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Ions are separated in ion mobility spectrometry (IMS) by their characteristic motion through a gas-filled drift tube with a static electric field present. Chemical dynamics, such as clustering and declustering of chemically reactive systems, and physical parameters, as, for example, the electric field strength or background gas temperature, impact on the observed ion mobility. In high kinetic energy IMS (HiKE-IMS), the reduced electric field strength is up to 120 Td in both the reaction region and drift region of the instrument. The ion generation in a corona discharge driven chemical ionization source leads generally to formation of proton-bound water clusters. However, the reduced electric field strength and therefore the effective ion temperature has a significant influence on the chemical equilibria of this reaction system. In order to characterize the effects occurring in IMS systems in general, numerical simulations can support and potentially explain experimental observations. The comparison of the simulation of a well characterized chemical reaction system (i.e., the proton-bound water cluster system) with experimental results allows us to validate the numerical model applied in this work. Numerical simulations of the proton-bound water cluster system were performed with the custom particle-based ion dynamics simulation framework (IDSimF). The ion-transport calculation in the model is based on a Verlet integration of the equations of motion and uses a customized Barnes-Hut method to calculate space charge interactions. The chemical kinetics is modeled stochastically with a Monte Carlo method. The experimental and simulated drift spectra are in good qualitative and quantitative agreement, and experimentally observed individual transitions of cluster ions are clearly reproduced and identified by the numerical model.

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) are usually operated at an absolute pressure of 20 mbar reaching high reduced electric field strengths of up to 125 Td for controlled reaction kinetics. This significantly increases the linear range and limits chemical cross sensitivities. Furthermore, HiKE-IMS enables the ionization of compounds normally not detectable in ambient pressure IMS, such as benzene, due to new reaction pathways and the inhibition of clustering reactions. In addition, HiKE-IMS allows the observation of additional orthogonal parameters related to an increased ion temperature such as fragmentation and field-dependent ion mobility, which may help to separate compounds that have similar ion mobility under low field conditions. Aiming for a hand-held HiKE-IMS to carry its benefits into field applications, reducing size and power consumption of the vacuum system is necessary. In this work, we present a novel HiKE-IMS design entirely manufactured from standard printed circuit boards (PCB) and experimentally investigate the analytical performance in dependence of the operating pressure between 20 mbar and 40 mbar. Hereby, the limit of detection (LoD) for benzene in purified, dry air (1.4 ppmV water) improved from 7 ppbV at 20 mbar down to 1.8 ppbV at 40 mbar. Furthermore, adding 0.9 ppmV toluene the signal of the benzene B+ peak decreases by only 2 % at 40 mbar. Even in the presence of high relative humidity in the sample gas above 90 % or toluene concentrations of up to 20 ppmV, the LoD for benzene just increases to 9 ppbV at 40 mbar.

In contrast to classical ion mobility spectrometers (IMS) operating at ambient pressure, the high kinetic energy ion mobility spectrometer (HiKE-IMS) is operated at reduced pressures between 10-40 mbar. In HiKE-IMS, ions are generated in a reaction region before they are separated in a drift region. Due to the operation at reduced pressure, it is possible to reach high reduced electric field strengths up to 120 Td in both the reaction as well as drift region, resulting in a pronounced decrease in chemical cross sensitivities and a significant enhancement of the dynamic range. Until now though, only limited knowledge about the ionization pathways in HiKE-IMS is available. Typically, proton bound water clusters, H+(H2O)n, are the most abundant positive reactant ion species in classical IMS with atmospheric chemical ionization sources. However, at reduced pressure and increased effective ion temperature, the reactant ion population significantly changes. As the ionization efficiency of analyte molecules in HiKE-IMS strongly depends on the reactant ion population, a detailed knowledge of the reactant ion population generated in HiKE-IMS is essential. Here, we present a coupling stage of the HiKE-IMS to a mass spectrometer enabling the identification of ion species and the investigation of ion molecule reactions prevailing in HiKE-IMS. In the present study, the HiKE-IMS-MS is used to identify positive reactant ion populations in both, purified air and nitrogen, respectively. The experimental data suggest the generation of systems of clustered primary ions (H+(H2O)n, NO+(H2O)m, and O2+(H2O)p), which most probably serve as reactant ions. Their relative abundances highly depend on the reduced electric field strength in the reaction region. Furthermore, their effective mobilities are studied as a function of the reduced electric field strength in the drift region.

