by
lmh » Fri Jul 02, 2010 12:54 pm
I am no expert on this, but:
The way I see it, you have to consider what is going on at the end of the electrospray needle. There is a voltage creating a field where ions move. The idea of electrospray is that ions are dragged out of the tip of the needle in droplets of solvent with the ions towards the outside of the droplet and all repelling one another. Presumably the original counterions are dragged back towards the conductive needle itself. I.e. the needle is acting like an electrode in electrochemistry. As the ions are dragged away, the neutral, unionised molecules remaining will tend to ionise to restore the equilibrium.
So the relevant feature of a molecule isn't so much its actual extent of formation of ions at the pH of the solvent, but its ability to form ions. If it can, it will in electrospray, even if it normally wouldn't to any great extent.
Dibutylphthalate ionises nicely as do all the phthalate esters. I'd guess their oxygens can pick up a hydrogen by donating an electron pair, and I suspect (chemists correct me) that the charge can be delocalised over the benzene ring which would make it more stable. But this is only half the story; in electrospray, things that can exist easily on the outsides of droplets seem to do very well too, and anything with alkane chains has that hydrophobic side. Detergent-like molecules do well.
One can try to be too clever about this. My (very limited) experience is that a molecule with absolutely no polar bonds will not show up, ever, and a molecule with no way to behave acidically at all will probably not run well in negative mode, but almost everything with any polar bonds (i.e. containing oxygen or nitrogen) gives some sort of signal in positive mode if you try hard enough. But it does depend on solvent, instrument, and probably the phase of the moon.
