An open source ion gate pulser for ion mobility spectrometry

Excluding the ion source, an ion mobility spectrometer is fundamentally comprised of drift chamber, ion gate, pulsing electronics, and a mechanism for amplifying and recording ion signals. Historically, the solutions to each of these challenges have been custom and rarely replicated exactly. For the IMS research community few detailed resources exist that explicitly detail the construction and operation of ion mobility systems. In an effort to address this knowledge gap we outline a solution to one of the key aspects of a drift tube ion mobility system, the ion gate pulser. Bradbury-Nielsen or Tyndall ion gates are found in nearly every research-grade and commercial IMS system. While conceptually simple, these gate structures often require custom, high-voltage, floating electronics. In this report we detail the operation and performance characteristics of a wifi-enabled, MOSFET-based pulser design that uses a lithium-polymer battery and does not require high voltage isolation transformers. Currently, each output of this circuit follows a TTL signal with ~20 ns rise and fall times, pulses up to +/? 200 V, and is entirely isolated using fiber optics. Detailed schematics and source code are provided to enable continued use of robust pulsing electronics that ease experimental efforts for future comparison.     

Simulation aided design of a low cost ion mobility spectrometer based on printed circuit boards

Miniaturized low-cost drift tubes with high analytical performance are a key component for the design of powerful and mass-deployable hand-held ion mobility spectrometers. Thus, a simple model that estimates the influence of the geometrical dimensions on the analytical performance is highly desirable for an effective design process. In this work, we present a simple procedure to predict peak distortion based on only the electrical field distribution inside the drift tube, which can be rapidly simulated using the finite element method. A simulation of the ion motion is not required. Based on these results, we developed an ion mobility spectrometer manufactured entirely from standard printed circuit boards (PCB). Since no additional components were used apart from electrical and gas connectors, ion source and metal grids, the presented ion mobility spectrometer is very simple and inexpensive. Nevertheless, the design provides a resolving power of 82 at a drift length of 50 mm and a drift voltage of 3 kV using a tritium ion source and a field switching shutter. The limits of detection for one second of averaging are 80 ppt_v for acetone, 35 ppt_v for dimethyl methylphosphonate and 180 ppt_v for methyl salicylate.     

An alternative field switching ion gate for ESI-ion mobility spectrometry

In electrospray ionization (ESI)-ion mobility spectrometry, continuously generated ions must be desolvated in a first tube before short ion pulses are introduced into a second (drift) tube. Both tubes are separated by an ion-gate. The resolving power of the resulting drift time spectrum is strongly influenced by the design of the ion gate. In the case of the Bradbury-Nielsen gates typically used, an orthogonal field between oppositely charged, parallel wires blocks ions from entering the drift tube. However, the blocking field also distorts the entering ion cloud. One alternative, which eliminates these effects and therefore enables a potentially higher resolving power, is already known for spectrometers with small ionization volumes, where ions are formed between two electrodes and subsequently transferred into the drift tube by a high voltage pulse. Based on this setup, we introduce an alternative ion gate design for liquid samples, named field switching ion gate (FSIG). The continuous flow of ions generated by ESI is desolvated in the first tube and introduced into the space between two electrodes (repeller and transfer electrodes). A third (blocking) electrode prevents the movement of ions into the drift tube in the closed state. Ions are transferred during the open state by pulsing the voltages of the repeller and blocking electrodes. First results demonstrate an increase of the resolving power by 100% without intensity losses and further changes in the spectrometer setup. The parameters of the FSIG, such as electrode voltages and pulse width, are characterized allowing the optimization of the spectrometer’s resolving power.     

