Chromatography Forum

All. I have a question about the ESI mechanism which has been puzzling me. ESI is described as a solution based process whereby a MS response is seen where an ion is formed in solution prior to desolvation. As an example a molecule with an amine function will be protonated at low pH and therefore in theory works well in ESI positive. However I am not sure how molecules that do not appear to have obvious acidic or basic groups and are fairly lipophilic ionise via electrospray?

As an example the steroid testosterone exists in the unionised form throughout the whole pH range (almost) yet still works well in electrospray positive. Does anyone have any ideas on this? Is there a chemical ionisation process occurring in these cases?

Thanks

You may read the following review that should give you answers:

Kebarle P, Verkerk UH, ELECTROSPRAY: FROM IONS IN SOLUTION TO IONS IN THE
GAS PHASE, WHAT WE KNOW NOW, Mass spectrometry reviews, 2009.

absolutely definitely, the more good reviews you read, the better, but as a very simplistic answer:

Electrospray is a solution-based process in that it happens in solution, and forms the ions you would expect to see in solution, but it is also an electrochemical process. For electrospray to work, you don't need a chemical to be strongly ionised in solution (in the sense that most of it is ionised/that the equilibrium between the neutral and ionised form is biased towards the ionised form). You just need a chemical that can be ionised in solution. A hand-wavy way to think about it is that if there is an appropriate ion in solution, the electric field will draw it out, and the bulk left behind that is not ionised will then ionise (a little) to redress the equilibrium.

Incidentally, it's a good idea to read all the theory before getting too excited about manipulating ionisation efficiency by changing pH. Depending on your system, adding buffers to try to force a pH at which the expected analyte will ionise better can have the opposite effect, because it's also affecting the conductivity of the solution in the spray needle, and hence the electric field that's moving ions around. (Well, that's what I think is going on, but I'm not much of an expert on this).

One way I have seen it explained how ESI works is that the charge is applied to the droplets that form in the aerosol. Then as the solvent evaporates the charge per volume increases until the point where the solvent has completely evaporated and the charge has been transferred to the solute molecules which are then drawn into the MS by the charge differential leaving the non-ionized particles behind. The charge transfer can then cause the molecule to lose or gain protons depending on the mode of ionization (positive or negative).

As I say that was just one explanation I found online as I am studying up on this myself trying to learn more about LC/MS/MS.