help : How can i degassed the solvent by helium sparging

[Newchromatographer]

what is helium sparging ,,, how can i use it to degass my solvent to get it free from air

[tom jupille]

Bubbling a stream of helium through your mobile phase. Helium has very low solubility, so dissolved air diffuses into the helium bubbles and are swept out of the system, while very little helium diffuses into the system.

If you are using a recent instrument that incorporates a built-in degasser, helium sparge is superfluous.

The apparatus is simple: just hook up a sinker frit (similar to the one on the inlet tube) to a helium source. Bubble helium through the liquid at a good rolling bubble for about a minute or so, then turn down the helium flow so that there is only a fine stream of bubbles coming from the frit.

[Newchromatographer]

Tom ,

is sonication at 20 min will do the same job ? because i read in the trouble shooting of removing air from the solvent is to use helium but i have in line degasser in shimadzue system and i do sonication before ?


thanks Tom for your help i really appreciate that.

[qcChemist]

Sonication will do the same job, but be aware that sonicating something that long will heat up your mobile phase, which could cause some peak shift and resolution issues. He's right, most modern LC systems have inline degassers that eliminate the need for sparging.

[tom jupille]

The degassing requirements depend quite a bit on whether your are "pre-mixing" an isocratic mobile phase or using "on-line mixing" in a gradient-capable system.

The problem arises from the fact that the solubility of air in mixtures of polar organic solvents + water is a non-linear function of composition: the solubility of air in a 50/50 mixture of water/methanol is substantially less than the average of the solubilities in the pure solvents. Of course, when you mix the two, the amount of dissolved air *is* the average, and that excess comes out at bubbles.

If you are pre-mixing the mobile phase, you let the bubbles form and then get rid of them. At that point, everything is at equilibrium. Sonication provides nucleation centers to let the bubbles form efficiently; it is often used in conjunction with a slight vacuum (e.g., from a water aspirator).

If you are mixing "on-line" (i.e., inside the system), then those bubbles form inside the system. The only option is to decrease the dissolved air content in the pure solvents. Sonication cannot take the dissolved air below its equilibrium level. And even if it could, when you stopped sonicating, the solvent would begin to re-equilibrate with the atmosphere. Helium sparge or the in-line membrane degassers are the most effective ways to get the dissolved air low enough to prevent bubble formation when the solvents mix.

As I indicated in my initial reply, if you have an in-line degasser (and it is functioning properly!), then helium sparge is superfluous.

[danko]

Unfortunately, the on-line degassers do not remove more than 50 – 60 % (at their best) of the air dissolved in the pure liquids (e.g. water, ACN, Methanol etc.) and that means that bubbles will form upon in-line mixing of two or more liquids. One has to remember that the on-line degasser is installed prior to the mixing chamber.

Another story is, that sometimes (actually often) bubbles are formed long before the liquids reach the degasser (in the lines between the eluents’ reservoirs and the degasser) and when they (the bubbles) become large enough they create air pocket/s in the line/s. In such cases no liquid will come through and the flow will be obstructed.

So, the safest routines in that context are initial degassing (helium sparging or whatever) and most importantly pre-mixing.
If the above is observed, suddenly the on-line degasser gets superfluous. The latter could give a number of advantages – e.g. reduced price for the system, faster primes (with new eluents) etc.

Best Regards

[lmh]

Hm, I'm a big fan of online degassers. Priming pumps has never been a major bottle-neck in our lab, and we don't seem to waste a lot of time over gas bubbles appearing in the system after the degasser, either. I'm not sure what others' experiences are?

On sonication: I have no doubt it works if carried out correctly, but have a feeling that many people have wasted much sonicator time with feeble sonicators that don't do the job. A few years ago, frustrated by the noise of a sonicating water-bath a friend used to degas his solvents, I put a hand-held dissolved oxygen meter in a bottle of water that was degassing, before and after. Before: 100%. After: 99%. OK, oxygen is a minor gas compared to nitrogen, but clearly not much was removed by 30 min in this particular sonicating waterbath...

