Dissociation of Proton Bound Ketone Dimers in Asymmetric Electric Fields with Differential Mobility Spectrometry and in Uniform Electric Fields with Linear Ion Mobility Spectrometry

Reliable Detection of Chemical Warfare Agents Using High Kinetic Energy Ion Mobility Spectrometry

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) ionize and separate ions at reduced pressures of 10-40 mbar and over a wide range of reduced electric field strengths E/N of up to 120 Td. Their reduced operating pressure is distinct from that of conventional drift tube ion mobility spectrometers that operate at ambient pressure for trace compound detection. High E/N can lead to a field-induced fragmentation pattern that provides more specific structural information about the analytes. In addition, operation at high E/N values adds the field dependence of ion mobility as an additional separation dimension to low-field ion mobility, making interfering compounds less likely to cause a false positive alarm. In this work, we study the chemical warfare agents tabun (GA), sarin (GB), soman (GD), cyclosarin (GF) and sulfur mustard (HD) in a HiKE-IMS at variable E/N in both the reaction and the drift region. The results show that varying E/N can lead to specific fragmentation patterns at high E/N values combined with molecular signals at low E/N. Compared to the operation at a single E/N value in the drift region, the variation of E/N in the drift region also provides the analyte-specific field dependence of ion mobility as additional information. The accumulated data establish a unique fingerprint for each analyte that allows for reliable detection of chemical warfare agents even in the presence of interfering compounds with similar low-field ion mobilities, thus reducing false positives.

The Charge-State and Structural Stability of Peptides Conferred by Microsolvating Environments in Differential Mobility Spectrometry

The presence of solvent vapor in a differential mobility spectrometry (DMS) cell creates a microsolvating environment that can mitigate complications associated with field-induced heating. In the case of peptides, the microsolvation of protonation sites results in a stabilization of charge density through localized solvent clustering, sheltering the ion from collisional activation. Seeding the DMS carrier gas (N₂) with a solvent vapor prevented nearly all field-induced fragmentation of the protonated peptides GGG, AAA, and the Lys-rich Polybia-MP1 (IDWKKLLDAAKQIL-NH₂). Modeling the microsolvation propensity of protonated n-propylamine [PrNH₃]⁺, a mimic of the Lys side chain and N-terminus, with common gas-phase modifiers (H₂O, MeOH, EtOH, iPrOH, acetone, and MeCN) confirms that all solvent molecules form stable clusters at the site of protonation. Moreover, modeling populations of microsolvated clusters indicates that species containing protonated amine moieties exist as microsolvated species with one to six solvent ligands at all effective ion temperatures (T_eff) accessible during a DMS experiment (ca. 375-600 K). Calculated T_eff of protonated GGG, AAA, and Polybia-MPI using a modified two-temperature theory approach were up to 86 K cooler in DMS environments seeded with solvent vapor compared to pure N₂ environments. Stabilizing effects were largely driven by an increase in the ion's apparent collision cross section and by evaporative cooling processes induced by the dynamic evaporation/condensation cycles incurred in the presence of an oscillating electric separation field. When the microsolvating partner was a protic solvent, abstraction of a proton from [MP1 + 3H]³⁺ to yield [MP1 + 2H]²⁺ was observed. This result was attributed to the proclivity of protic solvents to form hydrogen-bond networks with enhanced gas-phase basicity. Collectively, microsolvation provides analytes with a solvent "air bag," whereby charge reduction and microsolvation-induced stabilization were shown to shelter peptides from the fragmentation induced by field heating and may play a role in preserving native-like ion configurations.

Measuring the Kinetics of Neutral Pair Evaporation from Cluster Ions of Ionic Liquid in the Drift Region of a Differential Mobility Analyzer

