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Short answer
Degassing in HPLC means removing dissolved gas, mainly air, from the mobile phase before it reaches the pump and detector, so that bubbles do not form and disturb the baseline and the flow. In two LCGC columns by John Dolan, helium sparging removes about 80% of the dissolved gas, vacuum degassing about 60%, and sonication on its own is “not very effective”. For a system with an in-line degasser, Dolan writes that the in-line unit “should be all that you need to use”.
Definition and scope
Mobile phases such as water and organic solvents hold dissolved gas. That gas can come out of solution as bubbles in the flow path. Degassing is the preparation step, or the in-line function of the instrument, that lowers the dissolved-gas content before this happens.
The reasons given for doing it are practical. A training article from Lab Training (Dr. Deepak Bhanot, 2013, updated 2014) lists unstable baselines, spurious detector peaks, less precise pump flow and, through pressure build-up, possible pump damage as consequences of bubble formation. This page covers the common degassing approaches and what published evidence says about how well each works. It does not cover instrument-specific settings, and it is not a procedure.
How degassing is used in practice
Three approaches appear repeatedly in the sources: purging the solvent with helium (sparging), applying vacuum to the solvent, and exposing it to ultrasound (sonication). Modern systems often add a fourth, an in-line membrane degasser fitted in the flow path, which treats the solvent continuously on its way to the pump.
Dolan, writing in LCGC, treats the in-line degasser as the default for systems that have one, and sees a stand-alone step as unnecessary unless bubble problems persist despite the in-line unit. He also reports that in-line degassing performs better than static degassing of the solvent bottle. Where a bench step is used, the same column notes that vacuum degassing is most effective in the first 5 to 10 minutes and that leaving it under vacuum overnight is unlikely to improve things.
Boiling the solvent does remove gas thoroughly, but the Lab Training article advises against it because it also drives off volatile components of the mobile phase and so changes its composition.
Methods compared
The figures below come from two LCGC International columns by John Dolan: “Degassing Mobile Phase Solvents” (6 December 2010) and “Degassing Water” (9 July 2010). The second column cites a 1981 study for the reflux, helium and vacuum figures. They are columnist-reported values, not manufacturer specifications.
| Method | Reported removal of dissolved gas | What the source says |
|---|---|---|
| Refluxing (boiling) | 100% (1981 study, cited in “Degassing Water”) | Thorough, but Lab Training advises against it because volatile solvent components are lost. |
| Helium sparging | About 80% | Called the “gold standard” in both columns. |
| Vacuum degassing | About 60% (2010-12 column); 60 to 70% (1981 study, as cited) | “Usually enough for most applications”; most effective in the first 5 to 10 minutes. |
| Sonication alone | About 30% (Lab Training) | Only the Degassing Mobile Phase Solvents column calls it “not very effective”. |
| Sonication plus vacuum | No figure given | Common wisdom says better than vacuum alone, but Dolan has “never seen any data on this”. |
| In-line membrane degasser | No figure given | Developers say it is equally effective as helium sparging for preventing bubbles in practice. |
One further point from the same author: he treats removal of more than about 50% of the dissolved gas as satisfactory, which is why a method that reaches 60% can be described as usually enough.
Where it matters for labs and clinics
Degassing matters most when bubbles show up as symptoms: a baseline that steps or spikes, pump flow that is less steady, or detector peaks that do not correspond to sample components. The CASRAI HPLC guide covers how to tell an air-bubble pattern from thermal or detector drift, and the ghost peaks guide covers peaks that appear without a corresponding sample component. When the symptom points to bubbles, the degasser is one of the places to look, alongside the rest of the flow path; the preventive maintenance schedule by module lists the modules.
For mobile-phase preparation in gradient work, see HPLC gradient elution design and transfer. For a comparison of instrument classes, see HPLC vs UPLC vs UHPLC. Sonication as a general laboratory tool is described in What is a sonicator.
Common misunderstandings
Sonication alone is enough
Dolan’s Degassing Mobile Phase Solvents column calls sonication alone “not very effective”, and Lab Training puts it at only about 30% of dissolved air, the least of the common methods. Dolan has also seen no hard data that sonication improves vacuum degassing.
Longer vacuum is better
According to Dolan, vacuum degassing works mostly in the first 5 to 10 minutes. An overnight vacuum is unlikely to improve things.
Sonication plus vacuum is proven to beat vacuum alone
This is common wisdom, not a demonstrated result. The column that mentions it states the author has not seen data on the combination.
You always need a stand-alone degassing step
For a system with an in-line degasser, Dolan says the in-line unit should be all that is needed, and a separate step is only worth adding if bubble problems continue.
FAQ
Why does HPLC mobile phase need to be degassed?
Dissolved gas can form bubbles in the flow path. Lab Training lists unstable baselines, spurious detector peaks, less precise pump flow and possible pump damage as the consequences.
Which degassing method removes the most gas?
Among the practical methods in Dolan’s columns, helium sparging is the best, at about 80%, ahead of vacuum at about 60%. Refluxing reached 100% in the 1981 study he cites but is not advised for mobile phases.
Does an in-line degasser replace helium sparging?
Developers of membrane degassers say the two are equally effective in practice at preventing bubbles, according to Dolan, who adds that the in-line unit should be all that you need.
Sources
- John Dolan, “Degassing Mobile Phase Solvents”, LC Troubleshooting, LCGC International, 6 December 2010. https://www.chromatographyonline.com/view/degassing-mobile-phase-solvents (checked 29 October 2026).
- John Dolan, “Degassing Water”, LCGC International, 9 July 2010. https://www.chromatographyonline.com/view/degassing-water (checked 29 October 2026).
- Dr. Deepak Bhanot, “Why is Degassing of HPLC Mobile Phase Necessary?”, Lab Training, 18 November 2013, updated 17 July 2014. https://lab-training.com/why-is-degassing-of-hplc-mobile-phase-necessary/ (checked 29 October 2026).
