Laserspray Ionization, a New Method for Protein Analysis Directly from Tissue at Atmospheric Pressure with Ultrahigh Mass Resolution and Electron Transfer Dissociation*□S

Laserspray ionization (LSI) mass spectrometry (MS) allows, for the first time, the analysis of proteins directly from tissue using high performance atmospheric pres-sure ionization mass spectrometers. Several abundant and numerous lower abundant protein ions with molecu-lar masses up to 20,000 Da were detected as highly charged ions from delipified mouse brain tissue mounted on a common microscope slide and coated with 2,5-dihy-droxyacetophenone as matrix. The ability of LSI to pro-duce multiply charged ions by laser ablation at atmo-spheric pressure allowed protein analysis at 100,000 mass resolution on an Orbitrap Exactive Fourier transformmass spectrometer. A single acquisition was sufficient to iden-tify the myelin basic protein N-terminal fragment directly from tissue using electron transfer dissociation on a linear trap quadrupole (LTQ) Velos. The high mass resolution and mass accuracy, also obtained with a single acquisi-tion, are useful in determining protein molecular weights and from the electron transfer dissociation data in con-firming database-generated sequences. Furthermore, mi-croscopy images of the ablated areas show matrix abla-tion of 15 m-diameter spots in this study. The results suggest that LSI-MS at atmospheric pressure potentially combines speed of analysis and imaging capability com-mon to matrix-assisted laser desorption/ionization and soft ionization, multiple charging, improved fragmenta-tion, and cross-section analysis common to electrospra