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

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 Stokes–Millikan 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 Stokes–Millikan 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 ion–induced dipole (polarization) potential. Predictions were compared to previous measurements in air near room temperature of multiply charged poly­(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

IMS–MS and IMS–IMS 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 10¹–10³ 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 (IMS–IMS) are used to examine thermostability, and finally a differential mobility analyzer coupled to an atmospheric pressure drift tube ion mobility spectrometer (also IMS–IMS) is used for water uptake measurements. IMS–MS measurements reveal that dry DMAS nanoclusters have densities of ∼1567 kg/m³ near 300 K, independent of the ratio of dimethylamine to sulfuric acid originally present in the electrospray solution. IMS–IMS thermostability studies reveal that partial pressures of DMAS nanoclusters are dependent upon the electrospray solution concentration ratio, <i>R</i> = [H₂SO₄]/[(CH₃)₂NH]. Extrapolating measurements, we estimate that dry DMAS nanoclusters have surface vapor pressures of order 10^–4 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, IMS–IMS 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 5–8.5 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 ± 0.01 cm² V^–1s^–1 at 175 °C. However, when the counter current drift gas of the system is doped with 2-propanol at 20 μL/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 μL/h, MPA demonstrated the greatest shift in mobility (1.58 cm²V^–1s^–1) compared the DMMP monomer (1.63 cm²V^–1s^–1). Meanwhile, the DMMP dimer experienced no change in mobility (1.45 cm²V^–1s^–1). 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 (∼10 μg/m³) 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 10² ng m^–3) 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 ∼21 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 0.05–0.08 nm^–2 range. However, at higher BSA concentrations, BSA induces SPION–SPION 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 Exciton–Plasmon 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 200–3300 nm and feature a tunable narrow (∼100 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 (“optical wall”) in the UV and blue-green range of the spectrum. Water itself acts as an opaque “wall” in the near-infrared to infrared. We explore the limits of the concept of a “simple” metafluid by computationally testing and validating the effective medium approach based on the Beer–Lambert 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