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Polycat A + dilute TFA
Posted: Sat Nov 07, 2009 9:21 pm
by DJ
Andy Alpert from PolyLC discourages use of TFA with any of their columns, including Polycat A.
Why is this? No problem using 3 M CH3COOH, which would reach a pH near that of 10 mM TFA.
What about H3PO4?
Posted: Sun Nov 08, 2009 9:03 am
by danko
Hi DJ,
I’m convinced that it’s some kind of myth rather than a documented fact that TFA is bad for columns and once you’ve used a column with TFA you should only use that particular column with TFA containing eluents.
The theory goes like this: TFA is an ion-pairing agent that sticks to the stationary phase almost permanently. This should alter the columns selectivity (positively or negatively depending on what one is planning to use the column for).
I haven’t observed such tendencies, but I sure would like to hear from people that have investigated the matter in a more controlled manner.
To the other question: Phosphoric acid and better yet salts of it should be OK to use.
Best Regards
Posted: Thu Nov 12, 2009 3:43 pm
by Andy Alpert
Actually that's not the objection to TFA. A 0.1% solution of TFA affords a pH of ~ 2.0. At so low a pH the Si-C bond of silane-based coatings starts to hydrolyze. That's why C-18 column lose capacity, slowly but steadily, when used with mobile phases containing 0.1% TFA.
TFA is a strong acid and so is fully dissociated in water. Acetic acid is only 5% dissociated and so columns can tolerate a lot more of it. However, leaving a silane-based coating in molar concentrations of unbuffered acetic or formic acid will eventually destroy it.
There are more objections to TFA. Basic residues are the most polar ones and don't promote retention much in RPC. TFA forms somewhat hydrophobic ion pairs tenaciously with them, thereby promoting retention of peptides. It has the opposite effect in both SCX and HILIC, where basic residues are the most important ones for retention.