Bradbury-Nielsen Gate Research Articles

Enhancing ion mobility spectrometry performance through a programmable ion swarm shaping method based on Bradbury-Nielsen gates

The ion gate is a critical element in drift tube ion mobility spectrometry (IMS) as it directly influences the resolving power and sensitivity of the system. However, the conventional Bradbury-Nielsen gate (BNG) often leads to deformation of the ion swarm shape, resulting in reduced resolving power and significant discrimination effects. To address these limitations, we propose a novel method that incorporates a cutting phase following the gate opening. This approach effectively reduces trailing edge deformation, resulting in a maximum resolving power of over 100 and increased signal intensity. Additionally, this method maintains high resolving power even during longer gate opening times. Remarkably, this method not only significantly reduces the mobility discrimination effect but also enables the achievement of reverse discrimination by adjusting the duration of the cutting phase. Consequently, it demonstrates the potential to selectively amplify the peak height of target ions. Our method offers straightforward implementation across all IMS systems utilizing the BNG, thereby significantly improving system performance.

Fast silicon carbide MOSFET based high-voltage push-pull switch for charge state separation of highly charged ions with a Bradbury-Nielsen gate.

In this paper, we report on the development of a fast high-voltage switch, which is based on two enhancement mode N-channel silicon carbide metal-oxide-semiconductor field-effect transistors in push-pull configuration. The switch is capable of switching high voltages up to 600V on capacitive loads with rise and fall times on the order of 10ns and pulse widths ≥20ns. Using this switch, it was demonstrated that, from the charge state distribution of bunches of highly charged ions ejected from an electron beam ion trap with a specific kinetic energy, single charge states can be separated by fast switching of the high voltage applied to a Bradbury-Nielsen Gate with a resolving power of about 100.

Modeling the modulation characteristics of the Bradbury-Nielsen gate in ion mobility spectrometers.

The Bradbury-Nelson gate (BNG) is a common device used for ion control in time-of-flight mass spectrometry and ion mobility spectrometry (IMS). A dual-location control model was employed in order to better understand the behavior of ions around a modulated BNG. This model illustrated that the ions are released from the starting location and truncated at the cutoff location. The shapes of the starting and cutoff locations are both curved with similar curvature, and the cutoff location is situated further back. Therefore, the distance between the two locations is a key parameter leading to the ion loss during modulation and is influenced by the gating voltage difference. Through simulations and experiments, the ion loss is verified to increase with the increase in the gating voltage difference. Taking a Fourier transform IMS as an example, by reducing the gating voltage difference from 150 to 50V, the signal-to-noise ratio of the time domain result was improved from 91.7 to 386.5 and the resolving power was improved from 40.9 to 63.6. In addition, the superposition effect of multicycle modulation is shown and explained by the model. When the modulated frequency is too rapid and the closing time is insufficient for all the ions to be consumed, some ions continue to exist between the two locations, and the residual ions then enter the drift region during the next few cycles. This phenomenon needs to be avoided because the total number of ions entering the drift region will then increase uncontrollably.

Large Bradbury–Nielsen Gate Optimized for Electrospray Time of Flight Spectroscopy

A large, , low-cost Bradbury–Nielsen gate (BNG) was developed and optimized to reduce diagnostic noise and high-energy ion impingement damage to electrospray thrusters during time-of-flight spectroscopy (TOFS). This new BNG increased diagnostic ion transparency by 633%, significantly increased the signal-to-noise ratio in the raw data collection, and reduced thruster damage due to high-energy ionic bombardment during ion diversion. By deflecting the thruster’s ionic plume, as opposed to the conventional method of reflection, the BNG can be operated at 50–75% lower cyclic voltages, thus reducing high-frequency noise induced by the required high-voltage, high-frequency gate operation. The BNG reduces high-energy ion impingement on the thruster’s extraction surface by up to 98.9%. This significant reduction in return current allows for unaltered measurements of performance and enables repeated diagnostic use without deleterious effects during long test campaigns.

Field-Switching Repeller Flowing Atmospheric-Pressure Afterglow Drift Tube Ion Mobility Spectrometry.

