Chromatography Forum

Dear All,

I need to prepare a mobile phase 0.01 M ammonium formate in acetonitrile for a HILIC method. However it was not possible to dissolve this salt in acetonitrile although I sonicated the solution for long time. I would appreciate if anyone could give me some advice on how to dissolve this salt in acetonitrile.

Thanks

HILIC usually requires a certain amount of water, so the first question is why you would need straight ACN.

In addition, buffer concentrations are often specified in the aqueous part of the mobile phase (before addition of water).

If you are running an existing (presumably validated) method, check the documentation and procedure carefully to make sure you are interpreting it as intended. A well-documented method should have an explicit "recipe" for preparing all solutions.

Finally, if you *really* need 0.01M in ACN, try heating the solution (then cooling back down to ambient).

I am not using a validating method, but I am trying to developing my own method. I need to have acetonitrile 0.01 M ammonium formate and water 0.01 M ammonium formate as mobile phase.
I could try warming up acetonitrile to help dissolving the salt hope it will work

HILIC methods require some water on the surface of the packing. If you start with straight acetonitrile, you wont get a HILIC method.

Use acetonitrile with 5% water as your starting mobile phase and dissolve your salt in this mixture. Your problems will disappear.

You should get yourself the HILIC brochure from Waters. It has a lot of explanations and do's and don't's for HILIC.

It would be practical to use a 95:5 ACN:water if you're trying to buffer your mobile phase. There should be no practical use for 100% ACN on a HILIC column, even if you're using a gradient. The reason is; there needs to be water present during injection or your compounds will pass right through the column. The enriched water region is the only thing holding up your polar compounds, so without it you're essentially turning your column into an overpriced union.

Some methods may require lower volumes of water so you can go down to a lower percentage of water if need be, but remember, the solubility of your salt will go down with increasing organic composition. The best of luck to you my good sir.

I was having an issue with this as well. My goal was to have 0.1% ammonium formate on hilic column so I made up a buffer at 1% formic acid and ph'd to 4.3 with ammonium hydroxide. I made up my mobile phase by adding 100mL of this buffer to 900 mL of ACN. When adding the buffer the solution became hazy and never cleared up even after the temperature of the solution reached room temp. I also tried it reverse, adding the ACN to the buffer, but it also produced a hazy solution.

1% Formic acid seems to be high, 0.1% formic acid is generally used.

Try using triethylamine instead of ammonium hydroxide to adjust the pH. The retention times obtained with triethylammonium salts are somewhat less in HILIC than the retention times obtained with the corresponding ammonium salts, but they are often good enough. In addition, solubility of triethylammonium salts in a predominantly organic solvent is appreciably better, and the salts are still volatile.