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Title: Distortion of Ion Structures by Field Asymmetric Waveform Ion Mobility Spectrometry

Abstract

Field asymmetric waveform ion mobility spectrometry (FAIMS) is emerging as a major analytical tool, especially in conjunction with mass spectrometry (MS) and/or conventional ion mobility spectrometry (IMS). In particular, FAIMS is used to separate protein or peptide conformers prior to characterization by IMS, MS/MS, or H/D exchange. High electric fields in FAIMS induce ion heating, previously estimated at <10 0C on average and believed too weak to affect ion geometries. Here we use a FAIMS/IMS/MS system to compare the IMS spectra for ESI-generated ubiquitin ions that have and have not passed FAIMS, and find that some unfolding occurs for all charge states. The analysis of those data and their comparison with reported protein unfolding in a Paul trap indicate that at least some structural transitions observed in FAIMS, or previously in an ion trap, are not spontaneous. The observed unfolding is overall similar to that produced by heating of ~40 - 50 0C above room temperature, consistent with the calculated heating of ions at FAIMS waveform peaks. Hence the isomerization in FAIMS likely proceeds in steps during “hot” periods, especially right after ions entering the device. That process distorts ion geometries and causes ion losses by a “self-cleaning” mechanism, andmore » thus should be suppressed as much as possible. We propose achieving that via cooling FAIMS by the amount of ion heating; in most relevant cases cooling by ~75 0C should suffice.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
901751
Report Number(s):
PNNL-SA-50928
20496; 400412000
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Analytical Chemistry, 79(4):1523-1528; Journal Volume: 79; Journal Issue: 4
Country of Publication:
United States
Language:
English
Subject:
FAIMS, IMS, protein structure; Environmental Molecular Sciences Laboratory

