Infrared laser ablation and ionization of water clusters and biomolecules from ice

We demonstrate the direct desorption and ionization of angiotensin II from frozen water ice samples without supplementary matrix, using a pulse train of approximately 340 intense (∼109 W/cm2) picosecond pulses from a tunable, mid-infrared free-electron laser, at a wavelength of 5.9 μm. The pulse train was delivered at equally spaced intervals over a total duration of 120 ns. Ions thus formed were detected using a reflectron time-of-flight mass spectrometer. Single-shot ablation at spatially separated locations on the ice surface produced parent ions as well as Na and K adducts. Multiple pulse impact at a single location on the ice generated the parent ion signal and also protonated water clusters of the form (H2O)n−1H3O+. Investigations of clusters produced by infrared laser ablation of frozen trifluoroacetic acid solution support a mechanism involving electrostatic ejection of pre-formed ions.