Hi,

Our company recently bought a LC-MS and we want to use our current HPLC method with the LC-MS. We are currently running a gradient of H2O 0.1% TFA (A) and ACN 0.1% TFA (B).

0 min 90% A 10% B
10 min 30% A 70% B

We are using a Luna C18(2) 75mm X 4.6mm.

I heard that TFA decrease sensitivity of the MS so I tried to use formic acid instead of TFA. The problem is that the absorbance of formic acid in ACN and in Water is not the same... so I have a baseline drift.

Any solution ?

I also noticed that the separation of differents amines with TFA is better than with formic acid.

Sorry for my english.

1. Yes, TFA will usually decrease MS sensitivity

2. Actually, the absorbance spectra of TFA solutions also changes as a function of ACN concentration. Whether formic acid will be better or worse depends on the wavelength. If you are switching to MS detection, this should not be an issue, unless you need to run the exact same separation by both MS and UV detection.

3. TFA acts as a weak ion-pairing agent in reversed-phase LC, so a change in selectivity upon changing to formic acid should not be surprising. If the method was originally optimized for TFA, it should also not be surprising to see the selectivity be worse if you change to formic acid but keep everything else constant. In most cases such as this, you should expect to re-optimize the conditions: gradient steepness, temperature, formic acid concentration. I'll sneak in a promo here: the company I work for (LC Resources) makes software for just this purpose (DryLab).

3a. On the other hand, you should be able to quantitate with less resolution by LC-MS compared to LC-UV.

4. No need to apologize for your English, it is perfectly acceptable (and much better than my French!).

Bonne chance!

-- Tom Jupille / LC Resources