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). Methyl deactivated surface eliminates the unwanted interaction. Improve peak shape by focusing and concentration of the sample components when introducing a large sample volume onto the column.