degassit or solvac?

[dougs]

Hello,

I'm having some problems with my degassit. I'm considering switching to relying on vacuum + membrane filter (like the solvac system) for degassing my mobile phase solvents. I would like to hear some opinions on switching from the degassit to membrane filtration.

thanks
DS

[Mark Tracy]

A sonic bath and a mild vacuum is quick and effective. My feelings on filtration is that if you need it, you must do it, but if you don't need it, it is just one more apparatus to get contaminated. The one drawback with all bulk degassing schemes is that over several hours, an unprotected bottle will re-absorb air. You will need 2-5 psi of helium in a sealed bottle to keep air out. The cost of the special bottle caps, reguators and helium cylinders might cover the repair cost of your degassit.

[tom jupille]

My 2¢ worth is that vacuum or vacuum+ultrasonic degassing is OK for "premixed" isocratic mobile phases only, not for "in-line mixing" or gradient mobile phases (I'm assuming reversed-phase chromatography here).

The reason is that the solubility of air in MeOH/water or ACN/water mixtures is a non-linear function of composition; air is less soluble in the mixture than you would expect from the average of the solubilities in the pure solvents. Hence, air bubbles tend to form when the solvents are mixed. If you are premixing your mobile phase, then you let the bubbles form and pull the exess air out with mild vacuum; having the mixed mobile phase re-equilibrate with the atmosphere from then on is no problem.

With solvents for on-line mixing, the problem is different. First of all, you have to get the dissolved air level in each solvent down low enough to guarantee that you will be below the solubility in the final mixture. Second, as Mark pointed out, you need to keep the solvent from re-equilibrating with the atmosphere. Some form of continuous degassing (either your degassit or helium) is required for this.

[Mark Tracy]

Based on some gas solubility data I found at NIST, <30% saturation will stay in solution for all compositions of water/methanol at room temperature and pressure. This is confirmed by the specifications on Rheodyne's new online degasser, >70% removal of nitrogen from methanol at 2mL/min. I don't have data for acetonitrile, but it should not be greatly different.

[dougs]

Since I am doing online mixing, perhaps it is best to stay with a degassit. Are there any suggestions on which ones to look for? Mark, I'm looking into the Dionex...

Doug

[Mark Tracy]

Since you ask, I'll give the commercial. We have the SOR-100A solvent organizer with 0, 2, 4, or 6 channels of vacuum-degassing built into the base. It uses the latest Teflon AF technology, so the holdup volume is only 2 mL. It is nice and quiet, too. I've been using a 6-channel model for the past year and it has been very good to me. (Given how finicky the Summit pump is about bubbles, I need a good one!) It is powered by the Summit pump, but if you don't have that model, there is an optional transformer.

Oh, yes... We have found that if you have a filter in the reservoir, it can get contaminated, or be too restrictive. Either of these can mimic the effect of a bad degasser.