Different brand of acetonitrile give a different separation

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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.
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.
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)
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...
4 posts Page 1 of 1

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