Mobile-Phase Degassing: What, Why, and How

Jul 01, 2014
Volume 27, Issue 7

Why should you be concerned about mobile-phase degassing — it's all done automatically, isn't it?

Degassing of the mobile phase in liquid chromatography (LC) applications is a topic that is seldom talked about today. However, this was not the case in the past. For the first 30 years or so of LC use, problems related to mobile-phase outgassing were some of the most common problems encountered in routine use. This was true in user surveys conducted by LCGC as well as by my informal polls based on readers' questions and direct interactions with users. Over the past 15–20 years, on-line degassers have moved from a novelty device to a standard part of most LC systems. As a result, I suspect that many users have never encountered problems related to degassing. I dropped an in-depth discussion of degassing from my popular LC troubleshooting class, but in a recent class the topic came up again. Although in-line degassing helps us avoid most solvent out-gassing problems, it does not solve all problems related to dissolved air in the mobile phase. In this month's instalment of "LC Troubleshooting" I would like to review what degassing is all about. What is it? Why do we need it? How is it accomplished? Are there times when we should be especially watchful for problems related to it?

Why Degas the Mobile Phase?

Figure 1: Solubility of oxygen in ethanol (dashed line); saturation concentration of oxygen in the mixture (solid line). See text for details. Adapted from reference 1.
When solvents are in contact with the atmosphere, air gradually dissolves into the solvent. Air, of course, is primarily nitrogen and oxygen. In reversed-phase LC, the most common solvents are water or buffer as the A-solvent and acetonitrile or methanol as the B-solvent. When the aqueous and organic solvents are mixed, they each contribute to the total dissolved air content of the mixture. This is illustrated by the dashed line in Figure 1 for the solubility of oxygen in mixtures of water and ethanol (which behaves in a similar manner for oxygen and nitrogen with methanol and acetonitrile for the present discussion). Whether we are making isocratic (constant %B) mixtures or gradients, the amount of gas in solution is in proportion to the respective solvent volumes. The problem with this situation is that the solubility of air in the mixture is less than that of individual components. This is shown as the solid curve in Figure 1. When such a situation exists, the solution is supersaturated with air, generating an unstable condition in which air will bubble out, or outgas, from the solution.

If mobile-phase outgassing occurs within the LC system, the most common problem areas are the pumps and detector. At the extreme, air in the pump will cause the pump to stop delivering mobile phase to the column. If only an occasional bubble is present, the flow rate will be erratic, causing retention time problems. Air in an optical detector, such as an ultraviolet (UV), fluorescence, or refractive index detector, will scatter light passing through the flow cell, causing noise spikes in the chromatogram or an off-scale signal. These problems can be eliminated if the air is removed from the mobile phase.

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