What is Mobile Phase Degassing?

Solvents equilibrate with atmospheric gases in the laboratory. Henry’s law states that ‘the mass of a gas which will dissolve into a solution is directly proportional to the partial pressure of that gas above the solution’ and describes the solubility of a gas in a liquid. When the solvents are mixed, the solubility of air is less than it is in same proportion of pure solvents and excess air will tend to come out of solution (bubble out). Outgassing is the term used to describe dissolved gases coming out of solution.

This phenomenon can also occur in a HPLC system where rough surfaces produce nucleation sites for bubble formation. Air bubbles can also modify the flow of mobile phase through the column due to the creation of dead volumes. In HPLC analysis the problems produced by bubble formation can largely be prevented, by degassing the mobile phase.

Outgassing in HPLC Systems

Bubble formation can cause a number of problems depending on the mixing system.

• Low Pressure Mixing: Low-pressure mixing systems are more prone to bubble formation. The solvents used for the mobile phase are mixed before the pump. Therefore, the mixed mobile phase is in a supersaturated condition before it gets to the pump. Outgassing is promoted if the system undergoes any slight drop in pressure or if sites of nucleation are encountered. Pressure drops can happen if there are fluctuations in the pumping rate; and nucleation sites can include tube ends and other non-smooth surfaces. If the mobile phase outgases and bubbles are formed, this can lead to pumping problems, which may lead to pump failure.
• High Pressure Mixing: In high pressure mixing, each solvent has its own pump and mixing takes place after the pumps. Because the solvent is pumped at atmospheric pressure, no outgassing at the pump is likely to occur. Mixing takes place at high pressure, and any dissolved gas will remain in solution until the pressure drops. This happens after the mobile phase leaves the column. Therefore any outgassing happens after the column, but before the mobile phase enters the detector. In this instance, outgassing can cause detection errors and spurious peaks can be shown on the chromatogram.

Degassing the Mobile Phase

The solution to the problem of outgassing is to remove the gas from the mobile phase solvents before they are used. It is only necessary to remove a fraction of the dissolved air to bring it below the supersaturation level in the mobile phase. Two of the most widely used methods to degas a mobile phase are:

• Helium purging: helium is bubbled through the solvent and removes up to 80% of dissolved air
• Vacuum degassing: the solvent is exposed to a vacuum and the reduced pressure removes more than 60% of the dissolved air
• Sonication: ultrasonic baths as a stand-alone technique only removes up to 30% dissolved air

It is recommended that sonication is used in combination with one of the other techniques. For practical purposes, it is advisable to degas the mobile phase in both low and high pressure mixing systems.

A simple set-up for carrying out helium purging is shown in the article: Introducing the Source for High Quality Chromatography Glassware.