Mixtures of Trifluoroacetic Acid (TFA) and Triethylamine (TEA) are available commmercially as ion pair reagents. How is this mixture applied? I am familiar with the retentive effect of TFA on basic compounds but don't understand how addition of TEA would benefit the separation. Is it used for pH control?

I don't know the answer to your question. But if you use that stuff, be 100% certain you will never need to run your method with LC/MS.

Isn't it used to improve peak shapes of basic compounds?

David

TEA is used at concentrations of 20 mM or more to reduce peak tailing.

Neither TEA nor TFA produces any substantial ion-pairing effect (an increase in retention of ionizable compounds on reverse SP).
TEA is a strong base that in fact significantly reduces tailing of less basic compounds.
Using TFA/TEA mixture is not convenient due to the number of limitations:

- Limited choice of detectors
- MeOH should not be used as organic modifier in your mobile phase
- High background UV signal at 210 nm and lower

TEA-phosphoric acid will be a better alternative if you have to use TEA because of its lower UV transparency and no ester formation. Or, even better, you can employ TEA-formic acid or TEA-acetic acid combinations. With these you can use ELSD and MS, but note that a background signal will be higher at low UV.

AllsepTech: Why can't you use MeOH with this mixture?