[yy]

In making Mobile Phase, why do we use a simple beaker and cylinder,
rather than volumetric flask?

Is it all right to be "about" for mobile phase?

[Arkcon]

Yes.  Can you think of reasons why, given how the LC run is performed?  Try to list a few, to help yourself understand.

[yy]

No, I have no idea why -
Do you have a source of links where I can do some study on it?

[yy]

Some list of things I find below:

• no gas dissolved in the liquid MP
• very consistent composition

• no interfering compounds

And these do not say why we can be about.[/list]

[Train]

I had a professor who suggested measuring out mobile phase components by weight rather than volume for greater consistency between runs.  However, at work I typically prepare 2-5 L or more, so it's not very practical.  Using volumetric glassware is also usually not practical because you end up using more than one flask.  For example, how would you measure out 1750 mL?  You would have to use at least three volumetric flasks and besides being inconvenient I doubt the accuracy would be that much better than a graduated cylinder.

[gilly]

What makes a good robust HPLC method? - The ability to withstand small changes in mobile phase composition is one factor. If you have a good method it is able to withstand small changes in mobile phase therefore graduated cylinders are sufficient to prepare mobile phase and volumetric flasks are just overkill. Also, all HPLC columns vary slightly depending on the manufacturer, how much use they have had and what mobile phase has been run through them so preparing a mobile phase in a volumetric flask wouldn't necessarily give you the exact same results each time anyway.

[JGK]

Also using volumetric glassware in certain cases is not accurate.

For example if you want to make 1L of acetonitrile:water (60:40 v/v) I would combine 600 mL acetonitrile with 400 mL of water. However the final volume of the mixture will not be 1000 mL (1L) as there is some sample contraction.

If I try the same exercise with a volumetric, adding 400 mL water to the flask then making up to volume with acetonitrile the composition will not be 60:40 v/v.