15 research outputs found
Recommendations for reporting ion mobility mass spectrometry measurements
© 2019 The Authors. Mass Spectrometry Reviews Published by Wiley Periodicals, Inc. Here we present a guide to ion mobility mass spectrometry experiments, which covers both linear and nonlinear methods: what is measured, how the measurements are done, and how to report the results, including the uncertainties of mobility and collision cross section values. The guide aims to clarify some possibly confusing concepts, and the reporting recommendations should help researchers, authors and reviewers to contribute comprehensive reports, so that the ion mobility data can be reused more confidently. Starting from the concept of the definition of the measurand, we emphasize that (i) mobility values (K0) depend intrinsically on ion structure, the nature of the bath gas, temperature, and E/N; (ii) ion mobility does not measure molecular surfaces directly, but collision cross section (CCS) values are derived from mobility values using a physical model; (iii) methods relying on calibration are empirical (and thus may provide method-dependent results) only if the gas nature, temperature or E/N cannot match those of the primary method. Our analysis highlights the urgency of a community effort toward establishing primary standards and reference materials for ion mobility, and provides recommendations to do so. © 2019 The Authors. Mass Spectrometry Reviews Published by Wiley Periodicals, Inc
Correlating Resolving Power, Resolution, and Collision Cross Section: Unifying Cross-Platform Assessment of Separation Efficiency in Ion Mobility Spectrometry
Here we examine the
relationship among resolving power (<i>R</i><sub>p</sub>), resolution (<i>R</i><sub>pp</sub>), and collision cross
section (CCS) for compounds analyzed in previous
ion mobility (IM) experiments representing a wide variety of instrument
platforms and IM techniques. Our previous work indicated these three
variables effectively describe and predict separation efficiency for
drift tube ion mobility spectrometry experiments. In this work, we
seek to determine if our previous findings are a general reflection
of IM behavior that can be applied to various instrument platforms
and mobility techniques. Results suggest IM distributions are well
characterized by a Gaussian model and separation efficiency can be
predicted on the basis of the empirical difference in the gas-phase
CCS and a CCS-based resolving power definition (CCS/ΔCCS). Notably
traveling wave (TWIMS) was found to operate at resolutions substantially
higher than a single-peak resolving power suggested. When a CCS-based <i>R</i><sub>p</sub> definition was utilized, TWIMS was found to
operate at a resolving power between 40 and 50, confirming the previous
observations by Giles and co-workers. After the separation axis (and
corresponding resolving power) is converted to cross section space,
it is possible to effectively predict separation behavior for all
mobility techniques evaluated (i.e., uniform field, trapped ion mobility,
traveling wave, cyclic, and overtone instruments) using the equations
described in this work. Finally, we are able to establish for the
first time that the current state-of-the-art ion mobility separations
benchmark at a CCS-based resolving power of >300 that is sufficient
to differentiate analyte ions with CCS differences as small as 0.5%
Investigation of the Complete Suite of the Leucine and Isoleucine Isomers: Toward Prediction of Ion Mobility Separation Capabilities
In this study we
investigated 11 isomers with the molecular formula
C<sub>6</sub>H<sub>13</sub>NO<sub>2</sub> (<i>m</i>/<i>z</i> 131) to ascertain the potential of utilizing drift tube
ion mobility mass spectrometry to aid in the separation of isomeric
mixtures. This study of small molecules provides a detailed examination
of the application of uniform field ion mobility for a narrow scope
of isomers with variations in both bond coordination and stereochemistry.
For small molecules, it was observed that in general constitutional
isomers are more readily separated by uniform field mobility in comparison
to stereoisomers such as enantiomers or diastereomers. Diastereomers
exhibited differences in their collision cross section (CCS), but
were unresolvable in a mixture, whereas the enantiomers studied did
not exhibit statistically different CCS values. A mathematical relationship
relating the CCS to resolving power was developed in order to predict
the required ion mobility resolving power needed to separate the various
isomer classes. For the majority of isomers evaluated in this study,
a uniform field-based resolving power of 100 was predicted to be sufficient
to resolve over half (∼60%) of all hypothetical isomer pairs,
including leucine and isoleucine, whereas their stereoisomers (d- and l-forms) are predicted to be significantly more
challenging, if not impossible, to separate by conventional drift
tube techniques
Evaluation of Collision Cross Section Calibrants for Structural Analysis of Lipids by Traveling Wave Ion Mobility-Mass Spectrometry
Collision cross section (CCS) measurement
of lipids using traveling
wave ion mobility-mass spectrometry (TWIM-MS) is of high interest
to the lipidomics field. However, currently available calibrants for
CCS measurement using TWIM are predominantly peptides that display
quite different physical properties and gas-phase conformations from
lipids, which could lead to large CCS calibration errors for lipids.
