37 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
A Dual-Column Solid Phase Extraction Strategy for Online Collection and Preparation of Continuously Flowing Effluent Streams for Mass Spectrometry
Current desalination techniques for mass spectrometry-based
protocols
are problematic for performing temporal response studies where increased
temporal resolution requires small samples and faster sampling frequencies,
which greatly increases the number of samples and sample preparation
time. These challenges are pertinent to cellular dynamics experiments,
where it is important to sample the biological system frequently and
with as little sample waste as possible. To address these needs, we
present a dual-column online solid phase extraction (SPE) approach
capable of preconcentrating and preparing a constantly perfusing sample
stream, with minimal to no sample loss. This strategy is evaluated
for use in microfluidic bioreactor studies specifically aimed at characterizing
suitable sample flow rates, temporal resolving power, and analyte
concentrations. In this work, we demonstrate that this strategy may
be used for flow rates as low as 500 nL/min, with temporal resolving
power on the order of 3 min, with analyte loadings ranging from femtomoles
to picomoles for metabolites. Under these conditions, recoveries of
ca. 80% are obtained even at femtomole loadings
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
Real-Time Cellular Exometabolome Analysis with a Microfluidic-Mass Spectrometry Platform
<div><p>To address the challenges of tracking the multitude of signaling molecules and metabolites that is the basis of biological complexity, we describe a strategy to expand the analytical techniques for dynamic systems biology. Using microfluidics, online desalting, and mass spectrometry technologies, we constructed and validated a platform well suited for sampling the cellular microenvironment with high temporal resolution. Our platform achieves success in: automated cellular stimulation and microenvironment control; reduced non-specific adsorption to polydimethylsiloxane due to surface passivation; real-time online sample collection; near real-time sample preparation for salt removal; and real-time online mass spectrometry. When compared against the benchmark of “in-culture” experiments combined with ultraperformance liquid chromatography-electrospray ionization-ion mobility-mass spectrometry (UPLC-ESI-IM-MS), our platform alleviates the volume challenge issues caused by dilution of autocrine and paracrine signaling and dramatically reduces sample preparation and data collection time, while reducing undesirable external influence from various manual methods of manipulating cells and media (<i>e.g.</i>, cell centrifugation). To validate this system biologically, we focused on cellular responses of Jurkat T cells to microenvironmental stimuli. Application of these stimuli, in conjunction with the cell’s metabolic processes, results in changes in consumption of nutrients and secretion of biomolecules (collectively, the exometabolome), which enable communication with other cells or tissues and elimination of waste. Naïve and experienced T-cell metabolism of cocaine is used as an exemplary system to confirm the platform’s capability, highlight its potential for metabolite discovery applications, and explore immunological memory of T-cell drug exposure. Our platform proved capable of detecting metabolomic variations between naïve and experienced Jurkat T cells and highlights the dynamics of the exometabolome over time. Upregulation of the cocaine metabolite, benzoylecgonine, was noted in experienced T cells, indicating potential cellular memory of cocaine exposure. These metabolomics distinctions were absent from the analogous, traditional “in-culture” UPLC-ESI-IM-MS experiment, further demonstrating this platform’s capabilities.</p></div
Mapping Microbial Response Metabolomes for Induced Natural Product Discovery
Intergeneric
microbial interactions may originate a significant
fraction of secondary metabolic gene regulation in nature. Herein,
we expose a genomically characterized <i>Nocardiopsis</i> strain, with untapped polyketide biosynthetic potential, to intergeneric
interactions via coculture with low inoculum exposure to <i>Escherichia</i>, <i>Bacillus</i>, <i>Tsukamurella</i>, and <i>Rhodococcus</i>. The challenge-induced responses of extracted
metabolites were characterized via multivariate statistical and self-organizing
map (SOM) analyses, revealing the magnitude and selectivity engendered
by the limiting case of low inoculum exposure. The collected inventory
of cocultures revealed substantial metabolomic expansion in comparison
to monocultures with nearly 14% of metabolomic features in cocultures
undetectable in monoculture conditions and many features unique to
coculture genera. One set of SOM-identified responding features was
isolated, structurally characterized by multidimensional NMR, and
revealed to comprise previously unreported polyketides containing
an unusual pyrrolidinol substructure and moderate and selective cytotoxicity.
Designated ciromicin A and B, they are detected across mixed cultures
with intergeneric preferences under coculture conditions. The structural
novelty of ciromicin A is highlighted by its ability to undergo a
diastereoselective photochemical 12-π electron rearrangement
to ciromicin B at visible wavelengths. This study shows how organizing
trends in metabolomic responses under coculture conditions can be
harnessed to characterize multipartite cultures and identify previously
silent secondary metabolism
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 hexakis(fluoroalkoxy)phosphazines
(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
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
Mapping Microbial Response Metabolomes for Induced Natural Product Discovery
Intergeneric
microbial interactions may originate a significant
fraction of secondary metabolic gene regulation in nature. Herein,
we expose a genomically characterized <i>Nocardiopsis</i> strain, with untapped polyketide biosynthetic potential, to intergeneric
interactions via coculture with low inoculum exposure to <i>Escherichia</i>, <i>Bacillus</i>, <i>Tsukamurella</i>, and <i>Rhodococcus</i>. The challenge-induced responses of extracted
metabolites were characterized via multivariate statistical and self-organizing
map (SOM) analyses, revealing the magnitude and selectivity engendered
by the limiting case of low inoculum exposure. The collected inventory
of cocultures revealed substantial metabolomic expansion in comparison
to monocultures with nearly 14% of metabolomic features in cocultures
undetectable in monoculture conditions and many features unique to
coculture genera. One set of SOM-identified responding features was
isolated, structurally characterized by multidimensional NMR, and
revealed to comprise previously unreported polyketides containing
an unusual pyrrolidinol substructure and moderate and selective cytotoxicity.
Designated ciromicin A and B, they are detected across mixed cultures
with intergeneric preferences under coculture conditions. The structural
novelty of ciromicin A is highlighted by its ability to undergo a
diastereoselective photochemical 12-π electron rearrangement
to ciromicin B at visible wavelengths. This study shows how organizing
trends in metabolomic responses under coculture conditions can be
harnessed to characterize multipartite cultures and identify previously
silent secondary metabolism