Microdroplets (50 μm diameter) of the sample solution are introduced into vacuum with the help of an on demand droplet generator. There they are irradiated one by one by a mid-IR laser which runs at the same frequency (10Hz).
The laser wave-length is tuned to the absorption maximum of water at around 2.8 μm - the H-O stretching vibration of water. The absorbed laser energy leads to the explosion of the droplet, releasing the biomolecular complex ions into vacuum, where they can be analysed by a time-of-flight mass analyser. The incomplete ion neutralisation process leads to gas phase ions whose charge state mimic the charge states of the complexes in solution.