Mobile phase / buffer preparation


My lab has traditionally used 10mM formic acid (MeCN/water) as our LCMS mobile phase (H-class with a QSM, Waters BEH C18 columns) but would like to make full use of our instrument by adding a high pH method. 

We're considering using 10mM ammonium acetate at pH 9 for this. All of the 'recipies' I find are to make up 10mM ammonium acetate and then titrate in ammonium hydroxide until the deisired pH is reached. 

As a hands-on chemistry lab, we don't routinely measure pH by anything other than dip-sticks, so I would like to avoid this method of preparation in favour of simply mixing ammonium hydroxide and acetic acid in the theoretically correct proportions (I accept that while this is unlikely to give an exact pH 9 solution, it at least creates a method that can be routinely replicated each week when we make up mobile phases). 

I've been looking for a guide as to the mathematics of doing this, but find very little (many examples seem to focus on preparation of acidic or biological buffers).  I did find the zirchrom Buffer Wizard (http://www.zirchrom.com/Buffer.asp#BufferWizard), but without understanding the maths i'm reluctant to trust this. 

Advice most appreciated. 
Thanks in advance  

Answers

  • Pony up the small to middling bucks for a stirplate and a pH meter. They're not that expensive. Neither are the calibration buffers.

    Not what you wanted, but it's my advice.
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