Highly Efficient Ion Manipulator for Tandem Ion Mobility Spectrometry: Exploring a Versatile Technique by a Study of Primary Alcohols.

In this work, we present a tandem ion mobility spectrometer (IMS) utilizing a highly efficient ion manipulator allowing to store, manipulate, and analyze ions under high electric field strengths and controlled ion-neutral reactions at ambient conditions. The arrangement of tandem drift regions and an ion manipulator in a single drift tube allows a sequence of mobility selection of precursor ions, followed by storage and analysis, mobility separation, and detection of the resulting product ions. In this article, we present a journey exploring the capabilities of the present instrument by a study of eight different primary alcohols characterized at reduced electric field strengths E/N of up to 120 Td with a water vapor concentration ranging from 40 to 540 ppb. Under these conditions, protonated alcohol monomers up to a carbon number of nine could be dissociated, resulting in 18 different fragmented product ions in total. The fragmentation patterns revealed regularities, which can be used for assignment to the chemical class and improved classification of unknown substances. Furthermore, both the time spent in high electrical field strengths and the reaction time with water vapor can be tuned precisely, allowing the fragment distribution to be influenced. Thus, further information regarding the relations of the product ions can be gathered in a standalone drift tube IMS for the first time.

A High Kinetic Energy Ion Mobility Spectrometer for Operation at Higher Pressures of up to 60 mbar.

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) are usually operated at absolute pressures around 20 mbar in order to reach high reduced electric field strengths of up to 120 Td for influencing reaction kinetics in the reaction region. Such operating points significantly increase the linear range and limit chemical cross sensitivities. Furthermore, HiKE-IMS enables ionization of compounds normally not detectable in ambient pressure IMS, such as benzene, due to additional reaction pathways and fewer clustering reactions. However, operation at higher pressures promises increased sensitivity and smaller instrument size. In this work, we therefore study the theoretical requirements to prevent dielectric breakdown while maintaining high reduced electric field strengths at higher pressures. Furthermore, we experimentally investigate influences of the pressure, discharge currents and applied voltages on the corona ionization source. Based on these results, we present a HiKE-IMS that operates at a pressure of 60 mbar and reduced electric field strengths of up to 105 Td. The corona experiments show shark fin shaped curves for the total charge at the detector with a distinct optimum operating point in the glow discharge region at a corona discharge current of 5 µA. Here, the available charge is maximized while the generation of less-reactive ion species like NOx+ is minimized. With these settings, the reactant ion population, H3O+ and O2+, for ionizing and detecting nonpolar substances like n-hexane is still available even at 60 mbar, achieving a limit of detection of just 5 ppbV for n-hexane.

Miniaturized Drift Tube Ion Mobility Spectrometer with Ultra-Fast Polarity Switching.

Ion mobility spectrometers (IMS) are well suited for detecting trace gases down to levels at ppbv and even pptv within 1 s of analysis time when using chemical ionization. The measuring principle is based on the separation and detection of the ionized constituents of a sample. Depending on the sample composition, certain ionization sources create both positive and negative analyte ions, but the simultaneous detection of both ion polarities usually requires two drift tubes. Contained within this effort, we present an alternative approach for detecting both ion polarities using one single drift tube that can switch the polarity of the drift tube within 12 ms. This technique allows for generating one positive and one negative ion mobility spectrum, each with a drift time range of 13 ms (minimum reduced ion mobility of K0 = 0.72 cm2 V-1 s-1), within a total experiment time of 50 ms. Additionally, ions are continuously generated in the ionization region during both the polarity switching and the analysis of one of the polarities, which allows for an effective ionization/reaction time of 25 ms. Comparable to the performance of similar instrument designs we reported previously, the presented device has a high resolving power of RP = 70 with a drift length of 51 mm. The limits of detection are for the monomers between 70 and 370 pptv and for the dimers between 450 and 800 pptv for 1 s of averaging for various ketones, methyl salicylate, and chlorinated hydrocarbons. Although this work focuses on applying ultra-fast polarity switching to an existing IMS, the techniques shown here may be applied to other IMS implementations for different applications.

Toward Compact High-Performance Ion Mobility Spectrometers: Ion Gating in Ion Mobility Spectrometry.

