Charge retention by peptide ions soft-landed onto self-assembled monolayer surfaces

Soft-landing of singly and doubly protonated peptide ions onto three self-assembled monolayer surfaces (SAMs) was performed using a novel ion deposition instrument constructed in our laboratory and a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially designed for studying collisions of large ions with surfaces. Modified surfaces were analyzed using in situ 2 keV Cs⁺ secondary ion mass spectrometry or ex situ 15 keV Ga⁺ time-of-flight-secondary ion mass spectrometry (ToF-SIMS). The results demonstrate that a fraction of multiply protonated peptide ions retain more than one proton following soft-landing on the FSAM surface. It is shown that the [M+2H]²⁺ ions observed in FT-ICR SIMS spectra are produced by desorption of multiply charged ions from the surface, while re-ionization of singly protonated ions or neutral peptides is a source of [M+2H]²⁺ ions in ToF-SIMS spectra. Differences in neutralization efficiency of soft-landed ions following exposure of surfaces to laboratory air has a measurable effect on the results of ex situ ToF-SIMS analysis of soft-landed ions on SAM surfaces.