Singly charged clusters [C⁺A^-]_nC⁺ or [C⁺A^-]_nA^- of two salts [C⁺A^-] are produced by electrospray ionization of alcohol solutions of the ionic liquids 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (EMI-FAP) and 1,2-dimethyl-3-propylimidazolium-methide (DMPI-Me). The rate of neutral pair evaporation into [C⁺A^-] + [C⁺A^-]_n-1C⁺ or [C⁺A^-]_n-1A^- is studied in atmospheric pressure as a function of temperature T for the positive trimer ion (n = 2) of DMPI-Me and the negative trimer ion of EMI-FAP. The trimer is separated from all other electrosprayed ions in a first differential mobility analyzer (DMA₁) and then transferred through a cooled tube to a second DMA whose drift gas is kept at a controlled temperature (25 °C < T < 100 °C). Singular characteristics of the DMA are a residence time τ of ∼0.1 to 1 ms, with essentially uniform temperature and τ. The decomposition occurring within DMA₂ results in a complex mobility spectrum associated with dimer product ions, with apparent mobilities intermediate between those of the dimer and the trimer, depending on the product of the reaction rate k and τ. A theoretical expression yielding k from the shape of the collected mobility spectrum is obtained by accounting for the deterministic reactive, convective, and diffusive evolutions of the parent and product ions within DMA₂. Observed and predicted mobility spectra agree well, yielding the reaction rate k with little ambiguity. Activation energies near 1 eV are determined for both trimer ions. Paradoxically, the evaporation process substantially heats up the cluster ion product. The theory developed enables measuring decay times much smaller and much larger than the residence time in the DMA.

Automated chemical identification and library building using dispersion plots for differential mobility spectrometry

Differential mobility spectrometry (DMS) based detectors require rapid data analysis capabilities, embedded into the devices to achieve the optimum detection capabiites as portable trace chemical detectors. Automated algorithm-based DMS dispersion plot data analysis method was applied for the first time to pre-process and separate 3-dimentional (3-D) DMS dispersion data. We previously demonstrated our AnalyzeIMS (AIMS) software was capable of analyzing complex gas chromatography differential mobility spectrometry (GC-DMS) data sets. In our present work, the AIMS software was able to easliy separate DMS dispersion data sets of five chemicals that are important in detection of volatile organic compounds (VOCs): 2-butanone, 2-propanone, ethyl acetate, methanol and ethanol. Identification of chemicals from mixtures, separation of chemicals from a mixture and prediction capability of the software were all tested. These automated algorithms may have potential applications in separation of chemicals (or ion peaks) from other 3-D data obtained by hybrid analytical devices such as mass spectrometry (MS). New algorithm developments are included as future considerations to improve the current numerical approaches to fingerprint chemicals (ions) from a significantly complicated dispersion plot. Comprehensive peak identifcation by DMS-MS, variations of the DMS data due to chemical concentration, gas phase ion chemistry, temperature and pressure of the drift gas are considered in future algorithm improvements.

Variables Affecting the Internal Energy of Peptide Ions During Separation by Differential Ion Mobility Spectrometry

Differential ion mobility spectrometry (DIMS) devices separate ions on the basis of differences in ion mobility in low and high electric fields, and can be used as a stand-alone analytical method or as a separation step before further analysis. As with other ion mobility separation techniques, the ability of DIMS separations to retain the structural characteristics of analytes has been of concern. For DIMS separations, this potential loss of ion structure originates from the fact that the separations occur at atmospheric pressure and the ions, during their transit through the device, undergo repeated collisions with the DIMS carrier gas while being accelerated by the electric field. These collisions have the ability to increase the internal energy distribution of the ions, which can cause isomerization or fragmentation. The increase in internal energy of the ions is based on a number of variables, including the dispersion field and characteristics of the carrier gas such as temperature and composition. The effects of these parameters on the intra-DIMS fragmentation of multiply charged ions of the peptides bradykinin (RPPGFSPFR) and GLISH are discussed herein. Furthermore, similarities and differences in the internal energy deposition that occur during collisional activation in tandem mass spectrometry experiments are discussed, as the fragmentation pathways accessed by both are similar. Graphical Abstract ᅟ.

Fragmentation, auto-modification and post ionisation proton bound dimer ion formation: the differential mobility spectrometry of low molecular weight alcohols

Ion-neutral interactions at atmospheric pressure result in significant complexity in the observed responses for alcohols by differential mobility spectrometry. Such behaviours ultimately enable enhanced selectivity and confidence in data assignments.

Understanding the reactivity of metallic nanoparticles: beyond the extended surface model for catalysis

Metallic nanoparticles (NPs) constitute a new class of chemical objects which are used in different fields as diverse as plasmonics, optics, catalysis, or biochemistry.