Stationary-phase selection for capillary gas chromatography

Many liquid phases for packed column purposes were unacceptable for capillary GC. Although they offered selectivity, overriding factors responsible for their disfavor were overall lack of thermal stability and the instability of the stationary phase as a thin film at elevated temperatures and during temperature programming. In the latter processes, it is crucial that the phase remain a thin uniform film otherwise, loss of both inertness and column efficiency results. Today, these problems have been solved and the refinements are reflected in the high performance of commercial columns. The impetus has been driven by the improvements in the sensitivity of mass spectrometers such that the MS detector is now the second most popular detector in GC (the FID is the most widely used detector). This rise in the use of GC-MS has also necessitated more thermally stable columns offering much less column bleed. 

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Calixarenes have been extensively studied as solvent extractants for inorganic ions, stationary phases for capillary gas chromatography (GC), selectivity modifiers in CE, and as components of HPLC. The calixarene structure is quite versatile in that the lower rim can be modified to adjust solubility and the phenol groups on the upper rim are suitable for many chemical modihcations that may introduce specific selectivity. Here, we describe the application of modified calixarenes to ion chromatographic separations. 

Glycerin is used in Nasonex primarily as a humectant. For its quantification, both capillary gas chromatography method and HPLC methods may be selected. The GC is equipped with a flame-ionization defector, a 0.53 mm x 30 m fused silica analytical column coated with 3.0-p,mG43 stationary phase, and a 0.53 mm x 5 m silica guard column deactivated with phenylmethyl siloxane. The carrier gas was helium with a linear velocity of about 35 cm/s. The injection port and detector temperature was maintained at 240 and 260°C, respectively. The injection mode is splitless. The column temperature is programmed to be maintained at about 40°C for 20 min, then to increase to 250°C at a rate of 10°C/min and to hold at 250°C for 15 min. 

Details on the methods used (extraction, clean-up, separation) are given in the certification report [26]. Extraction was either carried out ultrasonically or by Soxhlet using organic solvent, e.g. cyclohexane, acetone, methanol, and toluene. Clean-up was performed with alumina cartridge or silica with an elution with e.g. cyclohexane, toluene, n-pentane etc. Also HPLC and TLC on silica were used for the clean-up. Capillary gas chromatography was used, identifying the PAH compounds on the basis of their relative retention times and, in case of mass selective detection, on the basis of their ion masses. In all cases, at least two columns with different stationary phases and different polarity have been used for quantification. For each PAH, the participant selected the best suited column. 

Capillary gas chromatography has been found to be the most selective technique for the analysis of essential oils. This technique is now recommended in the European Pharmacopoeia as the standard method to analyze essential oils. The recommended stationary phase is polyfethylene glycol) 20000. GC... 

The use of dendronized silica as a stationary phase in capillary gas chromatography was pioneered in 2001 by Newkome et al.," who used the triply branched dendrons prepared in solution and immobilized on fused silica inside the capillary column. The dendritic architecture provides unique selectivity in separation of a Grob test mixture of compounds. The same dendron-silica hybrids were used by the authors for solventless capillary microextraction (CME) and preconcentration in chemical analysis. 

Liquid crystals have found widespread use as stationary phases in gas chromatographic applications due to the benefits of coupling the usual analytical strengths of gas chromatography with the unique structure and shape selective properties of the liquid crystalline phase. Interaction of solutes with the orientational order provided by the anisotropy of the liquid crystal stationary phase allows for the effective and selective separation of positional and geometric isomers. This remarkable solute structural discrimination is especially important for the separation of isomers that have similar physical properties and thus cannot be conveniently separated on conventional capillary columns that mainly differentiate on the basis of boiling point/molecular weight or polarity differences. The mechanism of separation in liquid crystalline stationary phases is based on specific intermolecular inter-... 

Gas-Liquid Chromatography. In gas-liquid chromatography (GLC) the stationary phase is a liquid. GLC capillary columns are coated internally with a liquid (WCOT columns) stationary phase. As discussed above, in GC the interaction of the sample molecules with the mobile phase is very weak. Therefore, the primary means of creating differential adsorption is through the choice of the particular liquid stationary phase to be used. The basic principle is that analytes selectively interact with stationary phases of similar chemical nature. For example, a mixture of nonpolar components of the same chemical type, such as hydrocarbons in most petroleum fractions, often separates well on a column with a nonpolar stationary phase, while samples with polar or polarizable compounds often resolve well on the more polar and/or polarizable stationary phases. Reference 7 is a metabolomics example of capillary GC-MS. 

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. 

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