Citation Formats

Shvartsburg, Alexandre A., Li, Fumin, Tang, Keqi, and Smith, Richard D. Distortion of Ion Structures by Field Asymmetric Waveform Ion Mobility Spectrometry. United States: N. p., 2007. Web. doi:10.1021/ac061306c.
Shvartsburg, Alexandre A., Li, Fumin, Tang, Keqi, & Smith, Richard D. Distortion of Ion Structures by Field Asymmetric Waveform Ion Mobility Spectrometry. United States. doi:10.1021/ac061306c.
Shvartsburg, Alexandre A., Li, Fumin, Tang, Keqi, and Smith, Richard D. Thu . "Distortion of Ion Structures by Field Asymmetric Waveform Ion Mobility Spectrometry". United States. doi:10.1021/ac061306c.
@article{osti_901751,
title = {Distortion of Ion Structures by Field Asymmetric Waveform Ion Mobility Spectrometry},
author = {Shvartsburg, Alexandre A. and Li, Fumin and Tang, Keqi and Smith, Richard D.},
abstractNote = {Field asymmetric waveform ion mobility spectrometry (FAIMS) is emerging as a major analytical tool, especially in conjunction with mass spectrometry (MS) and/or conventional ion mobility spectrometry (IMS). In particular, FAIMS is used to separate protein or peptide conformers prior to characterization by IMS, MS/MS, or H/D exchange. High electric fields in FAIMS induce ion heating, previously estimated at <10 0C on average and believed too weak to affect ion geometries. Here we use a FAIMS/IMS/MS system to compare the IMS spectra for ESI-generated ubiquitin ions that have and have not passed FAIMS, and find that some unfolding occurs for all charge states. The analysis of those data and their comparison with reported protein unfolding in a Paul trap indicate that at least some structural transitions observed in FAIMS, or previously in an ion trap, are not spontaneous. The observed unfolding is overall similar to that produced by heating of ~40 - 50 0C above room temperature, consistent with the calculated heating of ions at FAIMS waveform peaks. Hence the isomerization in FAIMS likely proceeds in steps during “hot” periods, especially right after ions entering the device. That process distorts ion geometries and causes ion losses by a “self-cleaning” mechanism, and thus should be suppressed as much as possible. We propose achieving that via cooling FAIMS by the amount of ion heating; in most relevant cases cooling by ~75 0C should suffice.},
doi = {10.1021/ac061306c},
journal = {Analytical Chemistry, 79(4):1523-1528},
number = 4,
volume = 79,
place = {United States},
year = {Thu Feb 15 00:00:00 EST 2007},
month = {Thu Feb 15 00:00:00 EST 2007}
}
  • Field asymmetric waveform ion mobility spectrometry (FAIMS) has emerged as an analytical tool of broad utility, especially in conjunction with mass spectrometry. Of particular promise is the use of FAIMS and 2-D ion mobility methods that combine it with conventional IMS to resolve and characterize protein and other macromolecular conformers. However, FAIMS operation requires high electric fields and ions are inevitably heated by above-thermal collisions with buffer gas molecules. This may induce ion isomerization and dissociation that distort separation properties determined by FAIMS and subsequent stages and/or reduce instrumental sensitivity. As FAIMS employs a periodic waveform, the ion temperature canmore » be characterized at the maximum or average field intensity (E). Which method is most applicable to temperature sensitive ions, such as protein ions, has been debated. Here we address this issue by measuring the unfolding of compact ubiquitin ion geometries as a function of waveform amplitude (dispersion field, ED) and gas temperature, T. The field heating is quantified by matching the dependences of structural transitions on ED and T. Increasing ED from 12 to 16 or from 16 to 20 kV/cm is equivalent to heating the (N2) gas by ~15 - 25 oC. The magnitude of field heating for any E can be calculated using the two-temperature theory, and raising ED by 4 kV/cm augments heating by ~15 - 30 oC for maximum and ~4 - 8 oC for average E in the FAIMS cycle. Hence, isomerization of ions in FAIMS appears to be governed by the maximum internal temperature at waveform peaks.« less
  • Approaches to characterization and separation of ions involving their mobilities in gases were developed since 1960-s. Conventional ion mobility spectrometry (IMS) measures the absolute mobility and the field asymmetric waveform IMS (FAIMS) exploits the difference between mobilities at high and low electric fields. However, all previous work was based on the orientationally averaged cross-sections Ωavg between ions and buffer gas molecules. Virtually all large ions are electric dipoles that will be oriented by a sufficiently strong electric field. At typical FAIMS conditions, that must happen for dipole moments > ~400 Debye, found for many macroions including most proteins above ~30more » kDa. Mobilities of aligned dipoles depend on directional cross-sections Ωdir (rather than Ωavg), which should have a major effect on FAIMS separation parameters. Here we study the FAIMS behavior of ESI-generated ions for ten proteins up to ~70 kDa. Those above 29 kDa exhibit a strong increase of mobility at high field, which is consistent with predicted ion dipole alignment. This effect expands the FAIMS peak capacity by an order of magnitude, allowing separation of up to ~102 distinct protein conformers and revealing information about Ωdir and ion dipole moment that is of potential utility for structural characterization. Possible means to extend the dipole alignment to smaller ions are discussed.« less
  • Field asymmetric waveform ion mobility spectrometry (FAIMS) has emerged as a powerful tool of broad utility for separation and characterization of gas-phase ions, especially in conjunction with mass spectrometry (MS). In FAIMS, ions are filtered by the dependence of mobility on electric field while carried by gas flow through the analytical gap between two electrodes of either planar (p-) or cylindrical (c-) geometry. Most FAIMS/MS systems employ c-FAIMS because of its ease of coupling to MS, yet the merits of two geometries have not been compared in detail. Here, a priori simulations have revealed that reducing the FAIMS curvature alwaysmore » improves resolution at equal sensitivity. In particular, the resolving power of p-FAIMS exceeds that of c-FAIMS, typically by a factor of 2 - 4 depending on the ion species and carrier gas. We have constructed a new planar FAIMS that incorporates a curtain plate interface for effective operation with an ESI ion source and is joined to MS using an ion funnel interface with a novel slit aperture. The resolution increases up to 4-fold over existing c-FAIMS, even though the analysis is ~3 times faster. This allows separation of species not feasible in previous FAIMS studies, e.g., protonated leucine and isoleucine or new bradykinin isomers. The improvement for protein conformers (of ubiquitin) is less significant, possibly because of multiple unresolved geometries.« less
  • Field Asymmetric waveform Ion Mobility Spectrometry (FAIMS) has significant potential for post-ionization separations in conjunction with MS analyses. FAIMS exploits the fact that ion mobilities in gases depend on the electric field in a manner specific to each ion, which allows one to fractionate ion mixtures. Nearly all previous work has used pure gases, for which FAIMS fundamentals are understood reasonably well. However, experiments in gas mixtures like N2/CO2 have uncovered unexpected phenomena that remained unexplained. Here we introduce a universal model for FAIMS separations in mixtures, derived from the formalisms that determine high-field mobilities in heteromolecular gases. Overall, themore » theoretical findings are consistent with the data in N2/CO2, though quantitative discrepancies remain. As a control, modeled results for N2/O2 fit Blanc's law, in agreement with measurements. Calculations for He/N2 are broadly consistent with observations, and show why adding He to the working gas enhances FAIMS performance. We predict spectacular non-Blanc effects in mixtures of extremely disparate gases such as He/CO2, which could improve the peak capacity and sensitivity of technique. Understanding the FAIMS operation in gas mixtures will enable rational design of media for both targeted and global analyses.« less