Selection, stationary-phase polysiloxane phases

Besides cyclodextrin derivatives, other mixed special selectivity stationary phases also had synergistic effects in the separation of isomers. Those reported include di-benzo-18-crown-6 + MPBHpB, dibenzo-18-crown-6 and resorcarene derivative,MPBHpB + resorcarene derivative, and 25,27-dibutoxy-5,ll,17,23-tetra-tert-butyl-26,28-diundecenyloxy cali[4]arene + oo-undecylo-methyl-18-crown-6 polysiloxane." "... 

The selective stationary phases were further divided in the scheme proposed by Horning et al. [79] for the steroid separations into the types selective to alcohols, ketones, multiple bonds, etc. Here, various selective interactions between the column and the separated solutes are utilized. Most importantly, various polyglycols, polyesters, polyphenyl ethers, polyamides and polyimides belong to these categories. In addition, substitution of an alkyl group in a polysiloxane polymer by a more polar... 

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. 

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,... 

Modified cyclodextrins are the most versatile and widely used chiral stationary phases. The most widely used columns contain 10% to 50% cyclodextrin dissolved in either OV-101 or SE-52 polysiloxane. They are thermally stable up to 230°C but require some care in use because cyclodextrins are soluble in many solvents and can be washed off the column if they are exposed to too much solvent. Considerations in selecting a column... 

A programmed temperature-vaporization (PTV) injector (with a sorbent-packed liner) was used to preconcentrate and inject the sample. Thermal desorption was performed and the analytes were passed to a primary column (16 m X 0.32 mm i.d., film thickness 5 p.m, 100% methyl polysiloxane) and separated according to analyte vapour pressure. Selected heart-cuts were transferred to a second column (15 m X 0.53 mm i.d., Al203/Na2S04 layer, open tubular column with 10 (im stationary phase) where final separation was performed according to chemical functionality. 

The Carbowax column is very sensitive to oxidation when the stationary phase is exposed to traces of water or air especially at temperatures above about 160°C. A new type of cross-linking has been reported to impart resistance to oxidative degradation of the stationary phase [5-7]. Two other phases which show promise are an oligo-(ethylene oxide)-substituted polysiloxane (glyme) and an 18-crown-6-substituted polysilox-ane [8]. The glyme column offers a polar phase with good operational conditions to a low of a least 20°C with the same selectivity of Carbowax. The crown polysiloxane selectivity is based on the interaction of the solute molecule with the cavity of the crown ether. 

Mechanistic considerations (e.g., the extensive work published on brush-type phases) or the practitioner s experience might help to select a chiral stationary phase (CSP) for initial work. Scouting for the best CSP/mobile phase combination can be automated by using automated solvent and column switching. More than 100 different CSPs have been reported in the literature to date. Stationary phases for chiral pSFC have been prepared from the chiral pool by modifying small molecules, like amino acids or alkaloids, by the deriva-tization of polymers such as carbohydrates, or by bonding of macrocycles. Also, synthetic selectors such as the brush-type ( Pirkle ) phases, helical poly(meth) acrylates, polysiloxanes and polysiloxane copolymers, and chiral selectors physically coated onto graphite surfaces have been used as stationary phases. 

For fused sihca columns, the majority of separations can be done with fewer than 10 bonded hquid stationary phases of varying polarity. This is because with their very high resolving power, and selectivity of the stationary phase is less critical. The stationary phases are high-molecular-weight, thermally stable polymers that are liquids or gums. The most common phases are polysiloxanes and polyethylene glycols (Carbowax), with the former the most widely used. The polysiloxanes have the backbone ... 

A 30 to 50 m fused silica capillary column with a 5% phenyl-95% methyl-polysiloxane chemically bonded stationary phase (DB-5, CP Sil-8, HP Ultra 2, PTE-5) is very often used, while several oven temperature programs have been applied for PCB analysis. Table 18.7 shows a selection of combinations of column lengths, stationary phases, oven temperature programs, and detectors. Table 18.8 shows the stationary phase composition of the capillary columns listed in Table 18.7. Cochran et al. " have reviewed the most recent developments for the capillary GC of PCBs with detailed lists of PCB retention times on common capillary columns. 

The unambiguous regioselective synthesis of chiral polysiloxane-containing CyDs as chiral stationary phases, with the mono-octamethylene spacer in either the 02, 03, or 06 position was performed and the products applied to enantio-selective GC separations [99]. Subtle differences in the chemistry of the hydroxyl groups at the 2-, 3-, and 6-positions of CyDs can be exploited to direct an electrophilic reagent to the desired site. Selective monoalkylation at the primary side of a-CyDs involves the reaction of a-CyD with 4-methylamino-3-nitrobenzyl chloride in 2,6-lutidine. Monoalkylation at the 2-position of j8-CyD is accomplished by the reaction of yS-CyD with l -bromo-4-methylamino-3-nitroacetophenone [100]. 

The first effective chiral stationary phase, as already referred to in chapter 1, utilized derivatized amino acids to provide chiral selectivity [7] and this was achieved as early as 1966. These types of stationary phases however, had very limited temperature stability and the optimum temperature for separation was often in excess of that at which the stationary phase was stable. The first relatively stable stationary phase, as already mentioned, was introduced by Bayer [6], who combined the derivatized optically active component of the stationary phase in a polysiloxane gum. Nevertheless, a number of small molecular weight materials were used as stationary phase in the early days of chiral gas chromatography. The first type introduced by Gil-Av are shown below. 

Despite the early successes with the polysiloxane based stationary phases carrying peptide chiral selection agents, the chiral selectors used in contemporary chiral GC are largely cyclodextrin based. 

Capillary GC is currently the method of choice for enantiomer analysis of essential oils and enantio-selective-GC (Es-GC) has become an essential tool for stereochemical analysis mainly after the introduction of cyclodextrin (CD) derivatives as chiral stationary phases (CSPs) in 1983 by Sybilska and Koscielski, at the University of Warsaw, for packed columns [80], and applied to capillary columns in the same decade [81,82]. Moreover, Nowotny et al. first proposed diluting CD derivatives in moderately polar polysiloxane (OV-1701) phases to provide them with good chromatographic properties and a wider range of operative temperatures [83]. 

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