Does ammonium fluoride assist ionisation? (and how?)

Hi I recently read a report in which 1mM ammonium fluoride was added to the LC-MS mobile phase to increase sensitivity/ionisation. Can anyone verify this is effective for ESI positive mode analysis of drugs? Can you point me to a reference that describes how this works (if it does) 

Cheers

Peter

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  • Hi Peter, 

     

    Using ammonium fluoride in your mobile phase will introduce free ammonium ions to create the ammonium adduct of your compounds. If you're detecting the ammonium adduct, having free ammonium ions is a prerequisite to good ionization.

     

    It is the same concept as using ammonium formate, but you get the added benefit of the free fluoride ions working to suppress free sodium ions. Every molecule of your compound that forms the sodium adduct won't be able to form the ammonium adduct! Le Chatelier's principle still applies. 

     

    Best,

    Ryan

  • Ok thanks, so it’s not much use if M+H is your main pseudomolecular ion then I take it…

    When you say fluoride will ‘suppress sodium’, I assume you mean the fluoride ions co-ordinate in solution with sodium ions better than say, chloride or non-ionised drug molecules. Effectively mopping up the sodium- it that the theory here?

    Cheers

    Peter

Reply
  • Ok thanks, so it’s not much use if M+H is your main pseudomolecular ion then I take it…

    When you say fluoride will ‘suppress sodium’, I assume you mean the fluoride ions co-ordinate in solution with sodium ions better than say, chloride or non-ionised drug molecules. Effectively mopping up the sodium- it that the theory here?

    Cheers

    Peter

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