7 research outputs found
Recommendations for reporting ion mobility mass spectrometry measurements
Ā© 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. Ā© 2019 The Authors. Mass Spectrometry Reviews Published by Wiley Periodicals, Inc
Ion Mobilities in Diatomic Gases: Measurement versus Prediction with Non-Specular Scattering Models
Ion/electrical mobility
measurements of nanoparticles and polyatomic
ions are typically linked to particle/ion physical properties through
either application of the StokesāMillikan relationship or comparison
to mobilities predicted from polyatomic models, which assume that
gas molecules scatter specularly and elastically from rigid structural
models. However, there is a discrepancy between these approaches;
when specular, elastic scattering models (i.e., elastic-hard-sphere
scattering, EHSS) are applied to polyatomic models of nanometer-scale
ions with finite-sized impinging gas molecules, predictions are in
substantial disagreement with the StokesāMillikan equation.
To rectify this discrepancy, we developed and tested a new approach
for mobility calculations using polyatomic models in which non-specular
(diffuse) and inelastic gas-molecule scattering is considered. Two
distinct semiempirical models of gas-molecule scattering from particle
surfaces were considered. In the first, which has been traditionally
invoked in the study of aerosol nanoparticles, 91% of collisions are
diffuse and thermally accommodating, and 9% are specular and elastic.
In the second, all collisions are considered to be diffuse and accommodating,
but the average speed of the gas molecules reemitted from a particle
surface is 8% lower than the mean thermal speed at the particle temperature.
Both scattering models attempt to mimic exchange between translational,
vibrational, and rotational modes of energy during collision, as would
be expected during collision between a nonmonoatomic gas molecule
and a nonfrozen particle surface. The mobility calculation procedure
was applied considering both hard-sphere potentials between gas molecules
and the atoms within a particle and the long-range ionāinduced
dipole (polarization) potential. Predictions were compared to previous
measurements in air near room temperature of multiply charged polyĀ(ethylene
glycol) (PEG) ions, which range in morphology from compact to highly
linear, and singly charged tetraalkylammonium cations. It was found
that both non-specular, inelastic scattering rules lead to excellent
agreement between predictions and experimental mobility measurements
(within 5% of each other) and that polarization potentials must be
considered to make correct predictions for high-mobility particles/ions.
Conversely, traditional specular, elastic scattering models were found
to substantially overestimate the mobilities of both types of ions
IMSāMS and IMSāIMS Investigation of the Structure and Stability of Dimethylamine-Sulfuric Acid Nanoclusters
Recent
studies of new particle formation events in the atmosphere
suggest that nanoclusters (i.e, the species formed during the early
stages of particle growth which are composed of 10<sup>1</sup>ā10<sup>3</sup> molecules) may consist of amines and sulfuric acid. The physicochemical
properties of sub-10 nm amine-sulfuric acid clusters are hence of
interest. In this work, we measure the density, thermostability, and
extent of water uptake of <8.5 nm effective diameter dimethylamine-sulfuric
(DMAS) nanoclusters in the gas phase, produced via positive electrospray
ionization. Specifically, we employ three systems to investigate DMAS
properties: ion mobility spectrometry (IMS, with a parallel-plate
differential mobility analyzer) is coupled with mass spectrometry
to measure masses and collision cross sections for <100 kDa positively
charged nanoclusters, two differential mobility analyzers in series
(IMSāIMS) are used to examine thermostability, and finally
a differential mobility analyzer coupled to an atmospheric pressure
drift tube ion mobility spectrometer (also IMSāIMS) is used
for water uptake measurements. IMSāMS measurements reveal that
dry DMAS nanoclusters have densities of ā¼1567 kg/m<sup>3</sup> near 300 K, independent of the ratio of dimethylamine to sulfuric
acid originally present in the electrospray solution. IMSāIMS
thermostability studies reveal that partial pressures of DMAS nanoclusters
are dependent upon the electrospray solution concentration ratio, <i>R</i> = [H<sub>2</sub>SO<sub>4</sub>]/[(CH<sub>3</sub>)<sub>2</sub>NH]. Extrapolating measurements, we estimate that dry DMAS
nanoclusters have surface vapor pressures of order 10<sup>ā4</sup> Pa near 300 K, with the surface vapor pressure increasing with increasing
values of <i>R</i> through most of the probed concentration
range. This suggests that nanocluster surface vapor pressures are
substantially enhanced by capillarity effects (the Kelvin effect).