Classical ion mobility spectrometers (IMS) operated at ambient pressure, often use atmospheric pressure chemical ionization (APCI) sources to ionize organic compounds. In APCI, reactant ions ionize neutral analyte molecules via gas-phase ion-molecule reactions. The positively charged reactant ions in purified, dry air are H3O+, NO+, and O2+•. However, the hydration of reactant ions in classical IMS operated at ambient pressure renders ionization of certain analytes difficult. In contrast to classical IMS operated at ambient pressure, High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) are operated at a decreased pressure of 10-40 mbar, allowing operation at high reduced electric field strengths of up to 120 Td. At such high reduced field strengths, ions reach high effective temperatures causing collision-induced cluster dissociation of the hydrated gas-phase ions, allowing ionization of nonpolar and low proton affinity analytes. The reactant ion population, consisting of H3O+(H2O)n, NO+(H2O)m, and O2+•(H2O)p with an individual abundance that strongly depends on the reduced field strength, differs from the reactant ion population in IMS operated at ambient pressure, which affects the ionization of analyte molecules. In this work, we investigate the influence of reduced field strength on the product ion formation of aromatic hydrocarbons used as model substances. A HiKE-IMS-MS coupling was used to identify the detected ion species. The results show that the analytes form parent cations via charge transfer with NO+(H2O)m and O2+•(H2O)p depending on ionization energy and protonated parent molecules via proton transfer and ligand switching with H3O+(H2O)n mainly depending on proton affinity.

Collisional damping quadrupole ion guides have become popular tools in mass spectrometry. In such devices, ions are cooled by collisions with a buffer gas and focused to trajectories near the axis of the ion guide under the influence of a radio frequency (RF) field. This produces a narrow beam of low-energy ions which can be transported with high efficiency. Typically, quadrupole ion guides are constructed of four parallel rods to which RF voltages are applied. To overcome the dampening of the axial velocity component resulting from collisions with neutral gas particles, an additional static axial field is provided by dividing the rods in several segments and applying an electric potential to each segment. However, this method is mechanically complex, requiring a precise alignment of all segments, and several separate connections for the DC und RF voltages to all segments. In this work, we present a simple and low-cost segmented quadrupole ion guide design that is based on standard printed circuit boards (PCB) including both the segmented electrodes as well as the signal distribution network. Furthermore, we present simulations of the ion movement inside this PCB quadrupole and experimentally evaluate the ion transfer. Our measurements show that the segmented PCB quadrupole with planar electrodes reaches similar ion transmissions in comparison with conventional quadrupole ion guides built from segmented circular rods.

Ion funnels are one of the key components for transferring ions from higher pressure into the vacuum. Typically, ion funnels are constructed of several different plate ring electrodes with a decreasing inner diameter where radio frequency (RF) voltages and electric DC fields are applied to the electrodes to focus and transport ion clouds. In this work, we developed and investigated a simple and low-cost ion funnel design that is based on standard printed circuit boards (PCB) with integrated planar electrodes including the signal distribution network. This ion funnel is capable of withstanding high electric fields with superimposed RF voltages due to its buried capacitors. To evaluate the ion focusing efficiency of the ion funnel, we simulated the movement of ions inside this funnel and experimentally evaluated the ion transfer. Our simulations show that a rectangular ion funnel like the PCB ion funnel has similar performance compared with conventional stacked ring funnels. Due to the hundredfold lower parasitic capacitance between the planar electrodes compared with conventional ion funnels, high RF voltage amplitudes up to 195 V and reduced electric DC field strengths up to 100 Td can be reached at a frequency of about 5 MHz. Thus, the funnel is appropriate to focus light ions at elevated pressures up to 20 mbar. Graphical Abstract .