[Consumer Products Guy]

Hm, I'm a big fan of online degassers.

Same here. And I've been around long enough to have hands-on experience degassing with:
membrane filtration
ultrasonic
heat/vacuum
helium sparging

[danko]

Did I overlook the arguments?

Best Regards

[tom jupille]

Unfortunately, the on-line degassers do not remove more than 50 – 60 % (at their best) of the air dissolved in the pure liquids (e.g. water, ACN, Methanol etc.)

danko, do you have any more information or a reference on the effectiveness of the membrane degassers relative to other techniques? All I can find (on an admittedly cursory search) is a figure in Dolan & Snyder's "Troubleshooting LC Systems" book (page 147) taken from a 1981 paper that shows He sparge removing about 80% of dissolved oxygen from methanol, and an anecdotal statement in the new edition of "Introduction to Modern Liquid Chromatography" (page 96) that membrane degassing is "not quite as effective as helium sparging".

In any case, the thing to remember is that it's not necessary to remove *all* the dissolved air, merely to get it down to the point that the average concentration in the mixed mobile phase is lower than than its solubility in that mobile phase.

[danko]

Hi Tom,

I don’t have the exact references for the mentioned figures on hand. I just remembered that number after reading some texts touching the topic. If it really is that important I’ll try to find a couple of references when my spare time allows it. My memory fails me seldom, so if there were an error in my statement, due to memory failure, it would be a marginal one.
Another matter that I remember even better, is the conclusion of several studies regarding the effect of different methods of degassing and that is: The most effective method for removing dissolved air is helium sparging, followed by temperature increase, then lowering the pressure and finally sonication. So, 50 – 60 % removed air by lowering the pressure is quite consistent with the latter statement.
If the above reflections sound too inaccurate for this particular purpose, I suppose more concrete figures and conclusions can be found, but I would be surprised if these deviated significantly from what I offered from the top of my head.
Otherwise I would agree with you that the need for complete removal of the dissolved air is not desirable at any cost, but in my experience; separately degassed ACN and water will release considerable amount of air bubbles upon mixing. So the better degassing prior to the mixing step, the more certain is the prevention of air bubbles released in the mixing chamber.
The latter is to underline the importance of the premixing routine.

Having discussed all that the remaining fact is that air bubbles can be (will be) found in lines, reservoirs and sinker frits long before the flow-path reaches the degasser. And then you’re paying for nothing (with regards to the degasser investment).

Another matter should be mentioned in this context and that is the sensitivity reduction due to inadequate degassing of the mobile phase. So if one doesn’t like noisy baseline, an extra fine degassing would be advisable.

Best Regards

[HW Mueller]

We have been through this several times. Some years ago someone gave the figures on He solubility in H2O (if I remeber correctly; maybe Tom can find this, I am not so fond of the search function). The concentration was higher than I expected, even though it agreed with my experiments which showed that He saturated water only dispersed light in my detectors slightly less than if saturated with N2. If these two gases were very vigorously bubbled through the water, I got a horribly noisy, spiky baseline for both, considerably worse for N2. My conclusion was that He sparging should be called a regassing, not degassing, and that He has a lesser tendecy to gas out (lower tendency to form bubbles). Since people don´t usually see what their baseline absorption is the He sparging appears to not leave any effects, in truth it raises the baseline almost as much as N2 (via refractive index?, light scattering?).
Clearly, degassing via vacuum is the most efficient, you can´t get more efficient as one can get all the gases out that way. Using ultrasound + vacuum should just speed this up (my test apparatus doesn´t allow to check on the latter). The trouble with the vacuum method is if one has solvent mixtures: by the time the gas is out the solvent composition has changed.
I don´t have a membrane degasser so I can not compare it with just vacuum, I can imagine, though, that the efficiency would be strongly dependent on flow rate and vacuum.

