LCMSMS ionisation

[Davidantony]

Dear All,
I am asking a basic question regarding LCMSMS ionisation.How know a compound became positive or negative ionisation looking on the structure and mass.

[shemesh199]

look for groups that can be protonated or deprotonated, basic functional groups (amines) ++ will protonate , acidic compounds --- deprotonate, look for (carboxylic groups, some hydroxides).

Most of the pharmaceuticals are basic compounds, so ESI+ works very well most of the time, with the addition of an additive in the mobile phase composition (formic acid) we can protonate these species for good ionization in the source.

Some of the acidic compounds have great signal under negative mode ESI , ideally you should investigate both polarities.