Alkyl-stationary phases

Cooke, N. H. C. and Olsen, K., Some modern concepts in reversed-phase liquid chromatography on chemically bonded alkyl stationary phases,. Chromatogr. Sci., 18, 512, 1980. 

Phases of extended length (C30) have been utilized for the separation of larger-size constrained solutes, such as carotenoids and steroids [27-29,93,106,107]. Apractical limit of alkyl chain length of C34 to C36 is imposed by the commonly employed silan-ization chemistry techniques [106]. Immobilization of longer alkyl stationary phases has been achieved through the use of poly(ethylene-co-acrylic acid) materials for use in carotenoid separations [27,28,93]. Rimmer et al. [28] have recently compared the selectivity of both alkyl and poly(ethylene-co-acrylic acid) stationary phases on the basis of separations of carotenoids in food matrices (Figure 5.12), in addition to mixtures of tocopherols and PAHs. 

A complete review of spectroscopic methods applied to the analysis of alkyl-modified surfaces with a comprehensive list of spectroscopic indicators of alkyl chain conformational order is provided elsewhere [9] this review will focus on the application of spectroscopic and other relevant experimental techniques for the characterization of shape-selective chromatographic materials. On the whole, it has been observed experimentally that any increase in alkyl stationary-phase conformational order promotes an increase in selectivity for shape-constrained solutes in RPLC separations [9], As a complement to the wealth of spectroscopic and chromatographic data, the use of molecular simulation techniques to visualize and characterize alkyl-modified surfaces may also provide new insights into molecular-level features that control shape selectivity. A review of progress in the field of chromatographic material simulations will also be discussed. 

Dynamic motion of the alkyl stationary phases can also be obtained from NMR studies through an analysis of line shapes, comparisons between single-pulse (SP) and CP-MAS spectra, and various relaxation time constants. Zeigler and Maciel... 

Analytical shape computation techniques were applied for the detection of cavities and the calculation of molecular surface properties of isolated cavity features and other ordered formations within these resultant alkyl stationary-phase simulation models [227]. Deep cavities (8-10 A wide) within the alkyl chains were identified for Cig polymeric models representing shape selective stationary phases (Figure 5.23). Similar-structure cavities with significant alkyl-chain ordered regions (>11 A) were isolated from two independent Cig models (differing in temperature,... 

Sander, L.C., Lippa, K.A., and Wise, S.A., Order and disorder in alkyl stationary phases, AnoZ. Bioanal. Chem., 382, 646, 2005. 

Pesek, J.J., Matyska, M.T., and Takhar, S., Synthesis and characterization of long chain alkyl stationary phases on a sdica hydride surface, Chromatographia, 48, 631, 1998. 

RPLC columns with chemically bonded (alkyl) stationary phases provide rapid equilibration, high efficiency and symmetrical peaks. 

The curvature in the log k vs. n plots, observal with micellar and hybrid mobile phases, was first attributed to the different locations (with different microenvironment polarities) in the micelle, for different members of a homologous series [6, 7], Methylene selectivity decreases as the difference between mobile and stationary phase polarities is reduced. For a given mobile phase composition, the larger and more hydrophobic homologous compounds are located in a less polar environment of micelles, it is then conceivable to assume that these compounds experience a smaller change in their microenvironment polarity upon being transferred from the micellar pseudo-phase to the bonded alkyl stationary phase. The a(CH2) value between n-pentylbenzene and n-butylbenzene is smaller than it is between ethylbenzene and toluene (Table 9.1), because the former pair is located in a more nonpolar environment than the latter. 

To determine the role of stationary phase structure in the retention process, it is of primary importance to understand its structure in various solvent environments. Studies of the interphase region using deuterium NMR have shown that it is the mobile phase composition that determines the structure of the stationary phase (d). Results have shown that water does not associate strongly wdth the alkyl chains. However, acetonitrile can associate strongly, even at low mole fractions because of the microheterogeneous environments that exist for the binary mixture. Previous in situ studies of the alkyl stationary phases using surface enhanced Raman spectroscopy have also concluded that little conformational change occurs with the addition of a polar solvent to the interface (7). 

Studies of order and disorder in alkyl stationary phase, as well as various aspects of alkyl chain conformation and motion investigated through the use of NMR and other spectroscopic techniques have been reviewed by Sander et al ... 

CM Bell, LC Sander, JC Fetzer, SA Wise. S5mthesis and characterization of extended length alkyl stationary phases for liquid chromatography with application to the separation of carotenoid isomers. J Chromatogr A 753 37-45, 1996. 

Sander LC, Lippa KA, Wise SA. Order and disorder in alkyl stationary phases. Anal... 

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