This is not entirely true. In particular, basic compounds can give you two peaks by HPLC if the pH is not low enough. I can't explain to you
precisely why this is, but I've observed it many times with basic compounds when using formic acid as a mobile phase modifier. | Quote: |
If you have a pH which is not acidic enough (when running HPLC of amines) then you will get an equilibrium of the amine freebase and the amine salt
forming. This can lead to two peaks or just one broad peak, depending on the kinetics and pH. But if the HPLC is set up like most I have seen, it
will have TFA or Formic acid as the modifier, which should provide a good pH. If that is not done, many compounds will give poor peak shape, not
just benzylamine.
My QC work was done mostly with a GC-MS, as benzylamines are hard to QC with an HPLC, due to low UV absorbance, and also poor ionization in MS, due to
low MW, unless you have a MS tuned for low MW, which we did not. But some benzylamines would turn to crap (more than 2 peaks) within a few weeks,
even in a sealed botle, if ever opened enough to get any air in them.
We tested this numerous times to confirm, which helped us to understand why reactions done with "pure" amines where giving lower yields and purity
over time. "Pure" is one of the most overused terms in chemisty, given that any analytical method can only determine within it limitations, and we
found many chemicals that had impurities that only showed up in certain methods, but looked good by others. Just look at the melamine in milk and
adulterated heparin for other examples of an analytical method being "fooled" by certain impurities. |
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