Background electrolyte additives for capillary electrophoresis of biomolecules

Capillary electromigration techniques can be performed by a variety of modes, based on different separation mechanisms that, in most cases, can be selected by simply changing the operational conditions, specifically the composition of the electrolyte solution (BGE). In a multiplicity of separation modes, the BGE consists of a buffered aqueous solution, which may contain one or more no-buffering additives, either neutral or charged, and can be mixed with an organic solvent. This communication discusses the influence of a variety of BGE additives that we have employed for the separation of biomolecules in capillary electrophoresis, using either uncoated or coated fused-silica capillaries. The investigated additives include buffering agents suitable to control the protonic equilibrium in a wide pH range and compounds effective at reducing untoward interactions of the analytes with a variety of active sites on the inner surface of bare fused-silica capillaries, which comprise inert siloxane bridges and ionisable silanol groups. Most of the compounds employed for this purpose act either as masking or competing agents for the active interacting sites on the inner wall of the bare fused-silica capillary, so that they are not accessible to the analytes. Others are expected to functioning as strong ion-pairing or competing agents for the interacting moieties of the analytes exposed to the BGE, subtracting their availability to the interacting sites on the capillary wall. Zwitterionic, anionic or cationic ion-pairing agents, as well as certain ionic liquids and organic solvents, have also been employed to affecting the electrophoretic mobility of analytes and, therefore, their selective separation. Also discussed is the influence of the different BGE additives on the generation of the electric double layer at the interface between the capillary wall and the electrolyte solution, determining direction and velocity of the electroosmotic flow, either in bare or coated fused silica capillaries.