Degassing

Dissolved gas can be removed from solution by:

Bubbling with helium - discontinued.
Sonication - discontinued.
Vacuum filtration

Nowadays, an online degassing kit does the job.

Figure : Degassing

If the mobile phase is not degassed, air bubbles can form in the high-pressure system resulting in problems with system instability, spurious baseline peaks etc.

Refer to: HPLC > Part 3 Application > Degassing flat film degasser.