The interior surface of silica capillary is pristine when it comes from the initial draw process.  It is considered to be inert as compared to other glasses, such as borosilicate and quartz, due to the low level of impurities. A list of impurities and their typical abundance can be found in the Appendix. During the manufacturing process, the silica has been heated to a temperature at which its viscosity allows it to be drawn down to the specified dimensions, typically in excess of 1,800ºC . It is well understood that after exposure to these elevated temperatures, the surface of the capillary is devoid of both chemically and physically absorbed water. The surface is rich in siloxane bridges (Si-O-Si) and the residual silanol groups (Si-OH) present are isolated. In many applications, the initial conditioning of the surface before use is extremely important and should be given due consideration.  Development of a protocol that achieves the desired surface chemistry is well advised.

Common terms used when discussing conditioning of capillary are surface activity and deactivation. These words have different meanings depending on the application and the background of the user. Some would say that the drawn capillary has high surface activity because it readily incorporates water back into the surface structure. It follows that a fully hydroxylated surface would be deactivated.

In GC column production, surfaces are sometimes said to be “too active”, and steps are taken to address this during internal coating operations. Some refer to these initial steps as deactivation. In other instances, such as in the case of capillary column connectors for GC, deactivation refers to the addition of covalently bound ligands to the surface to reduce analyte adsorption.

In the case of CE, surfaces are treated to generate a uniform silanol population, a process some refer to as activation, or more commonly as conditioning. Deactivation in CE is accomplished by the addition of coatings and dynamic additives that eliminate EOF or unwanted analyte adsorption. Hewlett Packard (HP), in High Performance Capillary Electrophoresis, suggests that “The most reproducible conditions are encountered when no conditioning other than with buffer is employed. However, adsorption of sample to the surface and changes in EOF often do not allow this.” The author further suggests “Base conditioning to remove adsorbates and refresh the surface by deprotonation of the silanol groups is most commonly employed. A typical wash method includes flushing a new capillary with 1N NaOH, followed by 0.1N NaOH and then buffer”.⁶