Alkyl groups, bonded, HPLC

Adsorption surface, SFC, solvent Interactions, 1>>9 Air monitors. Industrial chromatography, 200 Alditol acetates, chromatogram, 32f Alkyl groups, bonded, HPLC development, 82,83 Amides, enantiomeric, separation, >l3,>l>lf Amine compounds, RPLC, 8>l Amino acids HPLC, 86,87f Ion exchange, 81 ligand exchange, >>... 

The nomenclature of the RP is not consequent. The RP most often used contains octyl (RP C8) or octadecyl (RP C18) groups. There is no differentiation even when two methyl groups are introduced additionally with the silane (as with monofunctional silanes) or only one (difunctional) or none (trifunctional silane). Some manufacturer use silanes with bulky side groups (e.g., isopropyl groups) to improve the hydrolytic stability of the bonded phases, but here also, only the longest alkyl group is used in nomenclature. RP C8 and RP C18 are the work horses in HPLC. Shorter chains (RP4) are used in protein separations, and special selectivity can be obtained with bonded phenyl, cyano, amino or fluoro groups. 

Besides classical RPs with alkyl groups ranging from C4, frequently applied in peptide separations, to the work horses in HPLC C8 and C18, and up to C30 for the separation of long chain analytes, like carotinoids, many other organo silanes have been bonded to silica and used in HPLC. 

The sorbents most frequently chosen for current use are the porous silica derivatives. Most popular are the hydrophobic or "reverse phase" sorbents with bonded alkyl groups. These are ideally suited for the partly aqueous carrier phases that are suitable for separations related to the life sciences, an area that accounts for most of the current use of HPLC. 

Similar to other coupled methods of polymer HPLC, for example, LC CC (Section 16.5.2), the choice of the column packing and the mobile phase components for EG-LC depends on the retention mechanism to be used. Adsorption is preferred for polar polymers applying polar column packings, usually bare silica or silica bonded with the polar groups. The eluent strength controls polymer retention (Sections 16.3.2 and 16.3.5). The enthalpic partition is the retention mechanism of choice for the non polar polymers or polymers of low polarity. In this case, similar to the phase separation mechanism, mainly the solvent quality governs the extent of retention (Sections 16.2.2, 16.3.3, and 16.3.7). It is to be reminded that even the nonpolar polymers such as poly(butadiene) may adsorb on the surface of bare silica gel from the very weak mobile phases and vice versa, the polymers of medium polarity such as poly(methyl methacrylate) can be retained from their poor solvents (eluents) due to enthalpic partition within the nonpolar alkyl-bonded phases. 

The silica gel-based column packings are the active materials of choice for polymer HPLC employing both exclusion and interaction retention mechanisms. These are either bare or bonded with various groups. C-18 alkyls and -CH2-CH2-CH2-NH2 groups are most popular for reversed-phase and normal-phase procedures of polymer HPLC employing the nonpolar and polar interactions, respectively. 

---