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).

Fig. 3.2a. Preparation of bonded phases. Reaction of silica with substituted chlorosilanes to form (i) monomeric (ii) polymeric...

Before reaction, the silica is treated with acid (eg refluxed for a few hours with 0.1 mol dm-3 HC1). This treatment produces a high concentration of reactive silanol groups at the silica surface, and also removes metal contamination and fines from the pores of the material. After drying, the silica is then refluxed with the dimethylchlorosi-lane in a suitable solvent, washed free of unreacted silane and dried. This reaction produces what is called a monomeric bonded phase, as each molecule of the silylating agent can react with only one silanol group. 

More complicated surface structures can be produced by changing the functionality of the silylating agent and the conditions under which the reaction is carried out. The use of di- or trichlorosilanes in the presence of moisture can produce a crosslinked polymeric layer at the silica surface, as shown in Fig. 3.2a (if). Monomeric bonded phases are preferred, as their structure is better defined and they are easier to manufacture reproducibly than the polymeric materials. 

A depiction of a butyl-bonded phase is shown in Figure 3.3. Monochloro-silane reagents produce only monomeric phases however, trichlorosilane reagents can produce both monomeric and polymeric phases depending upon the concentration of silyl reagent and the surface area. It is difficult to make a multilayer bonded phase, even when a large quantity of trichlorosilane is used for the reaction. 

Hydrocarbonaceous bonded phases described in the literature have at vialues ranging frolm 2 to 4. It is believed that among monomeric phases, those having the highest ol values are the best for use in RPC. In Table III the pertinent clttihtcteristics of some monomeric bonded phases are listed. 

It is seen that the trichlorosilane reacts with the silanol groups to form siloxane bridges. Subsequently the residual chlorines are hydrolyzed. Under carefiiUy controlled reaction conditions it is possible to obtain a product in which the hydrocarbonaceous layer at the surface is similar to that in a corresponding monomeric bonded phase. However, the hydrolysis of chlorines that did not react with surface silanbis may result in a silanol concentration at the surface that is higher than that in the silica gel proper used as the starting material for the reaction with alkyltri-chlorosilanes. 

Stationary phases with a high density of bonded alkyl groups can differentiate between two molecules of identical size where one is planar and the other twisted out of plane. This shape selectivity has been described by Sander and Wise [53] for polymeric stationary phases, where in the preparation, water has been added on purpose and trichloro alkyl silanes have been used. The selectivity for the retention of tetrabenzonaphthalene (TEN) and benzo[a]pyrene (BaP) was taken as a measure to differentiate between polymeric and standard RP columns. With standard ( monomeric ) RP columns, the twisted TBN elutes after the planar BaP, which on the other hand is more strongly retarded as TBN on polymeric stationary phases. In these cases the relative retention of TBN/ BaP is smaller than 1, whereas with monomeric phases the value is >1.5. The separation of the standards on three different phases is shown in Figure 2.9. These stationary phases have superior selectivity for the separation of polyaromatic hydrocarbons in environmental analysis. Tanaka et al. [54] introduced the relative retention of triphenylene (planar) and o-terphenyl (twisted), which are more easily available, as tracers for shape selectivity. However, shape selectivity is not restricted to polymeric phases, monomeric ones can also exhibit shape selectivity when a high carbon content is achieved (e.g., with RP30) and silica with a pore diameter >15 nm is used [55]. Also, stationary phases with bonded cholestane moieties can exhibit shape selectivity. 

Stalcup, A. M., Martire, D. E., and Wise, S. A., A thermodynamic comparison of monomeric and polymeric Cjg bonded phases using aqueous methanol and acetonitrile mobile phases. /. Chromatogr., 422,1-14,1988. 

Van t Hoff plots of In k versus the inverse of temperature (generally 1000/T for convenience) are very often linear, especially with monomeric bonded phases. They can exhibit nonlinear behavior, and the transition temperature is often close to the undefined room temperature. Temperature optimization is one trend in LC. A rising temperature increase reduces viscosity and increases the diffusion rate, thereby enhancing mass transfer, which flattens the HETP curve at high velocities (31). Conversely, Sander and Wise (32) investigated the influence of temperature reduction. 

The physical structure of the stationary phase depends on the compatibility of the solvent with the bonded n-alkyl chains. Compatible nonpolar solvents tend to promote extension of the chains, allowing full penetration by the solvent. Conversely, fairly polar solvents tend to promote collapse of the chains upon each other, allowing negligible solvent penetration. The stationary phase therefore has the ability to adjust itself to maintain a relatively nonpolar character (113). Retention on monomeric bonded phases with octyl (C8) or longer chains are dominated by a partitioning-like mechanism (114). 

LC Tan, PW Carr. Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography. II. Partitioning vs. adsorption mechanism in monomeric alkyl bonded phase supports. J Chromat A 775 1-12, 1997. 

Bonded phases can be obtained as monomeric or polymeric coverage of an organic ligand group, R, on the silica surface depending on whether a monofunctional (R- SiX) or a trifunctional (RSiXj) reactant is used, respec-... 

A polymeric surface structure can result in slower mass transfer of the analyte in the polymer coating compared with the more brush- or bristlelike bonding of monomeric phases and thereby lead to higher efficiencies with monomeric phases. However, Thurman and Mills [75] note that the trifunctional reagent yields a phase that is more stable to acid because the... 

In the gas phase, monomeric SO3 has a >3h planar structure with bond length S-0 142 pm, as shown in Fig. 16.6.2(a). The cyclic trimer (803)3 occurs in colorless orthorhombic y-S03 (mp 16.9°C), and its structure is shown in Fig. 16.6.2(b). The helical chain structure of P-SO3 is shown in Fig. 16.6.2(c). A third and still more stable form, a-S03 (mp 62°C), involves cross-linkage between the chains to give a complex layer structure. The standard enthalpies of formation of the four forms of SO3 at 298 K are listed below ... 

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). 

Monofunctional reagents such as alkyldimethylmonochlorosilanes yield monomeric packings (first reaction in Fig. 1.8A). Such bonded-phase materials are well defined as one silanol group reacts with one silane molecule and exhibit high efficiency because of low mass-transfer resistance due to fast diffusion of molecules into the flexible fur - or brush -like structure of the alkyl chains on the silica surface. 

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. 

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