Stationary phases preparation

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

Di- and trifimctional silanes are also used for the preparation of alkyl-modified silicas to be employed as chromatographic sorbents. Under anhydrous reaction conditions, sorbents prepared from these silanes have similar surface coverages and chromatographic behavior to monomeric stationary phases prepared with monofunctional silanes. 

For reasons enumerated above, experimental results given in the literature do not lend themselves to unambiguous interpretation. The graphs in Fig. 11 illustrate the dependence of the retention tor on the carbon load of stationary phases prepared by Ca.Cu.Cu> and Cu ligates. It is seen that both the direct and die logarithmic plots of k versus C are linear, i.e., the increase in the retention factor with the carbon load can be due to an iiicieiise in either the phase ratitr or (he contact surface. 

In this reaction a direct surface Si-C bond is formed. This type of bond appears to be more stable than the normal sSi-O-Si-C bond. The stability of a Cg stationary phase, prepared both by hydrosilation and organosilanization are compared in figure 8.12. Both curves represent fraction of the initial surface coverage as a function of hydrolysis time. It is clear that the hydrosilated phase has a much better performance over the displayed time interval. 

The basic components of a preparative HPLC system shown in Figure 4.5 simplifies the overall process. In a more realistic form the colour coded schematic diagram of Figure 4.6 shows a typical plant layout for a facility housing a 30 cm diameter column. The section outlined in green covers solvent delivery, red is used for the post column solvent flow, sample feed is shown in blue and the stationary phase preparation area is in turquoise. 

Another method that is becoming very important is chromatography using a chiral phase. Often, a chiral stationary phase, prepared by covalently bonding a chiral compound to the surface of silica beads, is used. 

Cert, A. and Moreda, W. (1998) New method of stationary phase preparation for silver ion column chromatography application to the isolation of steroidal hydrocarbons in vegetable oils. J. Chromatogr., 823, 291-297. 

Lammerhofer et al. [127] demonstrated the use of a strong anion-exchange stationary phase, prepared in a monolithic format, for the separation of a mixture of four NSAIDs—ibuprofen, naproxen, ketoprofen, and suprofen. The separation, presented in Figure 29, was achieved in 13 min with high column efficiencies of up to 231,000 plates/m. 

Enantiomeric Stationary Phases. Chiral nonracemic chromatographic stationary phases prepared from p-cyclodextrin, derivatized with (R)- and (S)-NEI, and covalently bonded to a silica support are useful for the direct separation of enantiomers of a wide variety of compounds in both normal-phase and reversed-phase HPLC. ... 

Fig. 5.25. (A) Separation factor (a) and capacity factors k ) for D- and L-PA versus amount injected D,L-PA on columns packed with heat-treated L-PA imprinted stationary phases prepared using dichloromethane as porogen. (B) Corresponding elution profiles at a sample load of 50 nmol D,L-PA. Mobile phase acetonitrile/water/acetic acid 92.5/2.5/5 (v/ v/v). Note that the columns with the materials treated at 140 and 160°C were only of 5 cm length. From Chen et al. [14].

The method of stationary-phase preparation has a major effect on the resolution, column stability, retention time, reproducibility, and peak shape. When preparing Cjg or Cg, for example, it is important that the residual silanol groups are capped to prevent peak tailmg. The extent of capping must be consistently maintained between different batches of the stationary phase for reproducible results. 

Monofunctional silane reagents yield efficient stationary phases with flexible furlike or brushlike structure of the chains bonded on the silica surface. When bifunctional or trifunctional silanes are used for modification, Cl or alkoxy groups are introduced into the stationary phase, which are subject to hydrolysis and react with excess molecules of reagents to form a polymerized spongelike bonded phase structure. Stationary phases prepared in that way usually show stronger retention but lower separation efficiency (plate number) than mono-merically bonded stationary phases. 

In comparison, the stationary phases prepared using the coprecipitation process of Zhang, Feng, and Da with... 

Pioneering attempts at using cinchona alkaloids as a platform for chiral stationary phase preparation have been reported as early as in the mid-1950s by Grubhofer and Schleith [52]. The chiral anion exchange polymeric materials were prepared by immobilization of quinine (and other cinchona alkaloids) via the 9-hydroxyl group or quinuclidine nitrogen to a polymer support. However, this resulted in very low selectivities of these phases toward racemic mandelic acid as a test analyte. Results of the early studies have been reviewed in detail by Davankov [53]. 

Fischer, L. Muller, R. Ekberg, B. Mosbach, K. Direct enantioseparation of B-adrenergic blockers using a chiral stationary phase prepared by molecular imprinting. J. Am. Chem. Soc. 1991, 113, 9358-9360. 

Gasparrini, E., Misiti, D., Rompietti, R., Villani, C. New hybrid polymeric liquid chromatography chiral stationary phase prepared by surface-initiated polymerization,/. Chromatogr. A, 2005,1064, 25-38. 

Column 250 X 4.6 5 p,m CSP 2 polymeric chiral stationary phase (preparation details in... 

Chiral stationary phases prepared from silica-based coated poly(saccharide) derivatives... 

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