Again, a short note on the vacuum experiments: I emtied out an old prep column, 25x250mm, attached a SSI needle valve to one end, mounted the column upright such that the valve was on the bottom. This can be filled with liquid (here H2O) via the opened valve by drawing vacuum on the other end. One can then keep a vacuum on the system as long as desired, with the valve closed, or one can bubble in gases through the valve. The liquid can then be pushed through the UV detector by applying N2 pressure on the top end of the vessel.

[Bruce Hamilton]

The following only applies to water, not mixtures with other solvents..
Note the advantage of helium is not only the low solubility, but the low rate of change of solubility with temperature.

Not sure how this will format.. From a previous Usenet life...
[ begin extract ]
The following data is from Kaye and Laby, 13th edition, and the units are
the number of cm3 of gas at 0C and 760 mmHg which dissolve in 1 cm3 of water at the temperature stated ( when the gas is at 760 mmHg pressure and in equilibrium with the water ).

Temp.(C) 0 10 20 30 40 50 60
Helium 0.0098 0.0091 0.0086 0.0084 0.0084 0.0086 0.0090
Hydrogen 0.0214 0.0195 0.0182 0.0170 0.0164 0.0161 0.0160
Nitrogen 0.0230 0.0185 0.0152 0.0133 0.0119 0.0108 0.0100
Oxygen 0.047 0.037 0.030 0.026 0.022 0.020 0.019
Argon 0.054 0.041 0.032 0.028 0.025 0.024 0.023
CO2 1.676 1.163 0.848 0.652 0.518 0.424 0.360

[ end extract ]

From memory, the 1981 paper seemed quite comprehensive, but I don't have a copy handy. Also, from memory, the HP patent on their micro membrane degasser for HPLC gave no data on effectiveness.

Solvent degassing and other factors affecting liquid chromatographic detector stability.
Janet N. Brown, Michael Hewins, Joop H.M. Van Der Linden and Roderick J. Lynch
Journal of Chromatography A
Volume 204, 16 January 1981, Pages 115-122

Abstract
We have measured the absorbance of methanol in the region 200–300 nm as a function of dissolved helium, nitrogen and air, and shown that a 1% change in oxygen level can cause a 4·10−3 a.u. change at 210 nm. We have also measured the amount of oxygen that can be removed from different solvents by the degassing techniques of refluxing, helium sparging, vacuum degassing and ultrasonic agitation. Using these results we have determined the operating conditions for minimum baseline drift in high-performance liquid chromatographic detection at 0.005 a.u.f.s. sensitivity in the range 190–220 nm. We observed a residual change in the absorbance of methanol, related to temperature, even after removal of oxygen.

[Newchromatographer]

Thanks Guys i wish to have a good memory like you ( i touched the wood don't worry ) .Guys pleas can you help me with this as finally i know that my big unknown peak ( i posted before here but unfortunately i found that the oxygen is disloved in methanol and not from contamiantion .After i changed completely the whole system accessories but with Dr.Donald advise every things solved ! ) now the unknow peak eluted after 5 min but in 5 min also my important compound is eluted with other compound eluted after 10 min because i am analysing two pesticides one at 5 min and the other 10 min . I can make the oxygen peak between my two peaks , the question is will the oxygen peak be acceptable in the middle between the two compounds becuase my supervisor is not happy with that and always want any contaminant or un important peak before the two peaks , please provide me with help here as i am wasting time with no success .

Ok , i was able to to elute the oxygen before the two compound but the proplem the analysis time is more than ( 55 min ) nearly one hour for just two compounds ! .Therfore i was thinking to get red of this oxygen to keep my analysis short but it seems refering to your replies that much difficult , i think i would be comfortable if there is no proplem if i keep it in the middle .

Sorry for my english hope you can understand me before my seperation

[HW Mueller]

Why do you get a peak when the oxygen is in the mobile phase? Are you using a gradient? How do you know it´s oxygen? If it is really in the mobile phase and not injected with the samples, then you have been given more than enough information to get rid of it.