In electrospray ionisation the metabolites are sprayed out of a fine needle, positioned with its end poking into a “cage” formed by metal surround.

There is a big voltage between the needle and the cage, so the little sprayed droplets find themselves in a strong electric field.

There are various theories about how it works. This is the oldest (and best): The ions already floating around in the droplets repel each other, just like the hairs on a child’s head when a physics teacher attaches him/her to the Van der Graff generator.

They find themselves on the outside of the droplet. But meanwhile a stream of drying gas, often heated, is blown around the spray area. The droplets shrink as the solvent evaporates, and the charged ions are forced closer and closer together.

Eventually the repulsive forces between the ions overcome the surface tension of the droplet, and it “explodes” into many tiny droplets, whose solvent disappears even more quickly.

Soon the ions are left free in an atmosphere of drying gas. The important requirement is that the ions should be present in solution before the electrospray process begins. This method requires a high flow of drying gas. In contrast to APCI (Atmospheric Pressure Chemical Ionisation), the drying gas does all the work of removing solvent. The pressure at the nebulising needle needn’t be big, because the needle sticks right into the cage close to where the ions are released, and doesn’t have far to squirt.

The ions that are seen tend to be the ions that this metabolite would normally form in solution. Fortunately, the electrospray needle does act like the electrode in a battery and allow some electrochemistry to go on, creating higher concentrations of these ions than you might normally expect. This is good, as it means you can still detect carboxylic acids by their -COO group, even in the presence of some formic acid, which would normally force most of the groups into their -COOH uncharged state.

Incidentally, as in APCI, the need to form a fine spray does influence the choice of solvent. Some solvents that aren’t usually used in LC-MS are rejected because they don’t have the surface tension characteristics for ES (Electrospray) to work properly.

Atmospheric Pressure Chemical Ionisation

In APCI the sample is sprayed out of a fine needle that is usually a bit shorter than that used in Electrospray. The solvent is then dried away by a heater. The remaining solutes, hopefully also vaporised, are then blown towards a corona discharge, which I think is just a technical name for a spark.

In the spark the vaporised solvent and analyte are ionised. The solvent can play an important role here, because if it has a lower affinity for protons than the analyte, it can pass a proton onto an analyte molecule, creating an [M+H]+ ion.

Since the metabolite of interest has to float its way into the spark in a stream of gas, this method relies on some volatility (unlike electrospray). Ionization events are comparatively rare, so the chances of an ionized metabolite meeting another ionizing species and getting a second charge are even rarer: most ions in APCI are singly charged.

Written by : Dr. Lionel Hill (JIC, UK)

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