Why does the peak tail for my basic analyte on a reverse-phase column when using formic acid in my mobile phase but not when I use trifluoroacetic acid? - WKB236325

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Article number: 236325

ENVIRONMENT

Liquid chromatography
Peak tailing
Trifluoroacetic acid (TFA)
Formic acid (FA)

ANSWER

Basic tailing:
Tailing can be caused when a positively charged base undergoes ion exchange to negative surface silanols on a particle surface.

If using 0.1% formic acid (FA):

Surface silanols on the particle surface will still carry some negative charges in 0.1% formic acid because the pH is not low enough to protonate all of them.
Basic analytes will be positively charged in 0.1% formic acid provided the pKa of the base is greater than 5.0. If the pKa is less than 5.0, some of the amines on a basic analyte will still be positively charged.
When positively charged basic analytes ion-pair with negatively charged silanols, the result is tailing due to cation exchange.

If using 0.1% trifluoroacetic acid (TFA):

0.1% TFA will bring the pH of an aqueous solution down to about 2.0:
- At this pH, a majority of silanol groups on a particle surface are neutralized (but not all ot them).
TFA will ion-pair to positively charged amines present in the injected samples:
- This will, in effect, neutralize the positive charge on the amines.
  - This results in a more hydrophobic analyte
The combination of neutralizing most of the surface silanol group and ion-pairing to positively charged amines effectively prevents the cation exchange mechanism from occurring as it does with formic acid only.
- This results in dramatically less tailing with TFA in your mobile phase.