Degassing-Filter Caps and Helium Sparging

The answer to the following reader question about degassing mobile phase solvents was supplied by LCGC’s “LC Troubleshooting” columnist John Dolan.

Q: Just how good of a job do degasser–filter caps do on degassing HPLC solvent? Where can I find a technique or protocol to degas solvent with helium?

John Dolan: I have not seen data on the effectiveness of the caps, but I suspect that if you use them with a vacuum filtration device, they should be similar to vacuum degassing, which usually is adequate. Helium sparging is simple — just use a sparging frit (a solvent reservoir frit is fine) on the helium line — and bubble a stream of helium (for example, use 5 psi pressure) through the solvent for 4–5 min. Usually this is enough to keep the solvent degassed for a day. In difficult cases, or where any oxygen causes problems (for example, with reductive electrochemical detection), continuous sparging may be necessary. Just turn down the flow to a trickle to keep the solvent degassed. Most popular today are the in-line vacuum degassing systems, which are quite effective, but they might not remove sufficient oxygen for oxygen-sensitive methods.