Rationale: Ion mobility spectrometry (IMS) instruments are typically equipped with atmospheric pressure chemical ionization (APCI) sources operated at ambient pressure. However, classical APCI-IMS suffers from a limited ionization yield for nonpolar substances with low proton affinity (PA). This is mainly due to ion clustering processes, especially those that involve water molecules, inhibiting the ionization of these substances. Methods: High Kinetic Energy (HiKE)-IMS instruments are operated at decreased pressures and high reduced electric field strengths. As most clustering reactions are inhibited under these conditions, the ionization yield for nonpolar substances with low PA in HiKE-IMS should differ from that in classical APCI-IMS. To gain first insights into the ionization capabilities and limitations of HiKE-IMS, we investigated the ionization of four model substances with low PA in HiKE-IMS using HiKE-IMS-MS as a function of the reduced electric field strength. Results: The four model substances all have proton affinities between those of H2O and (H2O)2 but exhibit different ionization energies, dipole moments, and polarizabilities. As expected, the results show that the ionization yield for these substances differs considerably at low reduced electric field strengths due to ion cluster formation. In contrast, at high reduced electric field strengths, all substances can be ionized via charge and/or proton transfer in HiKE-IMS. Conclusions: Considering the detection of polar substances with high PAs, classical ambient pressure IMS should reach better detection limits than HiKE-IMS. However, considering the detection of nonpolar substances with low PA that are not detected, or only difficult to detect, at ambient pressure, HiKE-IMS would be beneficial.

Ion mobility spectrometers (IMS) separate ions mainly by ion–neutral collision cross section and to a lesser extent by ion mass and effective temperature. When investigating isotopologues, the difference in collision cross section can be assumed negligible. Since the mobility shift of isotopologues is thus mainly caused by their difference in mass and effective temperature, the investigation of isotopologues can provide important insights into the theory of ion mobility. However, in classical IMS operated at ambient pressure, cluster formation with neutral molecules occurs, which significantly influences the mobility shift of isotopologues and thus makes a sound investigation of the effect of ion mass and effective temperature on the ion mobility difficult. In this work, the relative ion mobility of several organic compounds and their 13C-labeled isotopologues is studied in a High Kinetic Energy Ion Mobility Spectrometer (HiKE-IMS) at high reduced electric fields up to 120 Td, which allows the investigation of nonclustered ion species and thus enables a sound investigation of the mobility shift of isotopologues. The results show that the measured relative ion mobilities of isotopologues having the same effective temperature and, thus, their ion mass dominating the relative ion mobility agree well with theoretical relative ion mobilities predicted by the theory of ion mobility.

In High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS), ions are formed in a reaction region and separated in a drift region, which is similar to classical drift tube ion mobility spectrometers (IMS) operated at ambient pressure. However, in contrast to the latter, the HiKE-IMS is operated at a decreased background pressure of 10–40 mbar and achieves high reduced electric field strengths of up to 120 Td in both the reaction and the drift region. Thus, the HiKE-IMS allows insights into the chemical kinetics of ion-bound water cluster systems at effective ion temperatures exceeding 1000 K, although it is operated at the low absolute temperature of 45 °C. In this work, a HiKE-IMS with a high resolving power of RP = 140 is used to study the dependence of reduced ion mobilities on the drift gas humidity and the effective ion temperature for the positive reactant ions H3O+(H2O)n, O2+(H2O)n, NO+(H2O)n, NO2+(H2O)n, and NH4+(H2O)n, as well as the negative reactant ions O2–(H2O)n, O3–(H2O)n, CO3–(H2O)n, HCO3–(H2O)n, and NO2–(H2O)n. By varying the reduced electric field strength in the drift region, cluster transitions are observed in the ion mobility spectra. This is demonstrated for the cluster systems H3O+(H2O)n and NO+(H2O)n.

