High performance charge detection mass spectrometry without ultra-high vacuum†

Abstract

Charge detection mass spectrometry (CDMS) measurements of individual ions using either Orbitrap or electrostatic ion trap-based instruments have heretofore been performed under ultra-high vacuum conditions (10⁻⁹ Torr or lower). The rationale for this expensive and often cumbersome requirement is that these measurements need to be performed in an environment where collisions with background gas do not adversely affect the measurements. Here, the use of an electrostatic trap that accepts a broad range of ion energies and a dynamic ion signal analysis method enables accurate CDMS mass measurements at pressures as high as 1 × 10⁻⁶ Torr, multiple orders of magnitude higher than previously demonstrated. Consistent, accurate masses were obtained for pentameric antibody complexes (∼800 kDa), adeno-associated viruses (∼4.8 MDa), and both ∼50 and ∼100 nm diameter polystyrene nanoparticles (∼35 MDa and ∼330 MDa, respectively) at pressures ranging from 1 × 10⁻⁸ Torr to 1 × 10⁻⁶ Torr. The relationships between ion mass, trap pressure, ion lifetimes, individual ion energies and survival rates were investigated over a 1 s trapping period. Larger ions are more robust to higher pressures. While the trapping lifetimes of smaller ions decrease with increasing pressure, enough survive long enough for accurate mass measurements to be made. Some ions are lost because collisional dampening decreases their energies below the minimum stability threshold of the trap, but others with sufficient energy are still lost due to collision-induced scattering that moves the ions too far from the central trapping axis.