Polysiloxane Phases

Polysiloxanes are the most widely used stationary phases for packed and capillary column GC. They offer high solute diffusivities coupled with excellent chemical and thermal stability. The thorough review of polysiloxane phases by Haken (67) and the overview of stationary phases for capillary GC by Blomberg (68,69) are strongly recommended readings. 

One measure of the polarity of a stationary phase is the cumulative value of its McReynolds constants, as described in Chapter 2. Because a variety of functional groups can be incorporated into the structure, polysiloxanes exhibit a wide range of polarities. Since many polysiloxanes are viscous gums and, as such, coat well on fused silica and can be cross-linked, they are ideally suited for capillary GC. The basic stmcture of 100% dimethylpolysiloxane can be illustrated as 

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Figure 2.21 shows the on-line extraction gas chromatogram of 2.25 ml of water spiked at 5 ppb levels with 14 different organic pollutants (40). In this case, the authors concluded that wall-coated open tubular traps (thick-film polysiloxane phases) can be used for the on-line extraction of organic compounds from water. However, when using swelling agents such as pentane, non-polar analytes can be trapped quantitatively, while for more polar compounds chloroform is the most suitable solvent. 

Polysiloxanes containing a-cyano and a- or ]]-fluorine substituents are. thermally labile at comparatively low temperatures and are not used in gas chromatography. Polysiloxane phases containing 3,3,3-trifluoropropyl groups are thermally Stable up to about 250-275 C and possess rather unique selectivity, particularly for electron-donor solutes, such as ketones and nitro groups. The electronegative trifluoropropyl... 

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]. 

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. 

Polysiloxanes (also known as silicone oils or gums) have a repetitive backbone that consists of two hydrocarbon chains per silicon atom (see Fig. 2.7). These phases are the most widely used for capillary columns because of their wide temperature range (-50 < T < 325 °C, with Rj = R2 = CH3). Approximately 20 types of polysiloxane phases have been commercialised worldwide. 

Some polysiloxanes form liquid-crystalline phases because of the presence of relatively stiff side chains.168-176 They have been much studied, particularly with regard to the effect of deformation of the elastomeric polysiloxane phase on the mesomorphic behavior exhibited by the side chains. 

Yancey, J.A., Liquid phases used in packed gas chromatographic columns. Part 1 Polysiloxane phases, J. Chromatogr. Sci., 23, 161, 1985. 

Stationare Phase Zusammensetzung der Polysiloxan-Phase Literatur zur Trennabfolge... 

The major breakthrough in the GC enantiomer separation has been the work of Bayer and associates [23,76], who synthesized a silicone-based chiral phase, stable up to 240°C. As shown in Fig. 3.14, a racemic mixture of 19 protein amino acids can be separated [23] on a glass capillary column coated with Chirasil-Val, a chiral polysiloxane phase. The phase was synthesized through coupling L-valine-tert-butylamide to a copolymer of dimethylsiloxane and carboxyalkylmethylsiloxane. 

Nevertheless, although the solution to the problem of chiral separations by GC was apparent, a thermally stable stationary phase was still not developed for over a decade. It was not until 1977 that Frank, Nicholson and Bayer [6] produced a chiral stationary phase by the co-polymerization of dimethylsiloxane with (2-carboxypropyl)methoxy-silane and L-valine- butylamide which proved to be far more thermally stable than those of Gil-Av. Frank et al. showed that by using polysiloxanes, the separation of most amino acids could be carried out in a much reduced analysis time. The polysiloxane phases had a much lower volatility and higher stability than previous chiral stationary phases and could be used up to temperatures of 175 °C. It is clear that the shorter retention times were directly a result of the separation being carried out at a much higher temperature which, in turn, was due to the greater thermal stability of the polysiloxane phases. 

Figure 6.14, with eight dimethoxypentylsilanes and eight methylsilanes bonded to the polysiloxane phase. 

Stationary phases in capillary SFC [30] are usually based on a polysiloxane backbone, cross-linked by means of a free-radical initiator, and methyl, phenyl, octyl, biphenyl, and cyanopropyl substituents are incorporated into them. These polysiloxane phases exhibit wide ranges between their glass transition and decomposition temperatures, and this feature sets them apart from most high molecular-mass polymers, as they allow useful operating conditions for chromatography. Polysiloxanes can be tailor-made for specific... 

Table 2 Commercially available asymmetrically substituted, OH-terminated polysiloxane phases, designed for gas chromatography, working temperatures (on glass) 350-370°C...

Table 5 Symmetrically substituted polysiloxane phases, specially designed for high-temperature work...

N,0-bis(trimethylsilyl)trifluoroacetamide are separated with a 30-m capillary column coated with 100% dimethyl polysiloxane phase. 

The majority of insect and mammalian pheromones are small and relatively simple molecules with low polarity. Nonpolar polysiloxane phases such as 100% polysiloxane or 5% polyphenylmethyl siloxane are the preferred column for pheromone research due to a broad range, thermal stability, and long lifetime. Five per cent (phenyl)methylpolysiloxane columns were used in the identification of dodecenyl acetate and dodecenol, the sex pheromones of the citrus fruit borer, and anisole, the sex pheromone of the scarab beetle. A 100% polysiloxane column was used in the analysis of dodecenyl acetate, the female sex pheromone of the Asian elephant. 

Wolff, R.L. Analysis of Alpha-Linolenic Acid Geometrical Isomers in Deodorized Oils by Capillary Gas-Liquid Chromatography on Cyanoalkyl Polysiloxane Phases A Note of... 

Although the polysiloxane phases carrying chiral peptides are still used in contemporary chiral GC, the presently popular phases are based on cyclodextrins. These materials are formed by the partial degradation of starch followed by the enzymatic coupling of the glucose units into... 

The columns based on cyanopropylphenyl/dimethylpropyl polysiloxane phases (commonly known as "624") are designed to perform EPA Method 624, but are also used in Method 524.2 Revision IV and Method 8260B. The main advantage is the complete separation of hig y volatile compounds such as vinyl chloride. 

Viscosity of the film of stationary phase after deposition under the thermal conditions of GC proved to be an important consideration. Wright and coworkers (103) correlated viscosity of a stationary phase with coating efficiency and stability of the coated phase. The results of their study supported the experimental success of viscous gum phases, which yielded higher coating efficiencies and had greater thermal stability than did corresponding nonviscous counterparts. The popularity of the nonpolar polysiloxane phases is due in part to the... 

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