Here we report the direct CCS measurement of a series of phosphatidylcholines
(PCs) and phosphatidylethanolamines (PEs) in nitrogen using a drift
tube ion mobility (DTIM) instrument and an evaluation of the accuracy
and reproducibility of PCs and PEs as CCS calibrants for phospholipids
against different classes of calibrants, including polyalanine (PolyAla),
tetraalkylammonium salts (TAA), and hexakisÂ(fluoroalkoxy)Âphosphazines
(HFAP), in both positive and negative modes in TWIM-MS analysis. We
demonstrate that structurally mismatched calibrants lead to larger
errors in calibrated CCS values while the structurally matched calibrants,
PCs and PEs, gave highly accurate and reproducible CCS values at different
traveling wave parameters. Using the lipid calibrants, the majority
of the CCS values of several classes of phospholipids measured by
TWIM are within 2% error of the CCS values measured by DTIM. The development
of phospholipid CCS calibrants will enable high-accuracy structural
studies of lipids and add an additional level of validation in the
assignment of identifications in untargeted lipidomics experiments
Solvent Composition Can Have a Measurable Influence on the Ion Mobility-Derived Collision Cross Section of Small Molecules
Ion mobility (IM)
is an important analytical technique for increasing
identification coverage of metabolites in untargeted studies, especially
when integrated into traditional liquid chromatography–mass
spectrometry workflows. While there has been extensive work surrounding
best practices to obtain and standardize collision cross section (CCS)
measurements necessary for comparing across different IM techniques
and laboratories, there has been little investigation into experimental
factors beyond the mobility separation region that could potentially
influence CCS measurements. The first-principles derived CCS of 15
chemical standards were evaluated across 27 aqueous:organic solvent
compositions using a high-precision drift tube instrument. A small
but measurable dependency of the CCS on the solvent composition was
observed, with the larger analytes from this study (m/z > 400) exhibiting a characteristic increase
in
CCS at the intermediate (40–60%) solvent compositions. Parallels
to the behavior of solvent viscosity and protonation site tautomers
(protomers) were noted, although the origin of these solvent-dependent
CCS trends is as yet unclear. Taken together, these findings document
a solvent dependency on CCS, which, while minor (<0.5%), identifies
an important need for reporting the solvent system when utilizing
CCS in comparative ion mobility studies
Determining Double Bond Position in Lipids Using Online Ozonolysis Coupled to Liquid Chromatography and Ion Mobility-Mass Spectrometry
The
increasing focus on lipid metabolism has revealed a need for
analytical techniques capable of structurally characterizing lipids
with a high degree of specificity. Lipids can exist as any one of
a large number of double bond positional isomers, which are indistinguishable
by single-stage mass spectrometry alone. Ozonolysis reactions coupled
to mass spectrometry have previously been demonstrated as a means
for localizing double bonds in unsaturated lipids. Here we describe
an online, solution-phase reactor using ozone produced via a low-pressure
mercury lamp, which generates aldehyde products diagnostic of cleavage
at a particular double bond position. This flow-cell device is utilized
in conjunction with structurally selective ion mobility-mass spectrometry.
The lamp-mediated reaction was found to be effective for multiple
lipid species in both positive and negative ionization modes, and
the conversion efficiency from precursor to product ions was tunable
across a wide range (20–95%) by varying the flow rate through
the ozonolysis device. Ion mobility separation of the ozonolysis products
generated additional structural information and revealed the presence
of saturated species in a complex mixture. The method presented here
is simple, robust, and readily coupled to existing instrument platforms
with minimal modifications necessary. For these reasons, application
to standard lipidomic workflows is possible and aids in more comprehensive
structural characterization of a myriad of lipid species
Determining Double Bond Position in Lipids Using Online Ozonolysis Coupled to Liquid Chromatography and Ion Mobility-Mass Spectrometry
The
increasing focus on lipid metabolism has revealed a need for
analytical techniques capable of structurally characterizing lipids
with a high degree of specificity. Lipids can exist as any one of
a large number of double bond positional isomers, which are indistinguishable
by single-stage mass spectrometry alone. Ozonolysis reactions coupled
to mass spectrometry have previously been demonstrated as a means
for localizing double bonds in unsaturated lipids. Here we describe
an online, solution-phase reactor using ozone produced via a low-pressure
mercury lamp, which generates aldehyde products diagnostic of cleavage
at a particular double bond position. This flow-cell device is utilized
in conjunction with structurally selective ion mobility-mass spectrometry.