Due to the operation at background pressures between 10-40 mbar and high reduced electric field strengths of up to 120 Td, the ion–molecule reactions in High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) differ from those in classical ambient pressure IMS. In the positive ion polarity mode, the reactant ions H+(H2O)n, O2+(H2O)n, and NO+(H2O)n are observed in the HiKE-IMS. The relative abundances of these reactant ion species significantly depend on the reduced electric field strength in the reaction region, the operating pressure, and the water concentration in the reaction region. In this work, the formation of negative reactant ions in HiKE-IMS is investigated in detail. On the basis of kinetic and thermodynamic data from the literature, the processes resulting in the formation of negative reactant ions are kinetically modeled. To verify the model, we present measurements of the negative reactant ion population in the HiKE-IMS and its dependence on the reduced electric field strength as well as the water and carbon dioxide concentrations in the reaction region. The ion species underlying individual peaks in the ion mobility spectrum are identified by coupling the HiKE-IMS to a time-of-flight mass spectrometer (TOF-MS) using a simple gated interface that enables the transfer of selected peaks of the ion mobility spectrum into the TOF-MS. Both the theoretical model as well as the experimental data suggest the predominant generation of the oxygen-based ions O–, OH–, O2–, and O3– in purified air containing 70 ppmv of water and 30 ppmv of carbon dioxide. Additionally, small amounts of NO2– and CO3– are observed. Their relative abundances highly depend on the reduced electric field strength as well as the water and carbon dioxide concentration. An increase of the water concentration in the reaction region results in the generation of OH– ions, whereas increasing the carbon dioxide concentration favors the generation of CO3– ions, as expected.

Due to the operation at background pressures between 10-40 mbar and high reduced electric field strengths of up to 120 Td, the ion-molecule reactions in High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) differ from those in classical ambient pressure IMS. In the positive ion polarity mode, the reactant ions H+(H2O)n, O2+(H2O)n, and NO+(H2O)n are observed in the HiKE-IMS. The relative abundances of these reactant ion species significantly depend on the reduced electric field strength in the reaction region, the operating pressure, and the water concentration in the reaction region. In this work, the formation of negative reactant ions in HiKE-IMS is investigated in detail. On the basis of kinetic and thermodynamic data from the literature, the processes resulting in the formation of negative reactant ions are kinetically modeled. To verify the model, we present measurements of the negative reactant ion population in the HiKE-IMS and its dependence on the reduced electric field strength as well as the water and carbon dioxide concentrations in the reaction region. The ion species underlying individual peaks in the ion mobility spectrum are identified by coupling the HiKE-IMS to a time-of-flight mass spectrometer (TOF-MS) using a simple gated interface that enables the transfer of selected peaks of the ion mobility spectrum into the TOF-MS. Both the theoretical model as well as the experimental data suggest the predominant generation of the oxygen-based ions O-, OH-, O2-, and O3- in purified air containing 70 ppmv of water and 30 ppmv of carbon dioxide. Additionally, small amounts of NO2- and CO3- are observed. Their relative abundances highly depend on the reduced electric field strength as well as the water and carbon dioxide concentration. An increase of the water concentration in the reaction region results in the generation of OH- ions, whereas increasing the carbon dioxide concentration favors the generation of CO3- ions, as expected.

Ion mobility spectrometry provides information about molecular structures of ions. Hence, high resolving power allows separation of isomers which is of major interest in several applications. In this work, we couple our high-resolution ion mobility spectrometer (IMS) with a resolving power of Rp = 100 to a time-of-flight mass spectrometer (TOF-MS). Besides, the benefit of an increased resolving power such an IMS-MS also helps analyzing and understanding the ionization processes in IMS. Usually, the coupling between IMS and TOF-MS is realized by synchronizing data acquisition of the IMS and MS resulting in two-dimensional data containing ion mobility and mass spectra. However, due to peak widths of less than 100 μs in our high-resolution IMS, this technique is not practicable due to significant peak broadening during the ion transfer into the MS and an insufficient data acquisition rate of the MS. Thus, a novel but simple interface between the IMS and MS has been designed which minimizes ion losses, allows recording of ion mobility at full IMS resolving power, and enables a shuttered transmission of ions into the MS. The interface is realized by replacing the Faraday plate used in IMS by a Faraday grid that is shielded by two additional aperture grids. For demonstration, positive product ions of benzene, toluene, and m-xylene in air are investigated. The IMS is equipped with a radioactive 3H source. Besides the well-known product ions M+ and M·NO+, a dimer ion is also observed for benzene and toluene, consisting of two molecules and three further hydrogen atoms.