In this work, a field-switching (FS) technique is employed with a flowing atmospheric pressure afterglow (FAPA) source in drift tube ion mobility spectrometry (DTIMS). The premise is to incorporate a tip-repeller electrode as a substitute for the Bradbury-Nielsen gate (BNG) so as to overcome corresponding disadvantages of the BNG, including the gate depletion effect (GDE). The DTIMS spectra were optimized in terms of peak shape and full width by inserting an aperture at the DTIMS inlet that was used to control the neutral molecules' penetration into the separation region, thus preventing neutral-ion reactions inside. The FAPA and repeller's experimental operating conditions including drift and plasma gas flow rates, pulse injection times, repeller positioning and voltage, FAPA current, and effluent angle were optimized. Ion mobility spectra of selected compounds were captured, and the corresponding reduced mobility values were calculated and compared with the literature. The 6-fold improvements in limit of detection (LOD) compared with previous work were obtained for 2,6-DTBP and acetaminophen. The enhanced performance of the FS-FAPA-DTIMS was also investigated as a function of the GDE when compared with FAPA-DTIMS containing BNG.

3D-printing of a complete modular ion mobility spectrometer

Three-dimensional printing (3D-printing) is an asset for rapid prototyping. Therefore, this technology is applied in amateur, fundamental as well as in applied research communities. Moreover, the fused deposition modeling even allows a cheap and fast 3D-printing of functional filaments such as conductive or ferro-magnetic materials. Further applicable materials enable the production of electronic circuits, electrodes or even complete analytical sensors.Here, a 3D-printed stand-alone drift tube ion mobility spectrometer (IMS) completely manufactured in one process is presented. The ionization chamber, the Bradbury-Nielsen gate, the partly conductive drift tube, and the detector including an aperture grid and a Faraday plate were printed by dual extrusion 3D-printing. Therefore, non-conductive polylactic acid (PLA) was utilized for the housing and conductive PLA was used for the electrodes, the ion gate and the detector. Due to a magnetic quick lock system and a modular design, each part of this innovative ion mobility spectrometer can be exchanged in an instant. All parts were validated individually and as a whole by comparing them to an ion mobility spectrometer produced by computerized numerical control machining consisting of polytetrafluoroethylene (PTFE).To investigate the performance, parameters regarding the resolution, resolving power, signal-to-noise ratio, running-in period and long-term stability were used involving a set of isomeric volatile organic compounds (VOCs). Notably, every single 3D-printed component as well as the completely 3D-printed IMS could keep up with the traditionally produced ones. Furthermore, the signal intensity was comparable to the signal intensity of the PTFE-IMS in exemplary optimization steps. The resolution, resolving power and signal-to-noise ratio could be improved by the variation of the Bradbury-Nielsen gate design, thus demonstrating the potential of the 3D-printing for design optimization and rapid prototyping of analytical instrumentation.

Dropless ESI for IMS at ambient conditions

The generation and extraction of the charged particles (ions, clusters) from a fluid in a strong electric field at ambient conditions using a dynamic liquid flow splitter on the meniscus was examined. The existence of non-droplets regimes for obtaining charged particles for liquid flow rates up to 1000 μL/min, solute concentrations up to 1 M and acid content in solution up to 1% was displayed. Depending on the experimental conditions a conventional fluid electrospraying regime at ambient conditions in air as well as a dropless regime were demonstrated for obtaining charged particles current. The performance of the ion mobility spectrometer with a Bradbury-Nielsen gate at a dropless mode of charged particles generation was demonstrated for solutions of Human serum albumin (69,367 Da), Apoferritin (445,000 Da) and human epidermoid carcinoma A431 cells. The concentration of each substance in the solutions was ranging from 10−4 to 10−5 M. The concentration of cells in the suspension was 1 million/mL.

First off-line tests and performance of the Notre Dame multi-reflection time-of-flight mass spectrometer

A multi-reflection time-of-flight mass spectrometer (MR-TOF) will be a critical component for quickly removing radioactive contaminants produced at the future “N = 126 factory” addition to ATLAS at Argonne National Laboratory (ANL). This unique thermalized ion beam facility will employ multi-nucleon transfer reactions to produce very neutron-rich isotopes relevant to the astrophysical r-process. Ion beams produced at this facility will include isobaric contaminants, while precision measurements of such rare isotopes typically require highly purified samples. Hence, to tackle this issue an MR-TOF has been built and commissioned at the University of Notre Dame in an off-line test setup comprising a thermionic source and a Bradbury-Nielsen gate (BNG) to create the ions bunches to be purified. Resolving power above 30,000, with a corresponding efficiency of 35%, has been achieved. Most of the ion beam losses could be the result of the large radial divergence of ion bunches produced by the BNG, hence greater efficiency and resolving power are expected once the MR-TOF is connected to an radiofrequency quadrupole (RFQ) cooler and buncher at ANL.

Pushing the Resolving Power of Tyndall-Powell Gate Ion Mobility Spectrometry over 100 with No Sensitivity Loss for Multiple Ion Species.

