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0.1% HCOOH, 0.01% TFA, counterion question
Posted: Thu Aug 11, 2011 1:29 am
by 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?
Re: 0.1% HCOOH, 0.01% TFA, counterion question
Posted: Thu Aug 11, 2011 2:10 am
by 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.
Re: 0.1% HCOOH, 0.01% TFA, counterion question
Posted: Thu Aug 11, 2011 4:46 am
by DJ
It's formic, but I see your point.
Re: 0.1% HCOOH, 0.01% TFA, counterion question
Posted: Thu Aug 11, 2011 8:28 am
by 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?