Expanded theory for the resolving power of a linear ion mobility spectrometer

An expanded theory for the resolving power of a linear ion mobility spectrometer (IMS) is derived. By definition, the resolving power is directly proportional to the total drift time for the ion through the drift tube divided by the full-width-at-half-height (FWHH) of the observed ion mobility peak. Two approaches to theoretically estimating these two parameters are possible, depending on the operating parameters of the IMS cell. The drift time is given by the first moment of the IMS response. If the electric fields (assumed uniform) are equal in both the shutter/aperture and aperture/collector region, the FWHH is given by a difference in error functions. If the electric fields (again assumed uniform) are not equal, the FWHH is given by the second central moment of the IMS response and can only be known to within a multiplicative factor. The effectiveness of these two approaches is demonstrated using IMS data from the published literature.The additional peak broadening often observed in a linear IMS has several possible sources. One depends on the construction of the cell and the parallelism (or lack thereof) that might exist between the aperture grid and ion collector. Another depends on electric fields used to bias the cell. If the electric field in the aperture/collector region is less than in the shutter/aperture region, peak broadening occurs. Induction effects in the aperture/collector region not only shorten drift times, but also create diffusion-like broadening of the peak. Shortening the distance between the aperture grid and ion collector, or using a higher electric field in that region, minimizes induction effects. Drift time calibration requires adjustments for induction effects.     

微波诱导等离子体离子迁移谱仪性能及影响因素的研究

离子迁移谱仪的性能受到多种因素的影响,如漂移管电场强度离子门脉冲宽度、离子源工作条件、漂移管尺寸、离子门加工工艺和屏蔽网透过率等。在实际应用中需要对漂移管电场强度和离子门脉冲宽度进行调整以平衡灵敏度和分辨率。本研究详细研究了漂移管电场强度和离子门脉冲宽度对微波诱导等离子体离子迁移谱( MIPI-IMS)分辨率和灵敏度的影响。实验结果表明,存在一个最佳电场强度值使得分辨率达到最大,而且不同离子门脉冲宽度对应的最佳电场强度值不同;增大电场强度和离子门脉冲宽度有利于灵敏度的提升。与其它离子流较弱的离子源相比,离子流较大的微波诱导等离子体离子源在实际应用中对离子门脉冲宽度和漂移管电场强度有更多的选择。此研究结果有助于MIPI-IMS仪器性能的提升。将异丙醇用于测试MIPI-IMS的性能,结果表明,MIPI-IMS在保持较低检出限(7.7×10-11, V/V)的同时,分辨率可以达到66。     

A novel surface ionization source for ion mobility spectrometer

A surface ionization (SI) source is designed and prepared for ion mobility spectrometer (IMS). The source acts not only as an emitter but also an ion injector which can inject ions periodically into the drift region of drift tube. Using the dual-role source, the dimension of the drift tube can be decreased and the circuit for high voltage can be simplified efficiently. The IMS with the SI source has a response range of ∼4 orders of magnitude and a good reproducibility to tri-ethylamine. Compared with radioactive ionization (RI), the ultra-short time for ion injection and the zero level base line of ion mobility spectrum are characteristics of the surface ionization.     

Monte Carlo simulation of ion transport in ion mobility spectrometry

A program for simulation of ion trajectories in ion mobility spectrometry (IMS) instruments has been developed and incorporated into SIMION 7.0 [Int. J. Mass Spectrom. 200 (2000) 3–25]. Simulations were based on elastic collisions between ions and gas particles and conducted for an IMS drift tube. The program was validated by comparing the reduced mobility of helium ions derived from the simulation with the experimental data for helium ions in neon drift gas in low electric fields. Typical IMS parameters, including pressure, temperature, and flow rate of the drift gas were taken into account in the simulations. The program demonstrates capabilities of generating IMS spectra and predicting ion transport efficiency and separating ions. For the IMS drift tube studied, a correlation between imperfection of the electric field distribution and low resolution has been observed.     