Ion gate is a key buildup for drift tube ion mobility spectrometry (IMS) and its combination with mass spectrometry. Bradbury-Nielsen gate, as the most commonly used ion gate in IMS, possesses a distinct ion mobility discrimination effect due to its depletion features. This impedes the scaling of the ion gate opening time to improve the separation capability of IMS while keeping its sensitivity for multiple ion species. In this work, a Tyndall-Powell gate (TPG) simply composed of two identical wire grids was used to develop an ion gate with nearly no ion mobility discrimination for IMS. Experimental results showed that the TPG features a gate region where the electric field for opening the gate could be enhanced to effectively solve the ion mobility discrimination problem related to it. Meanwhile, enhancing that electric field enabled the TPG-IMS to keep a resolving power over 106 at 100 °C for ion peak with a signal-to-noise ratio up to 800. With that TPG-IMS, baseline separation of two ion peaks, the hydronium and the acetone monomer peaks with a reduced mobility difference of only 0.04 cm2 V-1 s-1, was achieved with no sensitivity loss for the least mobile acetone dimer ions.

Stabilization of gas-phase uranyl complexes enables rapid speciation using electrospray ionization and ion mobility-mass spectrometry

Significant challenges exist when characterizing f-element complexes in solution using traditional approaches such as electrochemical and spectroscopic techniques as they do not always capture information for lower abundance species. However, provided a metal-complex with sufficient stability, soft ionization techniques such as electrospray offer a means to quantify and probe the characteristics of such systems using mass spectrometry. Unfortunately, the gas-phase species observed in ESI-MS systems do not always reflect the solution phase distributions due to the inherent electrochemical mechanism of the electrospray process, ion transfer from ambient to low pressures conditions, and other factors that are related to droplet evaporation. Even for simple systems (e.g. hydrated cations), it is not always clear whether the distribution observed reflects the solution phase populations or whether it is simply a result of the ionization process. This complexity is further compounded in mixed solvent systems and when multiply charged species are present. Despite these challenges, the benefits of mass spectrometry with respect to speed, sensitivity, and the ability to resolve isotopes continue to drive efforts to develop techniques for the speciation of metal complexes. Using an electrospray ionization atmospheric pressure ion mobility mass spectrometer (ESI-apIMS-MS), we demonstrate an approach to stabilize simple uranyl complexes during the ionization process and mobility separation to aid speciation and isotope profile analysis. Specifically, we outline and demonstrate the capacity of ESI-apIMS-MS methods to measure mobilities of different uranyl species, in simple mixtures, by promoting stable gas phase conformations with the addition of sulfoxides (i.e. dimethyl sulfoxide (DMSO), dibutyl sulfoxide (DBSO), and methyl phenyl sulfoxide (MPSO)). Addition of these sulfoxides, as observed in the mass spectrum and mobility domain, produce stable gas-phase conformations that enable the observation of the counter anion pair while minimizing the range of ligand exchange events as the ionized complex enters the gas-phase. Other enhancements include improved data acquisition times by applying multiplexing approaches to the IMS Bradbury-Nielsen (BN) gate to realize increased ion transmission and improve ion statistics measured at the m/z detector. Analyte identification using this approach is based on a multitude of combined measured gas-phase ion metrics, which include mass measurements, isotope profiling, and experimentally determined reduced mobilities measured at the low-field limit (<2 E/N). Though geared initially towards uranyl complexes, this approach may find application in fields where both chemical speciation and isotopic profiles provide diagnostic information for a given metal.

Construction and simulation of a multi-reflection time-of-flight mass spectrometer at the University of Notre Dame

One of the most significant problems in the production of rare isotopes is the simultaneous production of contaminants, often time isobaric. Thus, a high-resolution beam purification method is required which needs to be compatible with both the low yield and short half-life of the desired radionuclide. A multi-reflection time-of-flight mass spectrometer meets all these criteria, in addition to boasting a smaller footprint relative to traditional separator dipole magnets. Such a device is currently under construction at the University of Notre Dame and is intended to be coupled to the IG-ISOL source of the planned cyclotron facility. The motivation and conceptual design are presented, as well as the status of simulations to determine the feasibility of using a Bradbury–Nielsen gate for bunching ion beams during initial system testing.

A progressively reduced pretension method to fabricate Bradbury-Nielsen gates with uniform tension.

