Electron Capture-Induced Charge Reduction Benefits the Recording of Ultralong Transients in Orbitrap-Based Individual-Ion Mass Spectrometry

Recently, the use of ultralong transients has enabled exceptional resolution and sensitivity in Orbitrap-based charge detection mass spectrometry (CDMS). Nevertheless, measuring small analytes carrying a few charges remains a challenge. Prolonged trapping should, in theory, allow for the detection of lower charged ions (<10+) due to enhanced signal-to-noise (<i>S</i>/<i>N</i>) ratios. However, in practice, due to ion decay through frequency drifts, or collision-induced fragmentations, low <i>m</i>/<i>z</i> ions deviate from the ideal coherent trajectories in the Orbitrap. Here, by incorporating electron capture charge reduction (ECCR) in the gas phase prior to CDMS, we show that charge reduction significantly improves the stability of ion trajectories when ions are trapped for long periods in the Orbitrap analyzer. Using proteins with molecular weights ranging from 12 to 900 kDa, we demonstrate that ECCR-CDMS enhances ion survival by up to 60-fold, even enabling the detection of doubly charged individual ions from cytochrome c that typically elude conventional Orbitrap-based CDMS.