Bonded alkyl functional groups, HPLC

Bonded alkyl functional groups, HPLC development, 83... 

There is a wide diversity of available trade names, synthetic nuances, or geometric ones such as column size and shape. However, over three-fourths of HPLC is done with chromatographic columns prepared as porous silica derivatives with bonded alkyl functional groups. Other bonded groups that are noteworthy include diol functionality for aqueous size exclusion and bonded counterions for ion-exchange. The result is an increasingly well focussed interest in relatively few, but widely used, column types. It is known that nominally identical column types do not exhibit identical separation behavior in practice. This nonuniformity has provoked user interest in the question of the extent to which sorptive reproducibility occurs from column to column and, similarly, among different manufacturers. 

The ratio between the silanols and siloxane groups determines the more or less pronounced hydrophilic character of the phase. As with HPLC, it is possible to use bonded silica in which various chains are bound by covalent bonds to silanol groups on the surface. Some phases incorporate alkyl chains (RP-2, RP-8, RP-18), while others contain organic functional groups (nitrile, amine, or alcohol). 

Micropacked columns are available with most of the bonded-phase packings used in HPLC. Porous and non-porous silica particles are optionally functionalized with covalently ound silanes or other strongly adsorbed materials. ALkyl-bonded silicas produce separations, generally based on solute volatility, but with the potential for selectivity differences based on interaction with silanol groups. Underivatized silica is popular for petroleum separations of aliphatic and aromatic hydrocarbons. Silver-ion-containing silica columns are selective for olefin separations. Huoroalkyl-bonded... 

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