Monomeric phase

Sander and wise have proposed a test method to determine the bonding chemistry used to prepare octadecylsiloxane column packings based on the relative retention of three polycyclic aromatic hydrocarbons, benzo[a]pyrene (BaP), phenanthro-phenanthrene (PhPh), and l,2 3,4 5,6 7,8-tetrabenzonaphthalene (TBN) eluted with the mobile phase acetonitrile-water (85 15) [52,67,199,210]. On monomeric phases the test solutes elute in the... 

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... 

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. 

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. 

Fujimoto et al. [77] synthesized a novel phase by coupling a dodecylamino-substituted P-cyclodextrin (P-CD) to 3-glycidoxypropyl-derivatized silica gel. The surface coverage of this phase was reported as 0.37 xmol/m, which amounts to a surface coverage of 2.6 xmol/m for C12 chains (seven chains per fi-CD). An increase in shape selectivity was observed when compared with a conventional Cis monomeric phase as determined by selectivity differences between j9/m-terphenyl, j9/o-terphenyl, and coronene/phenanthro[3,4-c]phenanthrene solute pairs and was attributed to the localized high ligand density as constrained by the fi-CD platform structure. 

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. 

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. 

Fig. 9 Synthesis scheme for surface modification of silica. Formation of (A) a monomeric phase and (B) a polymeric phase. (From Ref. 112.)...

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... 

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 miniemulsion polymerization, the initiator can be either oil- or water-soluble. In the case of an oil-soluble initiator, the initiator is dissolved in the monomeric phase prior to miniemulsification. Then the reaction starts within the droplets. This is comparable to suspension polymerization where the initia-... 

Haeupke et al. found that gel and half-porous copolymers may become porous after being modified with a second monomeric phase containing an inert diluent. Such modified copolymers are called double porous copolymers (Haeupke et al. ) or hybrid copolymers (Barrett and Clemens ). 

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. 

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. 

Monomeric phases (10-15 gum thickness) They are obtained by reaction of an alkyl-monochlorosilane in the presence of an alkaline agent with... 

---