A Bradbury-Nielsen gate (BNG) is often used to modulate ion beams. It consists of two interleaved and electrically isolated sets of wires with uniform tension, which ideally keep parallel, equidistant, and coplanar over a wide temperature range, making the BNG reliable and robust. We have previously analyzed the non-uniformity problem of wire tensions with sequentially winding method and developed a template-based transfer method to solve this problem. In this paper, we introduced a progressively reduced pretension method, which allows directly and sequentially fixing wires onto the substrate without using a template. Theoretical analysis shows that by applying proper pretension to each wire when fixing it, the final wire tensions of all wires can be uniform. The algorithm and flowchart to calculate the pretension sequence are given, and the fabrication process is introduced in detail. Pretensions are generated by weight combination with a weaving device. A BNG with stainless steel wire and a printed circuit board substrate is constructed with this method. The non-uniformity of the final wire tensions is less than 2.5% in theory. The BNG is successfully employed in our ion mobility spectrometer, and the measured resolution is 33.5 at a gate opening time of 350 μs. Compared to the template-based method, this method is simpler, faster, and more flexible with comparable production quality when manufacturing BNGs with different configurations.

Field Switching Combined with Bradbury-Nielsen Gate for Ion Mobility Spectrometry.

Bradbury-Nielsen gate (BNG) is commonly used in ion mobility spectrometers. It, however, transmits only a small fraction of the ions into the drift region, typically 1%. In contrast, all ions in the ionization chamber could be efficiently compressed into the drift region by the field switching gate (FSG). We report in this paper on the simultaneous use of BNG and field switching (FS) to enhance ion utilization of the BNG. In this technique, the FS collects the ions existing in the region between the FS electrode and the BNG and drives them quickly, going through the BNG in the period of gate opening. The BNG acts as the retarding field in the reported FSG to stop ions from diffusing into the drift region in the period of gate closing. Using this technique, an increase of at least 10-fold in the ion peak height without any loss of resolution is achieved for acetone compared with the BNG-only approach at a gate pulse width of 150 μs, and an even larger improvement factor of 21 is achieved for heavier DMMP dimer ions. This technique can be adapted to the current BNG-based ion mobility instruments to significantly enhance their sensitivity without any modification of the drift tube hardware.

Study of ion transmission for a linear mode Bradbury–Nielsen gate in ion mobility spectrometer

Fourier transform ion mobility spectrometer (FT-IMS) has the advantage of high ion utilization efficiency. Its two ion gates work in binary switch mode with square wave modulation function, which causes interference from high order scaled versions of the original spectrum. This problem can be avoided by using cosine modulation function, which requires the ion gates working in linear mode by quantitatively controlling its transmission in real time. In this paper, the influence of the structure parameters of Bradbury–Nielsen gate (BNG) and the electrical parameters on the ion transmission is investigated by SIMION simulation and experiment. The results show that the transmission in the open state is determined by the mechanical aperture ratio, and in the transition state is determined by the electric field ratio. In practical, the ion transmission can be quantitatively controlled by simply adjusting the voltage difference between adjacent wires when in the transition state. The nonlinearity between the transmission and the control voltage can be corrected by a simple calibration. This job proves the possibility of implementing a linear mode BNG, which can be used in further to improve the precision of the FT-IMS.

A simple template-based transfer method to fabricate Bradbury-Nielsen gates with uniform tension for ion mobility spectrometry.

A Bradbury-Nielsen gate (BNG) consists of two interleaved and electrically isolated sets of wires. It is usually used to gate or modulate ion beams. Uniformly tense wires can remain parallel, equidistant, and coplanar over a wide working temperature range, which is critical to reliable BNG performance. Hence, this study analyzes the non-uniform tension of wires wound using traditional sequential winding methods in which the elastic modulus of the metal wire is much larger than that of the insulation substrate. To address this problem, a simple and reliable template-based transfer method is developed. First, a template with large elastic modulus is used to fabricate a wire mesh with uniform tension. The mesh is then transferred to the substrate. Theoretically, this method reduces the non-uniformity of the tension in wires to less than 2%; therefore, it is used to construct a BNG with stainless steel wire, a stainless steel template, and a printed circuit board substrate. The BNG was installed in our homebuilt ion mobility spectrometer. To confirm that the performance of the BNG meets the requirements of portable ion mobility spectrometry, signal intensity and resolution (approximately 30) were experimentally determined.

