Broadband quadrupolar axialization of large multiply charged ions to enhance measurement and minimize conformational restrictions

For high-resolution Fourier transform mass spectrometry of electrosprayed proteins, the signal-to-noise ratio of measuring nozzle-skimmer fragment ions can be improved substantially by their broadband quadrupolar axialization (QA), even without increasing their concentration in the ion cyclotron resonance cell. Axialization of the product ions makes possible larger, more concentric ion orbits for measurements. QA allowed the identification of new sequence-indicative product ions from a 29 kDa protein. However, QA of large molecular ions gives little increase in signal, consistent with original trapping near the cell axis. By recentering of ions undergoing ion-molecule reactions, these can be carried out at much higher kinetic energy and pressures; for cytochrome c this increases the achievable H-D exchange by 40%, corresponding to exchange at all the active sites of its completely denatured conformer.