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Different brand of acetonitrile give a different separation
Posted: Fri Nov 09, 2018 8:04 am
by Mattias
Hi,
Have you ever experienced this before? I switched from JT Baker to VWR Acetonitrile Gradient Grade and two peaks changed place! This is a HILIC method that is very sensitive to pH so I assume that the pH is different in the two acetonitriles.
The pH in the mobile phase is controlled by a 10 mM ammonium acetate pH 4.5. I cannot measure the pH in the mobile phase (75% acetonitrile), but it appears as the JT Baker is more acidic than VWR.
Re: Different brand of acetonitrile give a different separation
Posted: Fri Nov 09, 2018 6:15 pm
by DR
Are you certain that both bottles had just ACN in them? One can also order ACN with 0.1% TFA or Formic acid mixed in. This is far more likely to have thrown your pH off than a difference in ACN lots as ACN would have essentially no buffering capacity compared to your buffer.
Re: Different brand of acetonitrile give a different separation
Posted: Mon Nov 12, 2018 7:57 am
by Mattias
My theory was questioned here as well, maybe one of the bottles did not contain what it was labelled as. I have ordered fresh bottles to repeat the experiment.
There is a alkalinity/acidity spec for acetonitrile, and the amounts are not extremly small (about 0.2 mM NaOH or HCl, respectively)
Re: Different brand of acetonitrile give a different separation
Posted: Mon Nov 12, 2018 1:13 pm
by Consumer Products Guy
During the acetonitrile shortage "back when", we intentionally ordered acetonitrile that contained acetic acid because that was available, and we added acetic acid to our mobile phase anyway.
Not saying you accidentally got some of this, but might be possible...
Re: Different brand of acetonitrile give a different separation
Posted: Mon Nov 19, 2018 11:31 am
by Mattias
Hi,
I ordered fresh bottles of acetonitrile from both suppliers, and there is absolutely no difference at all. So probably one of the opened bottles was not marked correctly...
Re: Different brand of acetonitrile give a different separation
Posted: Mon Nov 19, 2018 2:37 pm
by Consumer Products Guy
Mattias wrote:
Hi,
I ordered fresh bottles of acetonitrile from both suppliers, and there is absolutely no difference at all. So probably one of the opened bottles was not marked correctly...
If me, and I had any of the suspect stuff left, I'd run GCMS and try to see what was different. Just out of curiosity.