A simulation model study of the coupled field in the IMS drift tube

The performance of the IMS was influenced by many parameters, like temperature, gas flow rate, etc. in the drift tube. An exact and comprehensive simulation model was very useful for the IMS design and optimization. A combined simulation model was build up for the parameters simulation in the drift tube. Based on this simulation model, the heat transfer, velocity distribution, humidity and ion transportation inside the drift tube in bidirectional flow stream was simulated, and the impact on the IMS was studied. And the simulation was also validated using an IMS constructed in our laboratory. The experiment showed that the RIP intensity weakened as the humidity increasing, but the signal intensity of NO was enhanced first, and then decreased with the humidity increasing sequentially. This can be explained from the simulation results. The simulation results showed that the distribution of the velocity and temperature was not uniformed in the drift tube. And this phenomenon was more clearly when the gas flow velocity increased. It can be seen from the simulation that the humidity in the drift tube region was smaller than the sample moisture, and the resolution of the ion mobility spectrometry will be reduced by the humidity. But in the region rich in water molecules, ultraviolet photons re-acting with acetone would be obviously decreased and fewer re-agent ions were produced owing to the strong absorption of photons by water neutrals. The results showed that the coupled field simulation model can be used to study parameters effects on the IMS.     

Decomposition of overlapping plasmagram peaks by spectral subtraction

Low field atmospheric pressure Ion Mobility Spectroscopy (IMS) involves the careful analysis of plasmagrams with multiple peaks which can mask one another when they are closely spaced in drift time or corresponding reduced mobility. A typical signal processing approach to decomposing overlapped peaks would be to use an orthogonal decomposition technique, but unfortunately Gaussian-like functions are not orthogonal, so no unique decomposition can be guaranteed. However, each ion species in the drift tube will arrive at the Faraday plate with a known statistical distribution determined by the IMS instrument’s drift tube design, electric field strength, reagent gas flow and other instrument-specific factors such as the ion gate function. This paper presents a straightforward algorithm for decomposing plasmagrams into distinct peaks using a subtractive technique that independently estimates the statistical parameters of each peak, rejecting spurious peaks and electrical noise. The results show that for relatively short gate times, the plasmagram peaks are nearly Gaussian-shaped, but slightly fatter and asymmetric. We show that including of the gate rise and fall times is also significant in matching the plasmagram peak shape. We also show that the diffusion effects on resolution can be attributed to combinations of non-uniform ion distributions in the reaction chamber as well as detritus effects in the drift tube. Given the known peaks statistical parameters, one can then separate overlapping peaks using a straightforward spectral subtractive technique.     

Evaluation of micro-electrospray ionization with ion mobility spectrometry/mass spectrometry

In recent years, the resolving power of ion mobility instruments has been increased significantly, enabling ion mobility spectrometry (IMS) to be utilized as an analytical separation technique for complex mixtures. In theory, decreasing the drift tube temperature results in increased resolution due to decreased ion diffusion. However, the heat requirements for complete ion desolvation with electrospray ionization (ESI) have limited the reduction of temperatures in atmospheric pressure ion mobility instruments. Micro-electrospray conditions were investigated in this study to enable more efficient droplet formation and ionization with the objective of reducing drift tube temperatures and increasing IMS resolution. For small molecules (peptides), the drift tube temperature was reduced to ambient temperature with good resolution by employing reduced capillary diameters and flow rates. By employing micro-spray conditions, experimental resolution values approaching theoretically predicted resolution were achieved over a wide temperature range (30 to 250 °C). The historical heat requirements of atmospheric pressure IMS due to ESI desolvation were eliminated due to the use of micro-spray conditions and the high-resolution IMS spectra of GLY-HIS-LYS was obtained at ambient temperature. The desolvation of proteins (cytochrome c) was found to achieve optimal resolution at temperatures greater than 125 °C. This is significantly improved from earlier IMS studies that required drift tube temperatures of 250°C for protein desolvation.     

