Hey,
I was under the impression that just sonicating a solution would not do very much in the way of degassing a solution. I thought you had to at least pull a vacumn on the solution while sonicating. Am I wrong on this. This simple question has started a long discussion.

Degassing a solution via sonication is not as effective as degassing plus vacuum. I have found through my experience that when I degas acetonitrile/H2O solutions that it works fine, but more active degassing is required with, say, methanol/H2O solutions. A great way of degassing is by helium sparging if you want to go that far.

I agree with HMichele. Degassing with sonication plus vacuum is effective, but...Degassing 50:50 methanol/H2O solutions is quite difficult and requires degassing for a long time. That might change the content of MeOH in your solution.
By the way...Does anybody know anything about degassing by sonication+vacuum versus filtration through 0.45 µm filter. Which is the better method?

Thomas (and the poster of this message). According to Dolan & Snyder (1989), in order of effectiveness for degassing, the best techniques from most effective to least effective are : Refluxing, helium degassing, vacuum degassing and ultrasonic degassing.

As for using filtration under vacuum, I would use the above order again as a guide. The "degassing" component of the filtration is still the vacuum. I personally use the filtration under vacuum in our lab as the bottled water (EM Science) quite often contains traces of particulates. I like it because it filters and degasses at the same time.

Mike, thanks for your comments, but I'm still a bit confused. If filtration of your sample isn't required, then I would guess that the combination of sonication AND vacuum should be better than the vacuum filtration (because this is, as you say, just vacuum). Has studies been done on this. If anybody has got knowledge of such study, I would be very happy

The only studies on degassing that I know (as mentioned above in the reference to our book) are:

Brown et al, J. Chromatogr, 204 (1981) p 115;
Bakalyar et al, J. Chromatogr. 158 (1978) p 277;
Snyder, J. Chromatogr. Sci, 21 (1983) p 65

These give reflux > helium sparge > vacuum > sonication as the effectiveness. Interpreting the data of Bakalyar and Brown makes me believe that degassing will be effective (from a bubble standpoint) if >50% of the gas is removed. Brown't chart shows that this doesn't happen with sonication alone. I know that lots of people use sonication and vacuum together (place the vacuum flask in a ultrasonic bath during degassing) with the claim that this is better than vacuum alone. I believe the preponderance of opinion on this, but I have not seen any hard data. If someone has a reference or data to share, we'd all appreciate it. Finally, remember that degassing is one of those processes that there aren't hard and fast rules for. That is, reliability in terms of bubble problems depend on the LC system design, so some systems are more sensitive to bubbles (or thoroughness of degassign) than others. Similarly, some analyses are more sensitive to dissolved air in terms of chromatographic problems. However, as I've said before and will likely say again, I have yet to meet an LC system that won't be more reliable with degassed solvents.

Something to remember when using Filter/Degassing: If you use 0.2 um or 0.45 um filter membranes for your HPLC water you will not have degassed correctly. This is due to the Bubble Point of water. For the degassing under vacuum you need a 1.0 or 1.5 um filter.

The folklore related to ultrasonic-degassing of mobile phases should not be a point of discussion any more. In 1988 J.Doehl (J.Chrom Sci. 26, pp 7-11 (1988)), reported very clearly that ultrasonicating does nothing in terms of degassing solvents. The effectiveness of methods was, Helium sparging first, vacuum degassing second.

Another bit of folklore is the higher solubility of atmospheric gases in polar solvents. This can be easily corrected reading J. Chrom. Vol 158, pp 277-293 (1978).

Folklore? Maybe, but the fact remains that when a mobile phase with 50:50 Meoh:Water + 25mM H3PO4 + 1 mM SDS is put under vacuum you can't (visually) see any effect.

If you then start your sonication you will see a lot of bubbles being formed in your solution, and THEN the vacuum will remove the bubbles.

As I said: It maybe folklore, but I'm still not convinced, that there's no effect of sonication!

Just put a carbonated beverage in a sonic bath and you'll quickly see what sonication can do. I haven't pulled the Doehl paper yet, but I'd wager there's some flaw in the experimental design if it showed that sonication does nothing!

Sonication definitely does something! We do research on CEC (capillary electrochromatography). If you read any research paper on CEC, you will know bubble formation is a big problem in CEC and thoroughly degassing solvent is critical for successful separations. Guess what we use for degassing the solvent? Sonication ONLY! There may be a small difference to the sonication degassing discussed in the previous messages,we don't use big reservoirs in CEC and only degas solvents in small (2-4 mL) glass or plastic vial. Newer sonicators has a degas option which works better than regular sonication mode.