Quantitative Collision Cross-Sections from FTICR Linewidth Measurements: Improvements in Theory and Experiment

Abstract

Two corrections to the equation used in the cross-sectional areas by Fourier transform ion cyclotron resonance ("CRAFTI") technique are identified. In CRAFTI, ion collision cross-sections are obtained from the pressure-dependent ion linewidths in Fourier transform mass spectra. The effects of these corrections on the accuracy of the cross-sections obtained using the CRAFTI technique are evaluated experimentally using the 20 biogenic amino acids and several crown ether complexes with protonated alkyl monoamines. Good absolute agreement is obtained between the CRAFTI cross-sections and the corresponding cross-sections obtained using both static drift ion mobility spectrometry and computational simulations. These results indicate that the CRAFTI cross-sections obtained using the updated equation presented here are quantitatively descriptive of the size and shape of the gas-phase ions. Cross-sections that differ by less than 3% are measured for the isobaric isomers n-butylamine and tert-butylamine complexed with the crown ethers. This level of precision is similar to what has been achieved previously using traveling wave ion mobility devices. These results indicate that CRAFTI can be used to probe subtle structural differences between ions with approximately the same precision as that achieved in traveling wave ion mobility devices. Graphical Abstract ᅟ.

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

“…The correlation between an ion’s CCS and the rate at which the time-domain signal decays in ICR mass analyzers has long been recognized, − and several methods have recently capitalized on this relationship . For example, CCS measurements of crown ethers and amino acids have been made based on the frequency domain spectral line widths obtained under elevated pressure conditions; a method dubbed “CRAFTI” (cross-sectional areas by Fourier transform ion cyclotron resonance). − A similar technique, performed using a custom-built FT-ICR featuring a more powerful magnet (9.4 T instead of 4.7 T), was used to measure CCS of biomolecules up to the size of ubiquitin (8.5 kDa), which to our knowledge is the largest analyte measured by this type of method so far . This approach directly measured the decay rate of the time-domain transient signal by performing a series of digital low-pass filtering and down-sampling steps followed by fitting an exponential decay function to the resulting decay profile. , …”

CHAPTER 1. Ion Mobility–Mass Spectrometry: an Overview

“…For example, techniques wherein each collision results in 100% efficient ion scattering away from its initial trajectory can directly probe collision cross-sections in the IUPAC sense (). This is the case of CRAFTI 14 (cross-sectional areas by Fourier transform ion cyclotron resonance), frequency shifts in charge detection mass spectrometry, 15 or ion dephasing in orbitraps. 16 However, the IUPAC definition is problematic for ion mobility spectrometry measurements described herein, as explained in the next paragraph.…”