+46 in Negative ESI?

[ersimonson]

Why do I see adducts of +46 in Negative electrospray and how can I minimize it? I use water/ACN mobile phase with 0.1% formic acid on a ZQ single quad.

[southp]

Possibly these are formic acid (from the mobile phase) adducts? Formic acid has a M.W. of 46.

Almost anything you do to reduce adduct formation may impact your target analyte sensitivity. But normally you try increasing the source temperatures (desolvation/source block) and gas flows (desolvation/cone) as well as your cone voltage (this last one will almost certainly affect your target analtye). I've been told that adjusting your probe position can also help.

If it's the formic acid giving the adducts, reducing the mobile phase flow rate into the source (splitting) or decreasing the % of formic in your mobile phase might be worth a try too.

[james little]

don't know why you see M+46 ions with formic. However, not unusual to see M+45 ions when using formic or M+59 ions when using acetic in electrospray.

Especially for things that don't easily form negative ions. For example, PPG 1000 forms very significant acid adducts ions in negative ion electrospray. We use them plus the acid dimer-H ion (m/z 119 for acetate case) to calibrate in negative ion electrospray.