Bradbury-Nielsen vs. Field switching shutters for high resolution drift tube ion mobility spectrometers

A key component in the design of every drift tube ion mobility spectrometer (IMS) is the ion shutter which controls the injection of ions into the drift tube. Especially, compact drift tube IMS require very short injection pulses to achieve high resolution and therefore require fast ion shutters. Thus, it is important to find an ion shutter principle that can be readily scaled towards these short injection widths without causing major non-idealities in the injection process, such as drift field inhomogeneities, ion loss and ion discrimination by mobility. In this paper, we compare different ion shutter principles, foremost the Bradbury-Nielsen gate and a field switching design. It is shown through theoretical considerations and field simulations that the Bradbury-Nielsen shutter is more universally applicable and typically less complex for long injections widths but field inhomogeneities associated with its operating principle impede the scaling process. Thus, the currently less used field switching shutters will become the superior principle when very short injection widths are required, as this shutter principle allows for single digit microsecond widths.

Bradbury-Nielsen gate electrode potential switching modes optimizing the ion packet time width in an ion mobility spectrometer

The methods minimizing duration of the output pulse of the ion mobility spectrometer at its maximum intensity were proposed in order to increase the instrument resolution. The optimal duration of the enabling pulse matched with the gate characteristics and ensuring the minimum time width of output ion packet with maximum intensity was chosen. The following operation modes of the Bradbury-Nielsen gate were considered: 1) time compression of the transmitted ion packet by pulsed increase of the potentials at the some set of wires; 2) instantaneous switching the potentials at the adjacent wires and 3) combination of the instantaneous switching of the potentials on the adjacent wires followed by the time compression of a transmitted ion packet. It was shown that the proposed gate modes ensure minimization of the output ion packet duration and increasing of the ion mobility spectrometer resolution. The estimated resolution of the ion peaks doubles and may reach 200 under considered conditions, space charge not taken into account.

Ion bunch stacking in a Penning trap after purification in an electrostatic mirror trap

The success of many measurements in analytical mass spectrometry as well as in precision mass determinations for atomic and nuclear physics is handicapped when the ion sources deliver “contaminations”, i.e., unwanted ions of masses similar to those of the ions of interest. In particular, in ion-trapping devices, large amounts of contaminant ions result in significant systematic errors—if the measurements are possible at all. We present a solution for such cases: The ions from a quasi-continuous source are bunched in a linear radio-frequency-quadrupole ion trap, separated by a multi-reflection time-of-flight section followed by a Bradbury–Nielsen gate, and then captured in a Penning trap. Buffer-gas cooling is used to damp the ion motion in the latter, which allows a repeated opening of the Penning trap for a stacking of mass-selected ion bunches. Proof-of-principle demonstrations have been performed with the ISOLTRAP setup at ISOLDE/CERN, both with 133Cs+ ions from an off-line ion source and by application to an on-line beam of 179Lu+ ions contaminated with 163Dy16O+ ions. In addition, an optimization of the experimental procedure is given, in particular for the number of ion bunches captured as a function of the ions’ lifetimes and the parameters of the experiment .

Bradbury–Nielsen–Gate–Grid Structure for Further Enhancing the Resolution of Ion Mobility Spectrometry

In our previous work we proposed a three-zone theory for the Bradbury-Nielsen (BN) gate and proved with a grid-BN structure ion mobility drift tube that enhancements of the three-zone features led to higher resolutions and sometimes higher sensitivities. In this work we continued to seek further improvements of the resolution performance by adopting a BN-grid structure in the same drift tube. The postgate grid works both for confinement of the BN gate induced electric field and for isolation of the injection field from the drift field. This makes it possible to obtain better resolutions by further enhancing the compression electric field and lowering the injection field. It was found in the following experiments that reducing the injection field led to higher resolutions yet lower sensitivities. At an injection field of 140 V/cm, the inverse compression coefficient was found to be much larger than that in the grid-BN structure at all gating voltage differences (GVDs). At GVD = 350 V and a gate pulse width of 0.34 ms, the ion mobility spectrometry efficiency R(m)/R(c) reached as high as 221% in the BN-grid structure, presenting a further increase compared to 182% in the grid-BN structure. Finally, two examples are given to show the separation power improvements with good resolutions.

On-line separation of short-lived nuclei by a multi-reflection time-of-flight device

A multi-reflection time-of-flight (MR-ToF) mass analyzer has been integrated into ISOLTRAP, the precision mass spectrometer for on-line mass determinations of short-lived nuclides at ISOLDE/CERN. The new instrument improves ISOLTRAP by providing a fast separation of isobaric contaminant species as well as subsequent ion selection using the fast Bradbury–Nielsen gate. Suppression ratios of up to 104 and mass-resolving powers of over 105 have been reached in off-line experiments. Preliminary data from on-line applications illustrate the benefit and performance of the device and its potential in the context of the ISOLTRAP setup.