Chromatography Forum

We are currently working on a method, and I think we need both a salt (to mediate interactions with the silanols) and also low pH. Typically I would do something like add around 30 mM of ammonium formate and then add formic acid to bring the pH down. But in this case, I want to use Difluoroacetic Acid.

Now I don't think one can buy Ammonium Difluoroacetate, so I am thinking to start with ammonium formate, and then use Difluoroacetic acid to bring the pH down.

I guess the question is: Does anyone see a problem here. There would be a lot of "stuff" floating around in this mix.....ammonium.....difloroacetate (and its acid).......formate (and its acid). Is there any concern with such a complex system of mobile phase additives?

Any thoughts are appreciated.

Thanks in Advance

why making it unnecessarily complex?

why not just use ammonia hydroxide and your difluoroacetic acid and titrate to your desired pH.

Adding ammonia formate just introduces another "player to the game", which may not disturb at all but has the potential to add some troubles (another chemical -> purity? -> handling/weighing etc...)

Well that's not a bad thought. But I do think there may be a problem with this approach, at least in our application.

I think, by this method, we would wind up with much less ammonium in the final solution. And we need the ammonium to mediate interactions with silanols.

Personally, I also like to keep it as simple as possible. That said, I don't see the problem with your mixture of additives, provided that:

- You stay in the pH range that the column/system can handle
- If gradient method: no precipitation at high organic levels
- The method works...

Just to be safe, I would include a wash at the end of every batch (with plain water/MeOH) to get rid of everything before shutting the instrument down.

If you're working with MS detection, try to keep the concentration of additives down as low as possible (typically <10mM salts).

I think, by this method, we would wind up with much less ammonium in the final solution. And we need the ammonium to mediate interactions with silanols.

You can have as much ammonium as you wish. Just add the desired amount of ammonia and then adjust the pH down to the desired value with the acid.
And by the way, don’t worry too much about silanols if the pH is low – say about 2 – 3.