The ion funnel is one of the key components for transferring ions from higher pressure into vacuum. While known for over twenty years and thoroughly analyzed by mathematical means and simulations, its working principle might be difficult to understand sometimes. In this work, we propose a simple thought experiment based on the centripetal force ions experience inside the ion funnel, which offers a simplistic explanation for the focusing force in an ion funnel. Our centripetal force model can predict the proportionalities of the resulting pseudopotential and should be a good didactical tool.

Measuring a mixture of acetone and perdeuterated acetone (acetone-d6) with an ultra-high resolution drift time ion mobility spectrometer (resolving power of Rp = 235) and ultraviolet ionization (10.6 eV) at ambient pressure reveals three separated peaks. Two of the peaks can easily be associated with acetone and perdeuterated acetone. In a former publication several findings indicated an exchange of a methyl group and the formation of a H3COCD3 related peak. In this work the formed ion species were analyzed with a high resolution drift time ion mobility time of flight mass spectrometer. The mass spectra clearly show the formation of three proton-bound dimer peaks whereas the peak between acetone and acetone-d6 is a proton-bound mixed dimer consisting of one acetone and one acetone-d6 molecule. The final publication is available at Springer via https://doi.org/10.1007/s12127-018-0234-2.

Ion mobility spectrometers can be divided by their principle of ion separation. For example, classical drift tube time-of-flight Ion Mobility Spectrometers (IMS) separate ions by the absolute value of their low field ion mobility; Field Asymmetric Ion Mobility Spectrometers (FAIMS) separate ions by the field dependence of their ion mobility. However, the low field mobilities and the field dependence of the mobility vary only within a limited range for different ions, leading to a limited peak capacity of stand-alone drift tube IMS and FAIMS. Combining both types leads to orthogonal data and thus enhances the selectivity in comparison with stand-alone devices. In this work, a new approach of enhancing the separation power of a classical drift tube IMS by integrating a field asymmetric waveform ion separation region in longitudinal direction into the drift tube is discussed. This additional separation region is realized by superimposing the constant drift field of a drift tube IMS with an asymmetric parallel AC field using two additional grids inside the drift tube. Since the ions are exposed alternately to high field and low field strengths on their way through the additional separation region, the resulting drift time is affected. Hence, two ion species having the same low-field mobility, but showing a different field dependence of the mobility have different drift times in the enhanced IMS. In order to analyze the ion movement inside such a modified ion mobility spectrometer, the finite element method (FEM) software Comsol Multiphysics is used. Therefore, an existing drift tube IMS model which perfectly agrees with experimental results and considers field inhomogenieties, diffusion, Coulomb repulsion and ion losses at metallic surfaces, is expanded in order to simulate the ion movement in AC fields. This enhanced model provides visualization of the location and shape of the ion cloud during DC/AC operation. Particular attention is given to the increased broadening of the ion cloud due to field inhomogenieties in the additional AC field. Furthermore, ion losses inside the drift tube caused by the AC field and the additional grids are considered. In this work, simulations are used to theoretically investigate our new separation approach to give a first impression of the possible analytical performance. The final publication is available at Springer via https://doi.org/10.1007/s12127-015-0179-7.

Drift tube ion mobility spectrometers (IMS) are widely used for fast trace gas detection in air, but portable compact systems are typically very limited in their resolving power. Decreasing the initial ion packet width improves the resolution, but is generally associated with a reduced signal-to-noise-ratio (SNR) due to the lower number of ions injected into the drift region. In this paper, we present a refined theory of IMS operation which employs a combined approach for the analysis of the ion drift and the subsequent amplification to predict both the resolution and the SNR of the measured ion current peak. This theoretical analysis shows that the SNR is not a function of the initial ion packet width, meaning that compact drift tube IMS with both very high resolution and extremely low limits of detection can be designed. Based on these implications, an optimized combination of a compact drift tube with a length of just 10 cm and a transimpedance amplifier has been constructed with a resolution of 183 measured for the positive reactant ion peak (RIP(+)), which is sufficient to e.g. separate the RIP(+) from the protonated acetone monomer, even though their drift times only differ by a factor of 1.007. Furthermore, the limits of detection (LODs) for acetone are 180 pptv within 1 s of averaging time and 580 pptv within only 100 ms.