I have purified a small organic molecule using 0.1% TFA/Acetonitrile and 0.1% TFA/Water. Now due to the presence of TFA in my solution, I am not able to obtain proper mass using ESI-MS (due to low ionisation of TFA). I tried evaporating under high vacuum but it did not help either. Moreover, when I purified the same sample using 0.1% HCOOH, I was able to obtain a proper mass but the resolution of HPLC separation is poor in case of HCOOH.
You can use zip tip for sample preparation before MS detection. I used zip tips to bind my peptide to the C18 resin into the tip then i washed it with water and 10% ACN to remove the TFA then i eluted with 45% ACN and it worked for me.
This method works with peptides and may help
1. Evaporate the sample from the TFA column
2. resdissolve in acetic acid (will usually displace the TFA counter ion)
3. Evaporate again for ESI-MS
If your desired compound is not an acid, you can use a bicarbonate resin to remove TFA, either before concentrating your water/ACN fractions, or afterwards (e.g. dissolving in methanol, DCM, or ACN, passing your solution through the cartridge and reconcentrating).
@jeffery: After replacing TFA with acetic acid counter ion were you able to obtain a clean ESI-MS without a bunch of acetate peaks? In the case of TFA all I could get were lot of peaks of TFA (dimer, trimer..etc.)
@jeffery: What percentage of Acetic acid did you use?
@Mihai: I have only 1-1.5 mg of compound and therefore want to avoid further processing!
Should have been more expllicit...
1. This is a peptide method to remove TFA for a variety of purposes. The ESI is clean in the region we looked at it (~600 m/z) and not detectable by 19F NMR (TFA-peptides normally are). There may still be TFA in the lower region ,which may be a problem with smaller molecules
2. 1% I think. 10% definitely works for peptides.
3. Samples are lyophillized, not evaporated (may make a difference)
4. You might try the post column techniques in refs 8-10 below that are specifically for small molecules in the link below. If you have a cheaper, roughly similar molecule try them on that one first
http://www.clinchem.org/content/49/7/1041.full
You can use zip tip for sample preparation before MS detection. I used zip tips to bind my peptide to the C18 resin into the tip then i washed it with water and 10% ACN to remove the TFA then i eluted with 45% ACN and it worked for me.
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