Formic acid for negative ESI?

[Ericz]

Is it feasible to use formic acid for negative ESI? I did read some papers using it. Thanks!

[MG]

Yes it is. In general you will lose sensitivity with higher amounts, but often you want to have an acidic mobile phase to get good chromatography for acidic compounds. You can experiment with the amounts and see what is the best compromise between sensitivity and good, reproducible chromatography. You can also try acetic acid and see if it gives better sensitivity without sacrificing chromatography.

By the same token, you can run positive ESI experiments at high pH.