0.1% HCOOH, 0.01% TFA, counterion question

[DJ]

If 0.1% HCOOH, 0.01% TFA is used in the mobile phase during prep purification of basic peptides on RP-HPLC, following lyophilization, will the TFA counter-ion frequency be greater than 1/10?

[tom jupille]

I'm too lazy to fire up the Henderson-Hasselbalch equation and calculate it exactly, but my gut feel is that the TFA- counterions will predominate. TFA is (pKa = 0.5) is a roughly 10,000 times stronger acid than is acetic acid (pKa = 4.8 ). That is going to overwhelm the 10X concentration difference.

[DJ]

It's formic, but I see your point.

[HW Mueller]

One would think that one has no idea what the trifluoroacetate will be as lyiophilization certainly will have removed TFA, or are you talking about HPLC of a lyophilized sample?