The retention gap technique in combination with on-column injection allows concentration of a large sample volume in the capillary column. The retention gap depends on the solvent used:

• Me-Sil retention gap is only for use with n-hexane and diethyl ether
• Phe-Sil retention gap is for all solvents except methanol and water
• CW retention gap is for all solvents and especially for methanol and water

A retention gap must be inert without any noticeable retention and it can also be used as transfer lines or precolumns. Me-Sil retention gaps are more inert than Phe-Sil, while Phe-Sil is less susceptible to contamination. Maximum temperatures for CW retention gaps 250 °C, for Me-Sil and Phe-Sil retention gaps 320 °C.

Deactivated capillary columns are recommended for preparation of capillary columns as precolumns, whenever a larger contamination capacity is required.
Untreated capillaries are recommended for capillary electrophoresis, for the preparation of capillary columns and capillary LC applications.