Stationary phase, chromatographic. 920

Most chiral chromatographic separations are accompHshed using chromatographic stationary phases that incorporate a chiral selector. The chiral separation mechanisms are generally thought to involve the formation of transient diastereomeric complexes between the enantiomers and the stationary phase chiral ligand. Differences in the stabiHties of these complexes account for the differences in the retention observed for the two enantiomers. Often, the use of a... 

Chiral separations present special problems for vaUdation. Typically, in the absence of spectroscopic confirmation (eg, mass spectral or infrared data), conventional separations are vaUdated by analysing "pure" samples under identical chromatographic conditions. Often, two or more chromatographic stationary phases, which are known to interact with the analyte through different retention mechanisms, are used. If the pure sample and the unknown have identical retention times under each set of conditions, the identity of the unknown is assumed to be the same as the pure sample. However, often the chiral separation that is obtained with one type of column may not be achievable with any other type of chiral stationary phase. In addition, "pure" enantiomers are generally not available. 

From the viewpoint of molecular interactions, the number of fundamentally distinct chromatographic stationary phases is very limited.17 One mechanism for adsorption to the stationary phase is solvophobic, or mobilestationary phase transfer free energy effects, in which the adsorption of an analyte to the stationary phase liberates bound solvent. There is often an accompanying enthalpic component to such binding through dispersion interactions. Another mechanism for adsorption is that of specific interactions,... 

Remcho, V.T. and Tan, Z.J., MIPs as chromatographic stationary phases for molecular recognition, Anal. Chem. News Features, 4/1, 248A, 1999. 

The third technique we used was a measurement of changes in the sorption behavior of a compound in the presence of humic materials. A thin film of OV-1, a methyl silicone gum used as a chromatographic stationary phase, was plated on the bottom of a 60 ml Hypo-Vial (Pierce Chemical Co., Rockford, 11.). A solution of radiolabeled pollutant was added to the vial in either buffered distilled water in a solution of humic materials. Again, it is assumed that the pollutant is solution consists of two fraction free and bound. It is also assumed... 

Of particular importance with the use of LC-NMR as an experimental technique is that it is suited for only a limited number of applications in reference to structure elucidation. As will be discussed in greater detail, the sensitivity issues that arise between the amount of compound one is able to load onto a particular chromatographic stationary phase, and hence elute into the flow-cell of an LC-NMR probe, limit what type of structural analysis that can be performed. It is this author s current opinion that most complete structure elucidations of unknown molecular entities are not amenable to LC-NMR. In these... 

FIGURE 5.5 Synthesis schemes for chromatographic stationary phases (a) monomeric synthesis where X represents reactive (e.g., chloro or alkoxy) or nonreactive (methyl) substituents, (b) solution polymerization, in which water is added to the slurry and (c) surface polymerization, in which water is added to the silica surface. 

The most common approach to chromatographic stationary-phase characterization is in terms of bulk-phase properties, such as percent carbon loading onto the silica substrate. This property together with the surface area of the substrate and the molecular characteristics of the bonded silane can be used to calculate the bonding density (A) of the chromatographic sorbent [60] ... 

The application of NMR spectroscopy to the study of the conformational structure of chromatographic stationary phases over the past 25 years is the topic of several reviews [145-147], The commonly measured nuclei of the stationary-phase are... 

Wirth, M.J. and Fatunmbi, H.O., Horizontal polymerization of mixed trifunctional silanes on silica—a potential chromatographic stationary-phase. Anal. Chem., 64, 2783, 1992. 

Sandoval, I.E. and Pesek, I.I., Synthesis and characterization of a hydride-modified porous silica material as an intermediate in the preparation of chemically bonded chromatographic stationary phases, AnaZ. Chem., 61, 2067, 1989. 

Doyle, C.A., Vickers, T.J., Mann, C.K., and Dorsey, J.G, Characterization of hqnid chromatographic stationary phases by Raman spectroscopy—effects of ligand type, J. Chromatogr. A, 779, 91, 1997. 

Klatte, S.J. and Beck, T.L., Molecular-dynamics simulations of tethered alkane chromatographic stationary phases, 7. Phys. Chem., 99, 16024, 1995. 

Valko, K., Papp, O., and Darvas, F. Selection of gas chromatographic stationary phase pairs for characterization of the 1-octanol-water partition coefficient, J. Chromatogr., 301 355-364, 1984. 

Moaddel, R., Clorx, J.-F., Ertem, G., Wainer, I. W. Multiple receptor liquid chromatographic stationary phases the co-immobilization of nicotinic receptors, y-amino-butyric acid receptors, and N-methyl-D-aspartate receptors. Pharm Res 2002, 19, 104-107. 

Therefore, direct Group I methods for the determination of enantiomeric purity are generally preferred. A most important merit of the direct methods is the fact that the chiral auxiliary compounds, e.g., the chromatographic stationary phase, need not be enantiomerically pure. 

In addition to providing highly selective separations, there are a multitude of other desired characteristics that a gas chromatographic stationary phase should possess. These properties include high viscosity, low surface tension allowing for wetting of the fused silica capillary wall, high thermal stability, and low vapor pressure at elevated temperatures. The stationary phase solvent should also not exhibit unusual mass transfer behavior. 

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