On-column injection employs the direct introduction of the liquid sample into the column. This is very useful for the analysis of thermally unstable samples and avoids boiling point discrimination. On-column injection requires special syringes with the following features:

Small outer diameter needles adjusted to the inner diameter of the column (0.17 mm needle for columns with 0.25 mm inner diameter, 32 gauge for columns with 0.25 mm inner diameter and 26 gauge for columns with a 0.53 mm inner diameter)
Longer needles tailored to pass through the injector and reach within the capillary column
Point style 2 adapted to the typical septum of the on-column injector
Small volumes of 5 or 10 µL