Printed circuit board (PCB) based drift tube ion mobility spectrometers enable the use of state-of-the-art production techniques to manufacture compact devices with excellent performance at minimum cost. The new PCB ion mobility spectrometer (PCB-IMS) presented here is equipped with either a 140 MBq tritium or a 95 MBq nickel-63 ionization source and consists of a combination of horizontally arranged 6-layer PCBs for the drift and reaction regions and vertically arranged PCBs for interfacing the ionization source, ion shutter, and detector. The design allows the reproducible manufacturing and thus comparison of different IMS topologies. Here, we investigate different ion shutters, field-switching, Bradbury-Nielsen, and tristate and their effects on resolving power and limits of detection considering two different ionization region geometries and ionization sources, tritium and nickel-63. It is shown that the high resolving power of RP > 80 at low drift voltage of 3 kV and short drift length of 50 mm can be achieved independent of the used ion shutter mechanism and reaction region geometry. While the resolving power of all ion shutters is excellent, the Bradbury-Nielsen shutter shows a pronounced discrimination of slow ion species when using short shutter opening times for small initial ion cloud widths, as required for high resolving power. Thus, the intensity of the proton-bound dimer of 2-pentanone is reduced by 30% compared to the signal intensity obtained with both the field-switching and tristate shutter. The detection limits employing the Bradbury-Nielsen shutter and a 50 mm reaction region as required for nickel-63 are 58 pptv for the protonated monomer and 3.4 ppbv for the proton-bound dimer of 2-pentanone. The detection limits achieved with the tristate shutter utilizing the same reaction region are slightly higher for the protonated monomer at 68 pptv, but lower for the proton-bound dimer at 2 ppbv due to the advanced ion shutter principle not discriminating slow ions. However, the lowest detection limits of 13 pptv and 301 pptv can be achieved with the field-switching shutter and a 2 mm reaction region, sufficient for a tritium ionization source.

Enhanced Resolving Power by Moving Field Ion Mobility Spectrometry.

Ion mobility spectrometry is a powerful detection method widely used in various applications. Particularly in field applications, ion mobility spectrometers (IMSs) are useful because of their extremely low detection limits at short measuring periods and their compact and robust design. However, especially small IMSs suffer from the consequences of low resolving power when compared to laboratory systems. Therefore, in this paper, we present a new approach to increase the resolving power of a drift time IMS without employing higher drift voltages and bulky power supplies. The so-called moving field IMS (MOF-IMS) presented here allows a more effective use of the available voltage because of a segmented drift region where only a small part is supplied with voltage. Even with the basic version of an MOF-IMS presented here, it was possible to increase the resolving power by 60% from 60 to 95 without increasing the required drift voltage.

High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS) at 40 mbar.

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 decreased 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 increased to 9 ppbV at 40 mbar.

Improving Ion Mobility Spectrometer Sensitivity through the Extended Field Switching Ion Shutter.

Field switching ion shutters allow generating short ion packets with high ion densities by first ionizing for several milliseconds in a field-free ionization region and then quickly pushing the entire ion population out into the drift region. Thus, they are an excellent choice for compact ion mobility spectrometers with both high resolving power and low limits of detection. Here, we present an improved setup, named the extended field switching ion shutter. By generating a second field-free region between the ionization region and the drift region, shielding of the ionization region is significantly improved, even when using grids with higher optical transparency to improve ion transmission into the drift region. Furthermore, it is shown that under certain conditions, ion transmission through multiple grids in series can even surpass transmission through a single grid of the same transparency. For the studied ions, the signal intensity at low concentrations increases by approximately a factor of 7 to 9 for protonated monomers, 10 to 14 for proton-bound dimers, and 25 for the proton-bound 1-octanol trimer compared to the classical field switching shutter. However, due to the nonlinear response for ions containing multiple analyte molecules, the limits of detection improve only by a factor of 3 to 4 for proton-bound dimers and 3 for the proton-bound 1-octanol trimer. Nevertheless, this still leads to single-digit pptv limits of detection for protonated monomers and hundred pptv limits of detection for proton-bound dimers measured for a series of ketones. However, for the most intense peaks such as the reactant ion peak, a significant loss of resolving power by a factor of up to 1.4 was observed due to Coulomb repulsion.

Ion Fragmentation and Filtering by Alpha Function in Ion Mobility Spectrometry for Improved Compound Differentiation.

