Chemically bonded phases monomeric

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 . 

Figure 4.4 Separation of SRM 1647 and SRH 869 polycyclic aromatic hydrocarbon test mixtures on a monomeric and polymeric reversed-phase octadecylsiloxane bonded phases by gradient elution. (Reproduced with permission from ref. 69. Copyright American Chemical Society).

A final important reproducibility specification should be considered which applies specifically to bonded-phase packings. First, the bonded-phase should be specified as being polymeric or monomeric. If polymeric,information on the % organic or % carbon for the packing and the chemical structure of the bonded phase should be provided. However, as shown before, this information is often not sufficient to determine lot-to-lot chromatographic reproducibility. If the bonded phase is monomeric, data on the % organic or % carbon and chemical structure are also useful, but in addition, the surface coverage calculated from these values (6) should also be provided (EQ. 4). 

Figure 3.13 Variation of the capacity factor with the length of chemically bonded alkyl chains of the stationary phase using monomeric phases (nj. Mobile phase methanol-water (80 20). Solutes n-alcohols and aromatic solutes as indicated in the figure. Asterisks indicate the critical chain length. Figure taken from ref. [319]. Reprinted with permission. 

For steric reasons bifunctional or trifunctional silanes can react with either one only or, at most, two silanol groups on the silica gel surface (second reaction in Fig. 1.8A). Thus, some of the functional (Cl or alkoxy) groups remain unreacted and easily hydrolyse to form new silanol groups. If the reaction mixture contains even traces of water, the hydrolysis occurs during chemical modification of silica and the new silanol groups react with excess molecules of reagents to form a polymerised surface layer (Fig. 1.8B). These bonded phases may be more stable and usually show stronger retention than monomeric phases at low pH. However, the reaction is difficult to reproduce and various batches of the same material may have different properties, so that the reproducibility of separation is poorer than with monomeric phases. Polymeric phases are more resistant to penetration of analytes and may show increased mass-transfer resistance and decreased efficiency (plate number) of separation [- 91. 

Fig. 1 (a) Scheme of bonding chemistry for chemically bonded Cig silica phase (i) synthesis of monomeric Cig (ii) synthesis of poly-... 

The difficulties encountered in LLC can be overcome by the use of chemically bonded stationary phases or bonded-phases. Most bonded phases consist of organochlorosilanes or organoalkoxysilanes reacted with micro-particulate silica gel to form a stable siloxane bond. The conditions can be controlled to yield monomeric phases or polymeric phases. The former provides better efficiency because of rapid mass transfer of solute, whereas the polymeric phases provides higher sample capacity. BPC can be used in solvent gradient mode since the stationary phase is bonded and will not strip. Both normal-phase BPC (polar stationary, non-polar mobile) and reversed-phase BPC (non-polar stationary, polar mobile) can be performed. The latter is ideal for substances which are insoluble or sparingly soluble in water, but soluble in alcohols. Since many compounds exhibit this behaviour, reversed phase BPC accounts for about 60% of published applications. The main disadvantage of silica bonded phases is that the pH must be kept between 2 to 7.5. However, bonded phases with polymer bases (polystyrene-divinylbenzene) can be used in the pH range of 0 to 14. 

Figure 4.7. CP-MAS NMR spectra of a series of octadecylsiloxane-bonded phases prepared by different reaction mechanisms and the silica substrate. 1 = Porous silica (structural elements Q -85 to -115 ppm) 2 = monomeric phase prepared with octadecyldimethylchlorosUane (M 15 to 0 ppm) 3 = polymeric phase prepared by the solution method with octadecylmethyldichlorosilane (D -5 to -25 ppm) 4 = polymeric phase prepared by the solution method with octadecyltrichlorosilane T 5 to -70 ppm) and 5 = surface polymerized phase using octadecyltrichlorosilane. (Adapted from ref. [110]. American Chemical Society).

Reversed-phase EC on chemically bonded Cig (octadecylsilane) stationary phases is by far the most popular liquid chromatographic mode for separation of PAH compounds. Resolution is greatly influenced by the type of synthesis used to prepare the bonded phase. Good resolution can be achieved for the 16 US EPA PAHs on polymeric Cig phases. However, some isomers are unresolved (chrysene and benzo[a] anthracene) or only partially resolved [htmo ght perylene and indeno[l,2,3-cd]pyrene, benzo[ ]fluor-anthene, and benzo[ ]fluoranthene, or fluoranthene and acenaphthene) when monomeric Cig phases are used. 

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. 

Fig. 1 a, Scheme of bonding chemistry for chemically bonded Cig silica phase (i) synthesis of monomeric Cig (ii) synthesis of polymeric Cig (iii) end-capping process, b, Surface structure of a monomeric Cig phase (i) monomeric Cig ligand (ii) end-capped trimethyl ligand (iii) residual silanol. 

In the NMR spectra of LVIa,b no spin-spin coupling has been observed between 205T1 and 1H, 13C, and nB nuclei. According to mass spectroscopic data the compounds are monomeric in the gas phase. From the similarity between the physical-chemical data of LVIa,b and Lilia similar bonding in both classes of compounds is anticipated (183). 

Fig. 1.8. Chemical modification of silica gel in the preparation of (A) a monomeric and (B) a polymeric non polar alkyl bonded stationary phase for RPC by reaction with mono-, di- and tri-funclional alkylchlorosilanes.

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