Mobility spectrometry of amino acids and peptides with matrix assisted laser desorption and ionization in air at ambient pressure

Gas phase ions for valine, glutamate, phenylalanine, angiotensin, bradykinin, LH-RH, and bombesin were formed through matrix assisted laser desorption-ionization (MALDI) in air at ambient pressure and were characterized by ion mobility spectrometry (IMS). The IMS drift tube was operated at 100 °C with air as the drift gas and without an ion shutter. Responses were obtained using α-cyano-4-hydroxycinnamic acid as the matrix and a Nd-YAG laser at 355 nm with an unfocused beam at 6 mJ per pulse and 7 mm² cross section. Matrix and analyte were applied to a borosilicate glass target and microgram amounts of sample provided responses lasting 10 to 15 s with the laser operated at 11 Hz. Detection limits for the peptides were estimated to be 10 to 100 pmol per laser shot. The mobility spectra for individual amino acids and peptides exhibited multiple peaks with spectral distortions and raised baselines. These features and calculated values for reduced mobilities were consistent with the existence of clusters between analyte ions and matrix neutrals and the dissociation of these clusters in the drift region of the analyzer. Mobility spectra with distinctive peaks were not obtained for MALDI-IMS of peptides larger than 5700 amu, though ion formation was suggested from the depletion of matrix signal.     

Ion Mobility Sensor In Environmental Analytical Chemistry—Concept And First Results

Abstract

Ion mobility spectrometry is a technique for generating ions at atmospheric pressure via ion-molecule reactions, and for analysing them in an ion drift tube.

The time required for the ions to traverse the length of the drift tube is mainly a function of the mass and the charge of the ions. Besides, ion shape and polarizability also affect the drift time.

Ion mobility spectrometry does not allow structural identification and quantification of unknown substances in mixtures. However, under certain boundary conditions it provides selective fingerprints of the substances to be observed, and operates at the ppbv concentration level and the millisecond time scale.

Through further miniaturization of a recently developed instrument of this type an ion mobility sensor is to be constructed. This sensor includes drift channel, operating shutter, collecting electrode, electronic data acquisition and translation board. The sensor makes possible to obtain real-time ion mobility spectra.

We present and discuss the concept of a small ion mobility spectrometer, its operation principle and first results on the way towards its further miniaturization.     

迁移管的电场强度对真空紫外电离-离子迁移谱仪性能的影响

离子迁移管是离子迁移谱仪的核心部分,它用来产生均匀的电场,以使不同迁移率的离子进行分离。本研究以丙酮为例,详细研究了本课题组所研制的真空紫外电离源-离子迁移谱仪中迁移管的电场参数对离子的灵敏度和分辨率的影响,发现电压的增大灵敏度增大,但是分辨率存在一个最佳的电压,这些结果可用于迁移谱的优化设计。     

Development of an ion mobility spectrometer for use in an atmospheric pressure ionization ion mobility spectrometer/mass spectrometer instrument for fast screening analysis

An ion mobility spectrometer that can easily be installed as an intermediate component between a commercial triple-quadrupole mass spectrometer and its original atmospheric pressure ionization (API) sources was developed. The curtain gas from the mass spectrometer is also used as the ion mobility spectrometer drift gas. The design of the ion mobility spectrometer allows reasonably fast installation (about 1 h), and thus the ion mobility spectrometer can be considered as an accessory of the mass spectrometer. The ion mobility spectrometer module can also be used as an independently operated device when equipped with a Faraday cup detector. The drift tube of the ion mobility spectrometer module consists of inlet, desolvation, drift, and extraction regions. The desolvation, drift and extraction regions are separated by ion gates. The inlet region has the shape of a stainless steel cup equipped with a small orifice. Ion mobility spectrometer drift gas is introduced through a curtain gas line from an original flange of the mass spectrometer. After passing through the drift tube, the drift gas serves as a curtain gas for the ion-sampling orifice of the ion mobility spectrometer before entering the ion source. Counterflow of the drift gas improves evaporation of the solvent from the electrosprayed sample. Drift gas is pumped away from the ion source through the original exhaust orifice of the ion source. Initial characterization of the ion mobility spectrometer device includes determination of resolving power values for a selected set of test compounds, separation of a simple mixture, and comparison of the sensitivity of the electrospray ionization ion mobility spectrometry/mass spectrometry (ESI-IMS/MS) mode with that of the ESI-MS mode. A resolving power of 80 was measured for 2,6-di-tert-butylpyridine in a 333 V/cm drift field at room temperature and with a 0.2 ms ion gate opening time. The resolving power was shown to be dependent on drift gas flow rate for all studied ion gate opening times. Resolving power improved as the drift gas flow increased, e.g. at a 0.5 ms gate opening time, a resolving power of 31 was obtained with a 0.65 L/min flow rate and 47 with a 1.3 L/min flow rate for tetrabutylammonium iodide. The measured limits of detection with ESI-MS and with ESI-IMS/MS modes were similar, demonstrating that signal losses in the IMS device are minimal when it is operated in a continuous flow mode. Based on these preliminary results, the IMS/MS instrument is anticipated to have potential for fast screening analysis that can be applied, for example, in environmental and drug analysis.     

