Electrospray droplet impact (EDI) secondary ion mass spectrometry (SIMS) is a desorption/ionization technique for mass spectrometry. Highly charged water clusters produced
via atmospheric-pressure electrospray are accelerated in a vacuum at several kV and impact the sample, which is deposited on a metal substrate. Two mechanisms for desorption/ionization in the EDI ionization process have been proposed, namely, low-energy and high-energy regimes. In the low-energy regime, the excess charges in the primary droplets play a role in the formation of secondary ions. In the high-energy regime, samples are ionized as the result of the supersonic collision of the primary droplets with the sample. In general, EDI/SIMS is very effective for the formation of [M+H]
+ and [M-H]
- for a wide variety of molecules. This characteristic of EDI/SIMS is probably due to the formation of the highly acidic H
3O
+ and highly basic OH
- ions, which are superb protonating and deprotonating reagents, respectively. In addition, EDI produces a much smaller number of hydrogen atoms compared with matrix-assisted laser desorption/ionization (MALDI) and fast atom bombardment (FAB).
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