I've tried to control the sodium cationization of compounds often with little success for compounds that favor this ionizatin mechanism.
Normally I add ammonium acetate post column in our LC-MS analyses (0.1 ml/min of 25 mmolar ammonium acetate in methanol) to flows of 1.5 ml/min of HPLC eluent. Split the flow to MS with T to get between 100-400 ul/min total flow to MS. This allows me to go to 100% organic eluent and still get M+NH4 or M+H ions.
This tends to give me fewer sodium adducts. Also, find the in-source fragmentation of M+NH4 more useful than M+Na adducts.
Also noted for many compounds that the M+Na ion maximizes at higher skimmer voltages than the M+NH4 or M+H, thus tend to run at the lower end of the skimmer voltage range.
I have added KOAc in methanol at about the 4 mmolar concentration post column to enhance metal cationization for some compounds that don't give useful data from M+NH4 or M+H adducts.
