The retention gap technique in combination with on-column injection allows concentration of a large sample volume in the capillary column. Retention gaps consist of uncoated (no stationary phase), deactivated fused silica capillary tubing. A retention gap must be inert without any noticeable retention (analytes are not retained). Phenylmethyl (low-medium polar) deactivated tubing as an additional choice of surface deactivation. Improve peak shape by focusing and concentration of the sample components when introducing a large sample volume onto the column.