadecyl polysiloxane (ODS) as high performance liquid chromatography (HPLC) stationary phase and mixtures of acetonitrile/HjO or methanol/HjO as mobile phases. Under these classical reverse-phase conditions, the resulting efficiencies were extremely poor because of the low solubility of 5 (G = 1) in both mobile phases. By contrast, mixed mobile phases which contained acetonitrile (ACN) with some percentages of a cosolvent such as tetrahydrofuran (THF) substantially improved... 

Another direct approach to chiral polymeric stationary phases is the modification of commercially available polysiloxanes which contain reactive side groups. Thus, the diamide phase was linked to a modified XE-60 polysiloxane phase (Table 2). In one case (XE-60-L-Val-(/ or 5)-a-pea)124 another center of stereogenicity (R or S configuration) has been introduced in the amide group. An XE-60-L-Val-(S)-x-pea column was used for the enantiomer separation of racemic. V-rert-butoxycarbonyl amino acids after their methylation with diazomethane (serine and threonine as the O-trimethylsilyl derivatives) (Figure 12)124. 

A limiting factor of complexation gas chromatography is the low temperature range (25-120°C). Therefore, improved thermostable polymeric stationary phases, e.g., Chirasil-Metal, in which the chiral metal chelates are chemically anchored to a polysiloxane backbone, have been prepared155 156. 

Using a chiral column, coated with a definite modified cyclodextrin as the chiral stationary phase, the elution orders of furanoid and pyranoid linalool oxides are not comparable [11, 12]. Consistently, the chromatographic behaviour of diastereomers and/or enantiomers on modified cyclodextrins is not predictable (Fig. 17.1, Table 17.1). Even by changing the non-chiral polysiloxane part of the chiral stationary phase used, the order of elution may significantly be changed [13]. The reliable assignment of the elution order in enantio-cGC implies the coinjection of structurally well defined references [11-13]. 

Over 100 stationary phases of various types have been described in the literature for packed columns, which are slowly being abandoned. However, for bonded phase capillary columns the choice of stationary phase is limited because the generation of the film at the surface of the column requires a different principle than impregnation. Generally, two families of compounds are used to modify the polarity polysiloxanes and polyethylene (silicones) glycols. Very special phases such as cyclodextrins can be used for enantiomeric separations. Stationary phases can be used between a minimum temperature under which equilibrium is too slow to occur and a maximum temperature above which degradation of the polymer occurs. The maximum temperature depends on the film thickness and the nature of the polymer. 

Calculate tM from the following experiment, employing the method above A mixture of linear alkanes, possessing six, seven and eight atoms of carbon, is injected into the chromatograph. The total retention times for these compounds were respectively, 271 s, 311 s, and 399 s, under a constant temperature of 80 °C. (Length of column 25 m, ID = 0.2 mm, <7f — 0.2 pm and the stationary phase is made up of polysiloxanes). 

Common chiral stationary phases for gas chromatography have cyclodextrins bonded to a conventional polysiloxane stationary phase.7-8 Cyclodextrins are naturally occurring cyclic sugars. P-Cyclodextrin has a 0.78-nm-diameter opening into a chiral, hydrophobic cavity. The hydroxyls are capped with alkyl groups to decrease the polarity of the faces.9... 

Programmed temperature (120 -200°C) chiral separation on a 0.25-mm x 25-m open tubular column with a 0.25-nm-thick stationary phase containing 10 wt% fully methylated p-cyclodextrin chemically bonded to dimethyl polysiloxane. [From W. Vetter and W. Jun, Elucidation of a Polychlorinated Bipyrrole Structure Using Enantioselective GC," Anal. Chem. 3002, 74,4287.]... 

The retention index of 657 for benzene on poly(dimethylsiloxane) in Table 24-3 means that benzene is eluted between hexane (7 = 600) and heptane (7 = 700) from this nonpolar station-aiy phase. Nitropropane is eluted just after heptane on the same column. As we go down the table, the stationary phases become more polar. For (biscyanopropyl)09(cyanopropylphenyl)0l-polysiloxane at the bottom of the table, benzene is eluted after decane, and nitropropane is eluted after -Cl4H30. 

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