Stationary phases polysiloxane based

Since Pasteur separated crystalline sodium ammonium tartrate manually in 1848, many researchers have worked on the subject of enantiomeric separation. In 1939 Henderson and Rule fully separated derivatives of camphor by column chromatography using lactose as a stationary phase material [1]. Gil-Av et al. [2] were able to separate amino acid derivatives on a polysiloxane-based stationary phase by gas chromatography (GC) in 1966. Since then many approaches for a successful distinction between enantiomers have been developed for capillary GC and liquid chromatography [3]. It is still a current topic for researchers searching for chiral separation with SciFinder [4] results in 812 hits and searching for chiral recognition leads to 285 hits for the year 2003 only. 

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. 

A review appeared on the practice and theory of enantioselective CGC with optically active selectors, e.g. 3-(perfluorobutyryl)-(17 )-camphorate residues forming complexes on a functionalized polysiloxane stationary phase (e.g. Chirasil, 65) SEC operates at temperatures lower than those of CGC, thus allowing better resolution, especially of thermally unstable enantiomers (e.g. those based on restricted free rotation, as is the case of dimethyl l,l -binaphthyl-2,2 -dicarboxylate, 66 ). Various analytical problems were addressed, such as determination of enantiomeric excess, assignment of absolute configuration, the elusive separation of protio- and deuterio-substituted enantiomers and the semipreparative separation of enantiomers. The following chromatographic parameters are related to the chemical and thermodynamic properties enclosed in parentheses of the enantiomeric system (i) peak retention (chemoselectivity, —AG), (ii) peak separation... 

Terfloth, G.J. Pirkle, W.H. L5mam, K.G. Nicolas, E.C. Broadly applicable polysiloxane-based chiral stationary phase for high-performance liquid chromatography and supercritical fluid chromatography. J.Chromatogr.A, 1995, 705, 185-194... 

More recently, some new kinds of modified CDs have been developed which expand the range of compounds which can be resolved and also extend their use in GC. Up to now, the GC columns based on derivatized CDs have realized commercialization. Sigma corporation had developed a series of stable derivatized CD stationary phases named DEX columns for high resolution analyses of optical and positional isomers. They prepared these functional columns by adding permethylated a-CD, /3-CD or y-CD to a phenyl-containing polysiloxane stationary cophase. The DEX columns could separate chiral compounds without derivatization, and enantiomers and positional isomers are resolved by slight... 

The Amino Acid-Peptide Polysiloxane Stationary Phases Peptide-Dimethylpolysiloxane Chiral Stationary Phases Peptide-Phenylpolysiloxane Chiral Stationary Phases Cyanopolysiloxane Peptide Chiral Stationary Phases The Polysiloxane-Cyclodextrin Based Stationary Phases Chiral Liquid Chromatography... 

Today, there are a number of effective chiral stationary phases suitable for GC, some based on amino acid derivatives, some on polysiloxane polymers and some, probably the more popular, based on cyclodextrin derivatives. Very few chiral columns, if any, are packed, the vast majority being open tubular columns having various lengths and an I.D. that ranges from about 250 pm or 320 pm There has, however, been a recent trend to smaller diameter columns cf. 125 pm) to achieve higher efficiencies and even shorter analysis times. 

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. 

There are two commonly used chiral GC stationary phases from which the analyst can choose. Firstly, there are the polysiloxane based stationary phases that contain amino acids or peptides included in the polymer or reacted with the polymer matrix, secondly, there are those that contain one of the cyclodextrins, or deiivatized cyclodextrins... 

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... 

Both gas/solid adsorption and gas/liquid partition chromatography can be used for GC-MS, but GC is by far the most common. Because, in GC, the stationary phase is a liquid, usually a polymer, its vapor pressure will cause a continual low flow, or bleed into the ion source of the mass spectrometer. This bleed, which usually consists of decomposed stationary phase, will produce a spectrum whose intensity increases with column temperature. Stationary phases should therefore be of the high-boiling, low-bleed type. Most currently used stationary phases for routine GC-MS are based on alkyl-polysiloxanes or alkyl-phenyl-polysiloxanes that are chemically bonded to the column wall to increase stability. Columns containing such phases can, in some cases, be used at temperatures of up to 400°C. One advantage, however, to the presence of bleed peaks in the spectrum is that they enable a continual check to be made on the mass spectrometer calibration. For the alkyl siloxanes, ion peaks are present, in decreasing relative abundance, at miz 73, 207, 281, 355, 429,... 

Because of the general nature of the descriptors used in the two models, no attempt was made to precisely interpret their physical significance. However, an intuitive explanation may be proposed. The column used in this study consisted of a SE-30, dimethyl-polysiloxane stationary phase. This relatively nonpolar stationary phase typically takes advantage of dispersive interactions as opposed to dipole-dipole, dipole-induced-dipole, or acid-base interactions. These dispersive interactions are, in turn, loosely related to the bulk properties of the solute molecules (e.g., boiling point). All of the descriptors utilized in this study may be generally classified as bulk property descriptors. We can... 

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