Causes and Methods of Mobile Phase Degassing

Mobile phase solutions often form bubbles due to dissolved oxygen or air contamination. These bubbles, upon entering the detector, appear as sharp noise peaks on the chromatogram. Small bubbles gradually aggregate into larger bubbles, which, when they enter the flow path or column, slow down or destabilize the mobile phase flow, causing baseline fluctuations. Once bubbles enter the column, removing them is time-consuming. In fluorescence detection, dissolved oxygen can also quench fluorescence. Dissolved gases can also cause oxidation of some samples or alter the pH of the solution.

Currently, offline ultrasonic degassing, online inert gas bubbling degassing, and online vacuum degassing are commonly used for mobile phase degassing in liquid chromatography.

Ultrasonic degassing: The prepared mobile phase container is placed in an ultrasonic water bath for 10-20 minutes for degassing. This method is relatively simple and generally meets the requirements of routine analytical operations, therefore, it remains widely used.

Inert gas bubbling degassing: Helium gas from a gas source (cylinder) is slowly and evenly introduced into the mobile phase in the storage tank. Helium molecules displace and eliminate other gas molecules, and since helium itself has very low solubility in the solvent, the small bubbles formed by trace amounts of helium are negligible for detection.

Vacuum degassing device: The mobile phase is passed through a tubing made of porous synthetic resin membrane. A vacuum container surrounds this tubing. When the vacuum pump operates, the pressure outside the membrane is reduced, allowing low molecular weight gases such as oxygen, nitrogen, and carbon dioxide to pass through and be removed. Typical vacuum degassing devices have multiple flow paths, allowing for the simultaneous degassing of multiple solutions.