Multi-CRAFTI: Relative Collision Cross Sections from Fourier Transform Ion Cyclotron Resonance Mass Spectrometric Line Width Measurements

Abstract

Determination of collision cross sections (CCS) using the cross-sectional areas by the Fourier transform ion cyclotron resonance (CRAFTI) technique is limited by the requirement that accurate pressures in the trapping cell of the mass spectrometer must be known. Experiments must also be performed in the energetic hardsphere regime such that ions decohere after single collisions with neutrals; this limits application to ions that are not much more massive than the neutrals. To mitigate these problems, we have resonantly excited two (or more) ions of different m/z to the same center-of-mass kinetic energy in a single experiment, subjecting them to identical neutral pressures. We term this approach "multi-CRAFTI". This facilitates measurement of relative CCS without requiring knowledge of the pressure and enables determination of absolute CCS using internal standards. Experiments with tetraalkylammonium ions yield CCS in reasonable agreement with the one-ion-at-a-time CRAFTI approach and with ion mobility spectrometry (IMS) when differences in collision energetics are taken into account (multi-CRAFTI generally yields smaller CCS than does IMS due to the higher collision energies employed in multi-CRAFTI). Comparison of multi-CRAFTI and IMS results with CCS calculated from structures computed at the M06-2X/6-31+G* level of theory using projection approximation or trajectory method values, respectively, indicates that the computed structures have CCS increasingly smaller than the experimental CCS as m/z increases, implying the computational model overestimates interactions between the alkyl arms. For ions that undergo similar collisional decoherence processes, relative CCS reach constant values at lower collision energies than do absolute CCS values, suggesting a means of increasing the accessible upper m/z limit by employing multi-CRAFTI.

Collision Cross-Section Measurements of Collision-Induced Dissociation Precursor and Product Ions in an FTICR-MS and an IM-MS: A Comparative Study

“…The CRAFTI sequence then started by leaking argon gas into the trapping cell again and using resonant RF excitation for ion detection. The resonant excite was the sum of two single frequency waveforms, designed to excite two ions simultaneously in a strategy known as multi-CRAFTI . Ion-neutral collisions shortened the time domain signal, causing frequency peak broadening.…”

“…The resonant excite was the sum of two single frequency waveforms, designed to excite two ions simultaneously in a strategy known as multi-CRAFTI. 16 Ionneutral collisions shortened the time domain signal, causing frequency peak broadening. The fwhm of both ion peaks were extracted and used in eq 1 to calculate the two ions' relative σ.…”

Ion Mobility and Fourier Transform Ion Cyclotron Resonance Collision Cross Section Techniques Yield Long-Range and Hard-Sphere Results, Respectively

“…Data were analyzed using the Igor Pro software package (version 7, Wavemetrics Inc.; Lake Oswego, OR) to collect a set of power spectra measured at various collision gas pressures at each collision energy for each of the ion pairs. The collision cross section ratio of the two ions ( ) (where CCS 1 = singly charged ion, and CCS 2 = doubly charged ion) was determined from the fwhm line widths of the ions using eq : …”

“…Because relative CCS values are the focus here, we report all results in terms of ratios in the form CCS[host+2M] 2+ /CCS[host+M] + . Absolute CCS values can be obtained by comparison with standards of known CCS 23 (and are reported in the Supporting Information), but are not required for the comparisons that follow.…”

“…In this study, we measure collision cross sections of CB [5], mc5, CB* [5], CB [6], and CB [7] complexed with alkali metal ions using both IM-MS and the "multi-CRAFTI" 23 technique (multi-CRAFTI is a method that performs CRAFTI on two or more ions at the same time, under identical pressure conditions at nearly identical center-of-mass kinetic energies). We compare the results obtained using the two techniques, and also compare the experimental results with computational methods using density functional theory for geometry optimization coupled with cross section calculations.…”