Collision induced dissociation (CID) is a widely used technique in mass spectrometry to better understand the structural composition of ions and to improve the identification of compounds beyond the analysis of m/z. By increasing the kinetic energy of ions in an electric field, the collisions with neutral molecules may result in bond breakage, dissociation, or fragmentation of the molecular ion into smaller fragments if the necessary onset energy is exceeded. In this work and for the first time, we demonstrate CID in a field asymmetric time of flight ion mobility spectrometer (FAT-IMS). In contrast to the commonly used devices in mass spectrometry, the FAT-IMS operates at ambient pressure and temperature. Furthermore, the FAT-IMS allows separation of ions prior to dissociation, employing the shift occurring in the FAT region and, thus, an improved assignment of fragment ions to selected precursor ions. In this work, the effect of the operation parameters on the fragmentation efficiency of the FAT separator is analyzed. As proof of concept, eight saturated alcohols were investigated. The results show the expected substance-specific fragmentation behavior, which can be used to generate additional orthogonal information about certain analytes via their fragmentation pattern. Furthermore, a method for prefiltering analytes in FAT-IMS by the alpha function is introduced to remove spectral interferences.

A Simple Printed Circuit Board-Based Ion Funnel for Focusing Low m/z Ratio Ions with High Kinetic Energies at Elevated Pressure.

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 .

Ultra-high-resolution ion mobility spectrometry-current instrumentation, limitations, and future developments.

With recent advances in ionization sources and instrumentation, ion mobility spectrometers (IMS) have transformed from a detector for chemical warfare agents and explosives to a widely used tool in analytical and bioanalytical applications. This increasing measurement task complexity requires higher and higher analytical performance and especially ultra-high resolution. In this review, we will discuss the currently used ion mobility spectrometers able to reach such ultra-high resolution, defined here as a resolving power greater than 200. These instruments are drift tube IMS, traveling wave IMS, trapped IMS, and field asymmetric or differential IMS. The basic operating principles and the resulting effects of experimental parameters on resolving power are explained and compared between the different instruments. This allows understanding the current limitations of resolving power and how ion mobility spectrometers may progress in the future. Graphical abstract.

Ion mobility spectrometer with orthogonal X-Ray source for increased sensitivity.

Ion mobility spectrometers (IMS) are compact devices for extremely sensitive detection of proton and electron affine volatile compounds down to low pptv concentrations within less than a second. The measuring principle requires ionization of the target analyte. Most IMS employ radioactive electron sources, such as 63Ni or 3H. These radioactive materials suffer from legal restrictions limiting the fields of application. Furthermore, the electron emission has a predetermined intensity and cannot be controlled or disabled. In a previous work, we replaced the axially mounted 3H source of our ion mobility spectrometer with a commercially available X-ray source operated at low acceleration voltage of 4.5 kV to be applicable in most application without legal restrictions. However, the high penetration depth of the radiation together with the statistical behavior of the X-ray ionization process led to an increase of Fano noise and thus a limited signal-to-noise ratio. Therefore, the X-ray source is now mounted orthogonal to the drift tube in order to avoid Fano noise. Here, we compare the analytical performance of this orthogonal setup with the axially mounted X-ray source. The noise level is significantly reduced. This improves the signal-to-noise ratio from 700 with the axially placed source to more than 3000 with the orthogonally placed source, while the resolving power still remains at R = 100. Furthermore, typical limits of detection for some model substances in the low pptv range in positive and negative ion mode are given.

Fast Orthogonal Separation by Superposition of Time of Flight and Field Asymmetric Ion Mobility Spectrometry.

Ion mobility spectrometry is a powerful and low-cost technique for the identification of chemical warfare agents, toxic chemicals, or explosives in air. Drift tube ion mobility spectrometers (DT-IMS) separate ions by the absolute value of their low field ion mobility, while field asymmetric ion mobility spectrometers (FAIMS) separate them by the change of their ion mobility at high fields. However, using one of these devices alone, some common and harmless substances show the same response as the hazardous target substances. In order to increase the selectivity, orthogonal data are required. Thus, in this work, we present for the first time an ambient pressure ion mobility spectrometer which is able to separate ions both by their differential and low field mobility, providing additional information for selectivity enhancement. This novel field asymmetric time of flight ion mobility spectrometer (FAT-IMS) allows high repetition rates and reaches limits of detection in the low ppb range common for DT-IMS. The device consists of a compact 44 mm drift tube with a tritium ionization source and a resolving power of 70. An increased separation of four substances with similar low field ion mobility is shown: phosgene (K0 = 2.33 cm2/(V s)), 1,1,2-trichlorethane (K0 = 2.31 cm2/(V s)), chlorine (K0 = 2.24 cm2/(V s)), and nitrogen dioxide (K0 = 2.25 cm2/(V s)). Furthermore, the behavior and limits of detection for acetonitrile, dimethyl methylphosphonate, diisopropyl methyl phosphonate in positive polarity and carbon dioxide, sulfur dioxide, hydrochloric acid, cyanogen chloride, and hydrogen cyanide in negative polarity are investigated.