I would like to have your opinion about the advantages of theese type of deggasing

At my company we have both: an older system with Helium and Alliance 2690 with vacuum membranes.

We have got a lot of trouble with our membranes but it was just a supplier problem, Waters has got now newer Degasser chambers, so the troubles are solved now. When I have to choose, I would choose for membranes, because you really degas, with helium you just substitute the oxygen for helium.

No question - He sparging is the best degasssing method there is.

Membrane degassers are, IMHO, third-best after ultrasonication. If Helium isn't available, try ultrasonication, you won't be disappointed.

S.K. Srinivas

Reply to A. Anonymous,

The reason helium is used is because it has much less solubility in water and solvents than air. Yes, air is replaced by helium, but because helium has much less solubility in liquids, the amount of helium in a helium saturated solution is normally insignificant to cause pumping problems.

Keep in mind, if degassing is simply being used to avoid pumping problems casue by out gassing during gradient formation, then it is not necessary to remove every molecule of gas present in the mobile phase. All the approaches work adequately and what is "best" is mainly a matter of convenience and reliability.

Sparging with He is a pain. Of course, you have to have a tank of He and the bits to go with it (regulator, tubing, chained to the wall). As well with mixed solvents, the constant flow of gas changes the composition of the mixture. And finally, the tanks need to be changed out on a regular basis. Personally, I would choose membrane degassing, hhok it up and turn it on. The are disadvantages to the membrane methods. The initial capital cost can be significant, the have an internal volume which needs to be flushed when changing solvents. All chambers (in multi chamber systems) need to have solvent in them at all times. If an unused chamber goes dry, the degassing effiency of all chambers suffers. Which is best or worst? No method is best or worst, all have advantages and disadvantages.

Vacuum degassing in the ultrasonic bath - half a minute - done - no problems.

Vacuum degassing in the ultrasonic bath - half a minute - done - no problems.

Wait a minute, as soon as you are done, the solvent begins to re-gas. This re-gassing process may or may not affect results depending on how long a given volume of solvent is used. As well, some hardware is much more affected by outgassing than others. I have studies that show a degradation of performance within a couple of hours of solvent prep using only vacuum degassing/ultrasonic bath (using a low pressure, gradient proportioning system). I also have data that shows no performance degradation using a high pressure mixing system 5 days after solvent prep. It show also be pointed out that some autosamplers benefit from solvents that have a constant level of degassing. This is due to the changes in compressibility as the solvent regasses.

If you stir, you get air.
No stir - no air - no problem.
:-)

We use the Gilson 321 pumps which mix two
solvents in the high pressure region so there
is no need to de-gas.

Gas problems are also highly dependant on the mobile phase. Generally, normal phase solvents have much fewer gas problems.

I agree actually with all of the statements made. If you are using premixed isocratic mobile phases, a simple one-minute degassing in the ultrasonic bath is more than sufficient. This is the way I degassed my mobile phase in the old times. If you use a high-pressure gradient system, this is all that you need to do. Of course, you do not want to stir in any more air.
With low-pressure mixing, you can still run into problems even if you have degassed all the components of your mobile phase. In addition, the modern on-line degassers are rather convenient.