Before pumping the freshly prepared mobile phase through the HPLC system, it should be thoroughly degassed to remove all dissolved gasses. Offline degassing can be performed via:
Bubbling with helium
Sonication
Vacuum filtration
Additionally, modern HPLC pumps have an integrated module for online degassing. If the mobile phase is not or incomplete degassed, air bubbles can form in the system resulting in problems like for example system instability or spurious baseline peaks.