Biomolecular ions generated by infrared laser desorption on microdroplets under vacuum.

A droplet beam-laser ablation gas phase source has been developed for the isolation of biomolecular ions ejected from liquid microdroplets under vacuum. Liquid microdroplets containing the biomolecular ions of interest are generated in a primary vacuum chamber and passed through differential pumping stages to interact with an infrared laser pulse in a secondary vacuum chamber. Absorption of the infrared radiation by the solvent causes the explosion of the droplets and the desorption of biomolecular ions into vacuum. These ions are then mass-analyzed by a homemade time-of-flight spectrometer, either through a delayed extraction line or through coupling with a Paul trap. We show that this desorption source allows for the transfer of fragile molecules into the gas phase and preserves small non-covalent complexes. It can be operated in the positive and negative modes. Mass spectrometry analysis by the coupling of the source with a Paul trap allows us to get rid of the broad initial velocity distribution of the desorbed species subsequently to the laser ablation and leads to an improved mass resolution. This is a very promising setup for the production of hydrated biomolecules. It opens the way to the subsequent gas phase structural analysis of the desorbed biomolecular ions.