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 installment 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?
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.
The data of Figure 1 suggest that to avoid mobile-phase outgassing we do not have to remove all the dissolved gas from solution, but only reduce the total amount of dissolved gas so that a plot for the resulting mixture would result in a line falling below the solid line in Figure 1 (1). If half of the dissolved gas is removed, we should be safe. One simple way to do this is to use a vacuum to degas the solvents. This can be done easily by placing the solvent in a vacuum flask and pulling a partial vacuum with a water aspirator or mechanical pump of similar capacity. To help facilitate the process, a stir bar or a few (clean!) boiling stones can be added to the flask. Although I don't have quantitative data on this, many workers find that sonicating the solution while vacuum degassing seems to be more effective than vacuum alone. In any event, a few minutes of vacuum degassing will remove about 60–70% of the dissolved gas (2). Sonication alone will only remove 20–25% of the gas (2), which is insufficient to avoid outgassing with most LC systems. For some LC systems, the amount of vacuum applied while filtering the mobile phase may remove enough gas to avoid problems. The most effective way to degas the solvents is to bubble helium through the mobile phase by sparging for a few minutes, which removes approximately 80% of the dissolved air (2). It seems contradictory to use a gas to degas a solution, but the solubility characteristics of helium in the mobile phase are such that outgassing is not a problem. Helium sparging was widely used for degassing, and although it is used less today because of the ease of in-line vacuum degassing and decreased availability of helium, it is still the most effective degassing technique.
Until the late 1970s, all LC systems either were run with premixed mobile phases in the isocratic mode or used high-pressure mixing to generate a gradient by mixing the mobile-phase components after the pumps. With these systems, the premixed mobile phase was degassed before use or the individual solvents were degassed for gradient applications. On-line mixing of isocratic mobile phases was also possible if the solvents were degassed first. The advantage of high-pressure mixing is that the pumps only pump degassed solvents or mobile-phase mixtures, and because the solvents were mixed under pressure, any bubbles that might otherwise form at atmospheric pressure would stay in solution because of the elevated pressure of the system. Often, care had to be taken to provide a small back pressure (for example, 50 psi) on the outlet of the detector to keep the mobile phase from outgassing in the detector cell.
In the late 1970s, Spectra-Physics (now a part of Thermo Fisher Scientific) introduced an LC system that incorporated low-pressure mixing. Solvents were blended in a proportioning manifold before they reached the pump, a technique that is used in low-pressure mixing systems offered by most LC manufacturers today. When low-pressure mixing is used, the solvents are mixed at atmospheric pressure (or sometimes a bit below atmospheric pressure), so outgassing of the mobile phase is a huge problem. The Spectra-Physics system included a built-in helium sparging system to degas the solvents before mixing, so outgassing was avoided. Although Spectra-Physics had a patent on helium sparging (3), which discouraged other manufacturers from offering the same technology, it was hard to prevent individual users from constructing their own helium sparging apparatus for personal use, so the technique became popular.