Counterion Exchange Process — TFA to Acetate/HCl Salt Conversion

1. Background

Peptides purified using TFA-containing mobile phases exist as TFA salts. For some applications, TFA must be exchanged to acetate or hydrochloride.

2. Methods

Method	Principle	Scale	Time
Lyophilization with HCl	Dissolve in 0.1 M HCl, lyophilize	Small scale	24 – 48 h
Ion-exchange chromatography	Pass through acetate-form resin	Prep scale	2 – 4 h
Dialysis	Dialyze against acetate buffer	Sensitive peptides	4 – 8 h

3. Acetate Exchange Protocol

1. Dissolve peptide in 10% acetic acid (10 mg/mL)
2. Freeze and lyophilize
3. Repeat 2 – 3×
4. Verify by HPLC (RT shift) or ion chromatography

4. Verification

Test	Method	Acceptance
TFA residual	¹⁹F NMR or IC	<1.0% (w/w)
Acetate content	IC or enzymatic	Report
Chloride content	IC or titration	Report