Monomeric stationary phase

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. 

FIGURE 5.15 Fourier transform infrared spectroscopy absorbance spectra at different temperatures for a C22 monomeric stationary-phase. (Reproduced from Sander, L.C., et al., Anal. Chem.,55, 1068, 1983. With permission.)... 

For the monomeric stationary phase. Figure 9.19 shows the SRM 869a ofTBN/BaP decreasing with increasing proportions of aceonitrile in the mobile phase, which should indicate increased shape selectivity with added acetonitrile. This is the same observed trend when organic solvents are added to water in a reversed-phase separation. However, the selectivity measured from the triphenylene/o-terphenyl pair and the chrysene/benzo[fl]anthracene pair showed exactly opposite trends in that the planarity and LIB selectivity was increased with decreasing proportions of acetonitrile. 

A study of a group of alkyl chemically bonded reversed-phase stationary liquids (Cg-Caa) concluded that Ci8 provided better PAH separations. With monomeric stationary phases, PAHs are usually separated according to the number of aromatic rings in their chemical structure while with polymeric Cig phases, selective retention of PAH isomers based on the molecular shapes Ub ratio) is observed. Phase selectivity of PAHs on polymeric Cig in LC and on smectic liquid crystal phases in GC show close correspondence of retention behavior. PAH specialty columns for separations in LC are now readily available in the market. 

On the monomeric LCSP called 4,4 -biphenylene-bis (4-n-butyloxy-benzoate) placed in a capillary column, 12 polynuclear aromatic hydrocarbon (PAHs) were separated in the mixture derived from coal tar (Fig. 4). The chromatogram of selected PAHs on metallomesogen stationary phase is presented in Fig. 5. Chromatograms of selected volatile aromatics compounds on monomeric stationary phases are presented in Figs. 6 and 7. 

Another type of synthetic polymer-based chiral stationary phase is formed when chiral catalyst are used to initiate the polymerisation. In the case of poly(methyl methacrylate) polymers, introduced by Okamoto, the chiraUty of the polymer arises from the heUcity of the polymer and not from any inherent chirahty of the individual monomeric subunits (109). Columns of this type (eg, Chiralpak OT) are available from Chiral Technologies, Inc., or J. T. Baker Inc. 

Bonded-phase chromatography (BPC). To overcome some of the problems associated with conventional LLC, such as loss of stationary phase from the support material, the stationary phase may be chemically bonded to the support material. This form of liquid chromatography, in which both monomeric and polymeric phases have been bonded to a wide range of support materials, is termed bonded-phase chromatography . 

It is interesting that the retention behavior of lycopene varies dramatically depending on the stationary phase synthesis lycopene usually elutes before a- and P-carotene in monomeric columns, whereas with polymeric C30 columns, lycopene elutes after these carotenes. ... 

Our efforts to concretely determine the relative stereochemistry of this dimer have been met by failure. We have made attempts to resolve several of the monomeric tetracyclic aminoaldehydes of type 100 by HPLC using chiral stationary phase, in order to know for sure the structure of the homodimer. The poor solubility of these compounds in typical HPLC solvents hampered these efforts to access enantiopure monomer. A few attempts at diastereomeric salt formation from compounds of type 101 using chiral carboxylic acids were also unsuccessful. Computational analysis corroborates the assumption that the homodimer should be formed preferentially. 

The retention of analyses in RP-HPLC markedly depends on the adsorption of the organic constituent of the mobile phase on the surface of the stationary phase. The excess adsorption isotherms of ACN, THF and methanol were measured on silica support modified with C, C6, C8, C10, C12 and C18 monomeric phase and a model was developed for the description of the retention of solutes from the binary mobile phase. The dependence of the retention factor on the partition coefficient can be described by... 

Some new benzanthrone dyes were synthesized and applied for the one-step colouration and stabilization of polystyrene. The chemical structures of monomeric benzanthrone dye (formula 1), the stabilizer TTMP 2,2-(2,2,6,6-tetramethylpiperidine-l-yl)-4,6-dichloro-l,3,5-triazine (formula 2) and the new synthetic product showing both colouration and stabillizer capacity (formula 3) are shown in Fig. 3.135. The synthesis process was controlled by TLC using a silica stationary phase and an n-heptane-acetone (1 1, v/v)... 

Another approach to preparing a stable reversed phase with fewer residual silanols is the use of polyfunctional silanes of the type R2SiX2. These react to form a polymeric stationary phase that shields the siloxane bonds and restricts access to residual silanols. Polymer phases have higher carbon loads and are typically more retentive than monomeric phases. However, they are more difficult to synthesize reproducibly and may exhibit batch-to-batch variability in their properties. They also exhibit poorer mass transfer kinetics and so provide poorer efficiency than monomeric phases. 

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. 

Sander et al. [63] investigated the effect of microparticulate silica pore size on the properties of solution-polymerized Cig stationary phases and observed both an increase in bonding density and shape recognition for wider pore (>120 A) silica. A size-exclusion mechanism was proposed, in which the reaction of the silane polymer on the surface is enhanced for wide pores and reduced for narrow pores. Polymeric Ci8 phases prepared on substrates with narrow pores exhibited monomeric-like chromatographic properties. This effect may be the result of an increase in competitive surface linkage with the less sterically hindered monomers that coexist with the bulkier oligomers that have polymerized in the reaction solution (Figure 5.13). 

Gritti, R and Guiochon, G, Adsorption mechanism in reversed-phase liquid chromatography—effect of the surface coverage of a monomeric C-18-sihca stationary-phase, 7. Chromatogr. A, 1115, 142, 2006. 

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