Mobile phase mixing

[chhubert]

Dear all,
Just for curiosity. Why HPLC people like to pre-mix solvents in binary pump system for gradient HPLC, say 10% inorganic and 90% organic for solvent A whilst 90% inorganic and 10% organic for solvent B? Is there any pros and cons? I use to using 100% aqueous for solvent A and 100% organic for solvent B in my binary pump system for gradient HPLC and am happy with this.
Thanks a lot.

[Peter Apps]

I was wondering the same thing, why pay for a gradient pump and then do half the work manually ?. The people who were doing it could not come up with a better reason than that was how they always did it.

The I read Tom Jupille's post on Retention time shifting in cation HPLC and the little light bulb switched on - pre-mixing gives better repeatability for very shallow gradients. It still does not make sense to me for the usual 80:20 to 20:80 in 20 min gradients though.

Peter

[Victor]

Some other possible advantages:

1) IF you have problems with bug growth (many people do not have this problem), the general opinion is that it will be inhibited in a bottle containing 10% organic solvent compared with pure water-there is a long discussion of this eleswhere in the forum.

2) IF you have problems with air bubbles in your system (many people do not have this probelm), some pre-mixing of the organic solvent and water may be beneficial-especially if you are using.

3) In method development for isocratic runs or in gradient runs, it is possible under certain circumstances to put any (inorganic) additives in both bottles so that the concentration of buffer is constant in the mix, independent of the proportion of A and B. Of course caution is necessary here not to exceed solubility limits. This point is of limited relevance to most practitioners, who do not care about the exact concentration of additives as long as it is constant from run to run-see a recent post by Wanda, and a reply by Tom Jupille.

[Paulhurleyuk]

Another possible advantage is that in some cases mixing pure acqueous with pure solvent can produce some precipitation at the point of mixing (even where the two would normally mix) due to the pressure changes at the point of mixing.

Premixing the mobile phase softens this sudden change and can stop problem of blocked frits / check valves.

We also use this to maintain buffer conc and pH constant across a gradient.

Paul.

[DR]

Then there are the check valve issues many suffer when pumping straight ACN...

[HW Mueller]

Paul, could that precipitation you mention be due to unfavorable concentrations near the interface of the two liquids? When one tests miscibility in a test tube one often sees this at the areas where the liquids meet (aqueous buffer and MeOH, for instance). Shaking (mixing) can redissolve these precipitate veils. If you are slow enough with the mixing the crystals can get big enough to last for some time.

[syx]

I feel that 100% of each component of mobile phase in each reservoir is more efficient way.

[DR]

Pumps work better when you run them in a 0-100% pattern. Also, there's the bugs, precipitation & ACN problems to consider...

Premix (once you've got your gradient optimized).

For method development, pure solvents work adequately, yield flexibility etc. I'll give in that much.