The lamp-mediated reaction was found to be effective for multiple
lipid species in both positive and negative ionization modes, and
the conversion efficiency from precursor to product ions was tunable
across a wide range (20–95%) by varying the flow rate through
the ozonolysis device. Ion mobility separation of the ozonolysis products
generated additional structural information and revealed the presence
of saturated species in a complex mixture. The method presented here
is simple, robust, and readily coupled to existing instrument platforms
with minimal modifications necessary. For these reasons, application
to standard lipidomic workflows is possible and aids in more comprehensive
structural characterization of a myriad of lipid species
Structural Characterization of Methylenedianiline Regioisomers by Ion Mobility-Mass Spectrometry, Tandem Mass Spectrometry, and Computational Strategies. 3. MALDI Spectra of 2-Ring Isomers
Characterization of methylenedianiline
(MDA) 2-ring isomers (2,2′-,
2,4′-, and 4,4′-MDA) is reported using matrix assisted
laser desorption/ionization–mass spectrometry (MALDI-MS), a
common technique used for characterizing synthetic polymers. MDA is
a precursor to methylene diphenyl diisocyanate (MDI), a hard block
component in polyurethane (PUR) synthesis. This work focuses on comparing
MALDI results to those of our previous electrospray ionization-mass
spectrometry (ESI-MS) studies. In ESI, 2-ring MDA isomers formed single
unique [M + H]<sup>+</sup> (199 Da) parent ions, whereas in MALDI
each isomer shows significant formation of three precursor ions: [M
– H]<sup>+</sup> = 197 Da, [M<sup>•</sup>]<sup>+</sup> = 198 Da, and [M + H]<sup>+</sup> = 199 Da. Structures and schemes
are proposed for the MALDI fragment ions associated with each precursor
ion. Ion mobility–mass spectrometry (IM-MS), tandem mass spectrometry
(MS/MS), and computational methods were all critical in determining
the structures for both precursor and fragment ions as well as the
fragmentation mechanisms. The present study indicates that the [M
– H]<sup>+</sup> and [M<sup>•</sup>]<sup>+</sup> ions
are formed by the MALDI process, explaining why they were not observed
with ESI
Structural Characterization of Methylenedianiline Regioisomers by Ion Mobility-Mass Spectrometry, Tandem Mass Spectrometry, and Computational Strategies: I. Electrospray Spectra of 2‑Ring Isomers
Purified methylenedianiline (MDA)
regioisomers were structurally
characterized and differentiated using tandem mass spectrometry (MS/MS),
ion mobility-mass spectrometry (IM-MS), and IM-MS/MS in conjunction
with computational methods. It was determined that protonation sites
on the isomers can vary depending on the position of amino groups,
and the resulting protonation sites play a role in the gas-phase stability
of the isomer. We also observed differences in the relative distributions
of protonated conformations depending on experimental conditions and
instrumentation, which is consistent with previous studies on aniline
in the gas phase. This work demonstrates the utility of a multifaceted
approach for the study of isobaric species and elucidates why previous
MDA studies may have been unable to detect and/or differentiate certain
isomers. Such analysis may prove useful in the characterization of
larger MDA multimeric species, industrial MDA mixtures, and methylene
diphenyl diisocyanate (MDI) mixtures used in polyurethane synthesis
Structural Characterization of Methylenedianiline Regioisomers by Ion Mobility-Mass Spectrometry, Tandem Mass Spectrometry, and Computational Strategies. 2. Electrospray Spectra of 3‑Ring and 4‑Ring Isomers
Building on results from our previous
study of 2-ring methylenedianiline
(MDA), a combined mass spectrometry approach utilizing ion mobility-mass
spectrometry (IM-MS) and tandem mass spectrometry (MS/MS) coupled
with computational methods enables the structural characterization
of purified 3-ring and 4-ring MDA regioisomers in this current study.
The preferred site of protonation for the 3-ring and 4-ring MDA was
determined to be on the amino groups. Additionally, the location of
the protonated amine along the MDA multimer was found to influence
the gas phase stability of these molecules. Fragmentation mechanisms
similar to the 2-ring MDA species were observed for both the 3-ring
and 4-ring MDA. The structural characterization of 3-ring and 4-ring
MDA isomers using modern MS techniques may aid polyurethane synthesis
by the characterization of industrial grade MDA, multimeric MDA species,
and methylene diphenyl diisocyanate (MDI) mixtures