Most common acid modifiers to the mobile phase in reversed phase separations are as follows:
- phosphoric acid
- trifluoroacetic acid (TFA)
- formic acid
- acetic acid
Phosphate salts can be used to buffer pH in the range of 1.1 to 3.1 and are commonly used with UV/Vis detection because of their low absorbance of light in the UV/Vis spectra.
TFA at a concentration of 0.1% is also commonly used to adjust pH in the 1.5-2.5 range. A major benefit of TFA is that it can weakly ion pair with basic functionalities in analytes and consequently improve peak shape.
TFA is also volatile can be used with detectors such as MS and ELSD that require volatile buffers. Formate and acetate ammonium salts are also volatile and most commonly used with MSD. Formate salts have a buffer range of 3 to 5 and acetate salts buffer from pH 4 to 6.
Note that which buffer to use depends on the pKas of any acidic or basic functionalities in analytes as the pH should be adjusted so that these functionalities remain neutral thus increasing retention and minimizing secondary ionic interactions.
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