Results of a transient simulation of a drift tube ion mobility spectrometer considering charge repulsion, ion loss at metallic surfaces and ion generation

With optimized geometry and operating parameters both IMS selectivity and sensitivity can be significantly increased. However, finding these parameters and geometry requires an accurate knowledge of the electrical field and the ion concentration within the IMS at any time of operation. Furthermore, the ion loss at metallic surfaces and space charge effects caused by the moving ion cloud must be considered. This is particularly true when using non-radioactive electron emitters which generate a comparably high space charge density at electron currents similar to radioactive beta-sources due to their smaller ionization volume. This can lead to a reduced IMS resolution mainly caused by coulomb repulsion. In this work a transient model which enables a detailed view on the electric field within the IMS considering ion diffusion and migration as well as ion loss and coulomb repulsion is presented. This finite element model provides excellent agreement between simulated IMS spectra and experimental data especially when considering space charge effects and coulomb repulsion respectively. The model is used to design a short drift tube IMS with significantly improved resolution. Furthermore, this model allows considering ion-ion and ion-neutral reactions, such as ion generation, charge transfer reactions and ion-ion recombination. Moreover, fluid dynamics can be considered as required for modeling aspiration type IMS.     

A critical review of ion mobility spectrometry for the detection of explosives and explosive related compounds

Ion mobility spectrometry has become the most successful and widely used technology for the detection of trace levels of nitro-organic explosives on handbags and carry on-luggage in airports throughout the US. The low detection limits are provided by the efficient ionization process, namely, atmospheric pressure chemical ionization (APCI) reactions in negative polarity. An additional level of confidence in a measurement is imparted by characterization of ions for mobilities in weak electric fields of a drift tube at ambient pressure. Findings from over 30 years of investigations into IMS response to these explosives have been collected and assessed to allow a comprehensive view of the APCI reactions characteristic of nitro-organic explosives. Also, the drift tube conditions needed to obtain particular mobility spectra have been summarized. During the past decade, improvements have occurred in IMS on the understanding of reagent gas chemistries, the influence of temperature on ion stability, and sampling methods. In addition, commercial instruments have been refined to provide fast and reliable measurements for on-site detection of explosives. The gas phase ion chemistry of most explosives is mediated by the fragile CONO(2) bonds or the acidity of protons. Thus, M(-) or M.Cl(-) species are found with only a few explosives and loss of NO(2), NO(3) and proton abstraction reactions are common and complicating pathways. However, once ions are formed, they appear to have stabilities on time scales equal to or longer than ion drift times from 5-20 ms. As such, peak shapes in IMS are suitable for high selectivity and sensitivity.     