Meanwhile, IMSāIMS water uptake measurements show clearly that
DMAS nanoclusters uptake water at relative humidities beyond 10% near
300 K, and that larger clusters uptake water to a larger extent. In
total, our results suggest that dry DMAS nanoclusters (in the 5ā8.5
nm size range in diameter) would not be stable under ambient conditions;
however, DMAS nanoclusters would likely be hydrated in the ambient
(in some cases above 20% water by mass), which could serve to reduce
surface vapor pressures and stabilize them from dissociation
Tuning Mobility Separation Factors of Chemical Warfare Agent Degradation Products via Selective Ion-Neutral Clustering
Combining
experimental data with computational modeling, we illustrate
the capacity of selective gas-phase interactions using neutral gas
vapors to yield an additional dimension of gas-phase ion mobility
separation. Not only are the mobility shifts as a function of neutral
gas vapor concentration reproducible, but also the selective alteration
of mobility separation factors is closely linked to existing chemical
functional groups. Such information may prove advantageous in elucidating
chemical class and resolving interferences. Using a set of chemical
warfare agent simulants with nominally the same reduced mobility values
as a test case, we illustrate the ability of the drift-gas doping
approach to achieve separation of these analytes. In nitrogen, protonated
forms of dimethyl methyl phosphonate (DMMP) and methyl phosphonic
acid (MPA) exhibit the reduced mobility values of 1.99 Ā± 0.01
cm<sup>2</sup> V<sup>ā1</sup>s<sup>ā1</sup> at 175 Ā°C.
However, when the counter current drift gas of the system is doped
with 2-propanol at 20 Ī¼L/h, full baseline resolution of the
two species is possible. By varying the concentration of the neutral
modifier, the separation factor of the respective clusters can be
adjusted. For the two species examined and at a 2-propanol flow rate
of 160 Ī¼L/h, MPA demonstrated the greatest shift in mobility
(1.58 cm<sup>2</sup>V<sup>ā1</sup>s<sup>ā1</sup>) compared
the DMMP monomer (1.63 cm<sup>2</sup>V<sup>ā1</sup>s<sup>ā1</sup>). Meanwhile, the DMMP dimer experienced no change in mobility (1.45
cm<sup>2</sup>V<sup>ā1</sup>s<sup>ā1</sup>). The enhancement
of separation factors appears to be brought about by the differential
clustering of neutral modifiers onto different ions and can be explained
by a model which considers the transient binding of a single 2-propanol
molecule during mobility measurements. Furthermore, the application
of the binding models not only provides a thermodynamic foundation
for the results obtained but also creates a predictive tool toward
a quantitative approach
Aerosol Analysis via Electrostatic Precipitation-Electrospray Ionization Mass Spectrometry
Electrospray ionization (ESI) is
the preferred mode of ion generation
for mass analysis of many organic species, as alternative ionization
techniques can lead to appreciable analyte fragmentation. For this
reason, ESI is an ideal method for the analysis of species within
aerosol particles. However, because of their low concentrations (ā¼10
Ī¼g/m<sup>3</sup>) in most environments, ESI has been applied
sparingly in aerosol particle analysis; aerosol mass spectrometers
typically employ analyte volatilization followed by electron ionization
or chemical ionization, which can lead to a considerable degree of
analyte fragmentation. Here, we describe an approach to apply ESI
to submicrometer and nanometer scale aerosol particles, which utilizes
unipolar ionization to charge particles, electrostatic precipitation
to collect particles on the tip of a Tungsten rod, and subsequently,
by flowing liquid over the rod, ESI and mass analysis of the species
composing collected particles. This technique, which we term electrostatic
precipitation-ESI-MS (EP-ESI-MS), is shown to enable analysis of nanogram
quantities of collected particles (from aerosol phase concentrations
as low as 10<sup>2</sup> ng m<sup>ā3</sup>) composed of cesium
iodide, levoglucosan, and levoglucosan within a carbon nanoparticle
