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Soft landing on a plasma-treated metal surface of multiply protonated protein ions from the gas phase results in a substantial retention of protein function, as demonstrated for trypsin and streptavidin. The majority of trypsin ions soft-landed at hyperthermal kinetic energies are undamaged and retain 72-98% of enzymatic activity after being washed into solution. A small fraction of trypsin ions that were landed at nominal kinetic energies of 130-200 eV remain tethered to the surface and show approximately 50% enzymatic activity. The streptavidin tetramer is found to dissociate to monomer units upon multiple charging in electrospray. The majority of soft-landed monomers can be washed into solution where they show affinity to biotin. The layer of streptavidin monomer that is immobilized on the surface can be detected if fluorescence-tagged and retains the ability to reversibly bind biotin. A mechanism is proposed to explain nondestructive protein ion discharge on the surface that considers proton migration from the soft-landed cations to the metal oxide layer and metal ion reduction by electron transfer from the bulk metal.

Soft landing of singly charged gas-phase ions on dry metal surfaces that were pretreated in situ by oxygen plasma results in 0.1-2% total yields of recovered intact compounds. Lysine, peptides, crystal violet dye, and a biotin conjugate are found to survive soft landing of hyperthermal ions of up to 50-eV kinetic energy. Soft landing at 40-50-eV ion kinetic energies of a fluorescence-labeled biotin conjugate results in an immobilized fraction that cannot be washed from the surface and is found to contain an intact biotin moiety. The present results represent an approximately 10(4) fold improvement in soft-landing efficiency and indicate that plasma-treated metal surfaces can be useful for preparative separation of organic and biological molecules by mass spectrometry. The substantial improvement in soft-landing yields results from a high transmission of electrosprayed ions into the vacuum system, efficient and nondestructive discharge of ions on the metal oxide surface, and facile analyte recovery in the absence of a matrix.