Offline vs. Online Degassing of HPLC Solvents

Gas bubbles and outgassing can significantly affect HPLC performance. Outgassing occurs when the dissolved gas concentration exceeds the solvent’s absorption capacity. Variables such as temperature, pressure, and solvent composition all influence gas solubility. Rough internal surfaces within the HPLC system can also act as nucleation sites for bubble formation. These bubbles can cause retention time shifts, unstable baselines, inconsistent peak areas, and even pump failures—reducing both accuracy and reproducibility.

Offline vs. Online Degassing of HPLC Solvents

Offline Degassing

Offline degassing is performed before the mobile phase enters the HPLC system. The most common method is helium sparging, where helium gas is bubbled through the solvent to displace dissolved air. While effective, offline degassing allows the solvent to gradually reabsorb gases once exposed to air, reducing its long-term stability.

To protect against contamination, use a solvent inlet filter—typically a 2 µm filter—to prevent particulates from entering the reservoir and improve sparging efficiency. Maintaining slight system back pressure helps prevent bubbles from reforming at atmospheric pressure. Installing a back pressure regulator after the detector minimizes bubble formation and ensures consistent flow (check your detector’s pressure rating before installation).

Online Degassing

Online degassing continuously removes gases during operation, eliminating reabsorption and ensuring consistent solvent quality. The most common technique uses a vacuum degasser integrated into the system. The mobile phase flows through a gas-permeable membrane where dissolved gases such as oxygen and nitrogen are extracted under reduced pressure. This results in stable baselines, consistent retention times, and improved reproducibility.

As with offline methods, bubbles can still form when pressure drops after the column. Installing a back pressure regulator downstream of the detector minimizes this issue and helps maintain steady flow through the cell.

Choosing the Right Degassing Method

Both offline and online degassing methods improve HPLC reliability, but their applications differ:

• Offline degassing methods (such as helium sparging, ultrasonication, or vacuum treatment) are simple and inexpensive but allow for reabsorption once solvents are exposed to air.
• Online degassing with a vacuum membrane device offers continuous, maintenance-free gas removal—making it the industry standard for modern HPLC systems.

For long analytical runs or gradient methods, online degassing provides the most reliable performance. If your current system lacks one, Chrom Tech can help integrate a high-quality vacuum degasser to improve your analytical precision and instrument stability.

Finally, remember that HPLC solvents are considered hazardous waste under FDA and EPA regulations. Always collect used solvents in approved solvent waste containers to ensure safe and compliant disposal.