matrix. With EP-ESI-MS, the integrated mass spectrometric signals
are found to be a monotonic function of the mass concentration of
analyte in the aerosol phase. We additionally show that EP-ESI-MS
has a dynamic range of close to 5 orders of magnitude in mass, making
it suitable for molecular analysis of aerosol particles in laboratory
settings with upstream particle size classification, as well as analysis
of PM 2.5 particles in ambient air
Ion-Mobility-Based Quantification of Surface-Coating-Dependent Binding of Serum Albumin to Superparamagnetic Iron Oxide Nanoparticles
Protein binding and protein-induced
nanoparticle aggregation are known to occur for a variety of nanomaterials,
with the extent of binding and aggregation highly dependent on nanoparticle
surface properties. However, often lacking are techniques that enable
quantification of the extent of protein binding and aggregation, particularly
for nanoparticles with polydisperse size distributions. In this study,
we adapt ion mobility spectrometry (IMS) to examine the binding of
bovine serum albumin to commercially available anionic-surfactant-coated
superparamagnetic iron oxide nanoparticles (SPIONs), which are initially ā¼21
nm in mean mobility diameter and have a polydisperse size distribution
function (geometric standard deviation near 1.4). IMS, carried out
with a hydrosol-to-aerosol converting nebulizer, a differential mobility
analyzer, and a condensation particle counter, enables measurements
of SPION size distribution functions for varying BSA/SPION number
concentration ratios. IMS measurements suggest that initially (at
BSA concentrations below 50 nM) BSA binds reversibly to SPION surfaces
with a binding site density in the 0.05ā0.08 nm<sup>ā2</sup> range. However, at higher BSA concentrations, BSA induces SPIONāSPION
aggregation, evidenced by larger shifts in SPION size distribution
functions (mean diameters beyond 40 nm for BSA concentrations near
100 nM) and geometric standard deviations (near 1.3) consistent with
self-preserving aggregation theories. The onset of BSA aggregation
is correlated with a modest but statistically significant decrease
in the specific absorption rate (SAR) of SPIONs placed within an alternating
magnetic field. The coating of SPIONs with mesoporous silica (MS-SPIONs)
as well as PEGylation (MS-SPIONs-PEG) is found to completely mitigate
BSA binding and BSA-induced aggregation; IMS-inferred size distribution
functions are insensitive to BSA concentration for MS-SPIONs and MS-SPIONs-PEG.
The SARs of MS-SPIONs are additionally insensitive to BSA concentration,
confirming the SAR decrease is linked to BSA-induced aggregation
Broadband Absorbing ExcitonāPlasmon Metafluids with Narrow Transparency Windows
Optical
metafluids that consist of colloidal solutions of plasmonic and/or
excitonic nanomaterials may play important roles as functional working
fluids or as means for producing solid metamaterial coatings. The
concept of a metafluid employed here is based on the picture that
a single ballistic photon, propagating through the metafluid, interacts
with a large collection of specifically designed optically active
nanocrystals. We demonstrate water-based metafluids that act as broadband
electromagnetic absorbers in a spectral range of 200ā3300 nm
and feature a tunable narrow (ā¼100 nm) transparency window
in the visible-to-near-infrared region. To define this transparency
window, we employ plasmonic gold nanorods. We utilize excitonic boron-doped
silicon nanocrystals as opaque optical absorbers (āoptical
wallā) in the UV and blue-green range of the spectrum. Water
itself acts as an opaque āwallā in the near-infrared
to infrared. We explore the limits of the concept of a āsimpleā
metafluid by computationally testing and validating the effective
medium approach based on the BeerāLambert law. According to
our simulations and experiments, particle aggregation and the associated
decay of the window effect are one example of the failure of the simple
metafluid concept due to strong interparticle interactions