Determination of Collision Cross Sections Using a Fourier Transform Electrostatic Linear Ion Trap Mass Spectrometer

Abstract

Collision cross sections (CCSs) were determined from the frequency-domain linewidths in a Fourier transform electrostatic linear ion trap. With use of an ultrahigh-vacuum precision leak valve and nitrogen gas, transients were recorded as the background pressure in the mass analyzer chamber was varied between 4× 10 and 7 × 10 Torr. The energetic hard-sphere ion-neutral collision model, described by Xu and coworkers, was used to relate the recorded image charge to the CCS of the molecule. In lieu of our monoisotopically isolating the mass of interest, the known relative isotopic abundances were programmed into the Lorentzian fitting algorithm such that the linewidth was extracted from a sum of Lorentzians. Although this works only if the isotopic distribution is known a priori, it prevents ion loss, preserves the high signal-to-noise ratio, and minimizes the experimental error on our homebuilt instrument. Six tetraalkylammonium cations were used to correlate the CCS measured in the electrostatic linear ion trap with that measured by drift-tube ion mobility spectrometry, for which there was an excellent correlation (R ≈ 0.9999). Although the absolute CCSs derived with our method differ from those reported, the extracted linear correlation can be used to correct the raw CCS. With use of [angiotensin II] and reserpine, the corrected CCSs (334.9 ± 2.1 and 250.1 ± 0.5, respectively) were in good agreement with the reported ion mobility spectrometry CCSs (335 and 254.3, respectively). With sufficient signal-to-noise ratio, the CCSs determined are reproducible to within a fraction of a percent, comparable to the uncertainties reported on dedicated ion mobility instruments. Graphical Abstract ᅟ.

Recommendations for reporting ion mobility Mass Spectrometry measurements

“…For these reasons, we devoted special attention to define K 0 and IM‐derived CCS values, and to clarify what influences these quantities. There are other methods than ion mobility spectrometry to determine experimental collision cross sections in a more direct way, for example, by monitoring the loss of ion signal due to collisions (Covey & Douglas, ; Chen, Collings, & Douglas, ; Anupriya, Jones, & Dearden, ; Anupriya et al, ; Dziekonski et al, ; Elliott et al, ; Sanders et al, ). These methods to determine collision cross sections are however not based on ion mobility measurements, and will not be covered here.…”

Fundamentals of ion mobility spectrometry

“…While the terms tend to be used interchangeably in IMS, they are in fact not identical in a wider context. Scattering or dephasing measurements carried out at very low pressures, wherein collisions eject the ions from stable trajectories [3][4][5][6], allow to determine true scattering collision cross sections, which can be adequately calculated by a projection approximation. Ion mobility is different: we still detect the ions after they had undergone collisions.…”

Determination of Collision Cross-Sections of Protein Ions in an Orbitrap Mass Analyzer

“…More recently, other mass spectrometers have been utilized for CCS measurements. For instance, CCSs of several tetraalkylammonium cations and small peptides were measured from spectral line widths obtained on a home-built FT electrostatic linear ion trap (FT-ELIT) with impressive accuracy . In another study, a charge-detection mass spectrometer (CDMS) was used to measure CCSs of proteins as large as bovine serum albumin .…”