Ion mobility spectrometer—field asymmetric ion mobility spectrometer-mass spectrometry

Since the development of electrospray ionization (ESI) for ion mobility spectrometry mass spectrometry (IMMS), IMMS have been extensively applied for characterization of gas-phase bio-molecules. Conventional ion mobility spectrometry (IMS), defined as drift tube IMS (DT-IMS), is typically a stacked ring design that utilizes a low electric field gradient. Field asymmetric ion mobility spectrometry (FAIMS) is a newer version of IMS, however, the geometry of the system is significantly different than DT-IMS and data are collected using a much higher electric field. Here we report construction of a novel ambient pressure dual gate DT-IMS coupled with a FAIMS system and then coupled to a quadrupole ion trap mass spectrometer (QITMS) to form a hybrid three-dimensional separation instrument, DT-IMS-FAIMS-QITMS. The DT-IMS was operated at ~3 Townsend (electric field/number density (E/N) or (Td)) and was coupled in series with a FAIMS, operated at ~80 Td. Ions were mobility-selected by the dual gate DT-IMS into the FAIMS and from the FAIMS the ions were detected by the QITMS for as either MS or MSⁿ. The system was evaluated using cocaine as an analytical standard and tested for the application of separating three isomeric tri-peptides: tyrosine-glycine-tryptophan (YGW), tryptophan-glycine-tyrosine (WGY) and tyrosine-tryptophan-glycine (YWG). All three tri-peptides were separated in the DT-IMS dimension and each had one mobility peak. The samples were partially separated in the FAIMS dimension but two conformation peaks were detected for the YWG sample while YGW and WGY produced only one peak. Ion validation was achieved for all three samples using QITMS.     

Letter: A simple ion source set-up for desorption/ionization on silicon with ion mobility spectrometry and ion mobility spectrometry-mass spectrometry

Using a simple ion source set-up, laser desorption/ionization on silicon (DIOS) was demonstrated with the use of a custom-made drift tube ion mobility spectrometer (IMS), mounted on a commercial triple quadrupole mass spectrometer, and with an IMS equipped with a Faraday plate detector. DIOS was tested by mobility measurement of tetrapropylammonium iodide, tetrabutylammonium iodide and tetrapentylammonium iodide, whilst 2,6-di-tert- butylpyridine was used as a standard. The reduced mobilities measured for the test halides are in concordance with previously obtained ion mobility spectrometry-mass spectrometry data.     

Evaluation of micro- versus nano-electrospray ionization for ambient pressure ion mobility spectrometry

The comparison of nanospray and microspray ionizations for detecting mixtures of compounds by ion mobility spectrometry has been investigated for sensitivity, ion transmission through a drift tube, and ion suppression effects when used as an ionization source for ambient pressure ion mobility spectrometry (IMS). Several articles have demonstrated that nano-electrospray ionization mass spectrometry (n-ESI-MS) has improved sensitivity, provides less background noise, and lower limits of detection than micro-electrospray ionization (μ-ESI) for IMS. Most importantly, data from n-ESI-MS is concentration-sensitive. Our laboratory previously published an article that observed a striking result when μ-ESI-IMS was investigated for a single compound in the positive ion mode. The data reported was mass-sensitive. In this new investigation, we have investigated mixtures, and experiments were designed to evaluate the effect of sensitivity, ion transmission and ion suppressions in μ-ESI-IMS and n-ESI-IMS. At an electrospray flow rate in the μL min⁻¹ range, compounds with higher proton affinities responded best while at the nanospray flow rates of nL min⁻¹, relative responses were more equal. This study observed that a decreased ESI flow rate resulted in a decreased ion signal. These trends demonstrated less sensitivity for ESI-IMS at reduced flow rates but suggest better quantification. At higher flow rates, relative ionization efficiencies were still uniform for all the components studied individually and in mixtures and sensitivity improved by about 78%. Concentration studies showed that at high concentrations, ion detection efficiencies were uniform at about 33% for all compounds studied individually and in mixtures. At low concentrations, the detection efficiency varied from 31% to 86%, depending on the proton affinity of the component in the mixture. Ion transmission through the IMS tube measured with a segmented Faraday detector that was incorporated into the IMS design indicated that most of the ion current for mixtures was transported through the IMS tube with a radius of less than 18 mm for both positive and negative ion modes.