Ion Optics Research Articles

Efficient Coupling of Structures for Lossless Ion Manipulations with Ion Trap Mass Analyzers Using Phase Modulation.

Phased structures for lossless ion manipulation offer significant improvements over the scanning second gate method for coupling with ion trap mass analyzers. With an experimental run time of under 1 min for select conditions and an average run time of less than 4 min, this approach significantly reduces experimental time while enhancing the temporal duty cycle. The outlined SLIM system connects to an ion trap mass analyzer via a PCB stacked ring ion guide, which replaces the commercial ion optics and capillary inlet. By applying a discrete and repeating injection pulse and solving a series of algebraic equations, the system reconstructs an arrival time distribution with a minimal degree of error with enhanced ion throughput. To demonstrate the feasibility of this approach, the 3.4-m SLIM system resolves gas-phase conformers for various small peptides and proteins. This system and methodology also enable direct implementation between SLIM and ion trap mass analyzers traditionally interfaced with front separation systems such as liquid chromatography.

Design of a compact, high-resolution velocity-map imaging spectrometer for attosecond spectroscopy.

We present the design of a VMI spectrometer optimized for attosecond spectroscopy in the 0-40eV energy range. It is based on a compact three-electrode configuration where the lens shape, size, and material have been optimized using numerical simulations to improve the spectral resolution by a factor of ∼5 relative to the initial design [Eppink and Parker, Rev. Sci. Instrum. 68, 3477-3484 (1997)] while keeping a flat spectral response in the 10-40eV range. The experimental performance is tested using an attosecond source based on high-order harmonic generation. A good agreement is observed between the measured and simulated spectral resolution. At low kinetic energy, the electrostatic lens remains the limiting factor, while the high energy range is mostly affected by the resolution of the camera objective.

A Computer Simulation Study on Ion Optics Aiming at the Realization of Projection-Type Mass Spectrometry Imaging

The projection mode in mass spectrometry imaging (MSI) obtains ion images by retaining the spatial distribution of desorbed ions immediately after their generation as the ion images, then projecting them onto an ion detector of time-of-flight mass spectrometry. Compared to conventional MSI, projection-type MSI can deliver overwhelmingly higher spatial resolution. The mass resolution of projection-type MSI can be improved by a technique named post extraction differential acceleration (PEDA). To assess the feasibility of projection-type MSI with minimal modification of a commercially available laser desorption/ionization time-of-flight mass spectrometer, ion trajectory simulations were performed using the ion optics simulation program SIMION. Under boundary conditions that reproduce the ion acceleration region of the commercial mass spectrometer with a high fidelity, the trajectory simulation results were obtained for a huge number of combinations of ion masses, initial conditions, PEDA parameters, etc. The data analysis provided useful findings that can lead to the realization of projection-type MSI based on the modified commercial mass spectrometer, for instance, the imaging conditions for ion images in the commercial equipment, guidelines for optimizing PEDA parameters, and the effects of differences in PEDA high-voltage rising waveforms.

Spatial resolution of a velocity-selected ion imaging microscope for surface reaction kinetics mapping.

Experimental validation of complex microkinetic models derived from quantum chemistry is crucial for the advancement of bottom-up approaches to heterogeneous catalysis. State-of-the-art velocity-resolved kinetics experiments have made tremendous progress in this arena but integrate reactivity over centimeter-scale single-crystal catalytic surfaces even when complex spatial phenomena may perturb the kinetic results. We report a new design, optimization, and analysis of an ion imaging microscope that can collect spatially resolved kinetic data from a catalytic surface. In its simplest configuration, gaseous reaction products are ionized by a laser line or sheet above a catalytic surface. The resulting ions are extracted and strongly lensed to an intermediate velocity-mapped plane where a pinhole of radius r only transmits ions produced from reaction products with desorption velocities within a narrow solid angle centered on the surface normal. Transmitted ions re-expand through an electrostatic zoom lens to form a spatial image of the initial reaction product distribution with reduced blur from desorption velocity components parallel to the surface. The ion hits that define the magnified and deblurred spatial image can be used to determine spatiotemporal flux and speed-distributions of gas leaving the catalyst surface. Electrostatic trajectory simulations are performed and verify that transmission is ∝r2/TSurface. However, calculated global point spread functions acting on the magnified image have a width that is ∝r and largely independent of TSurface. Thus, velocity-filtered ion imaging microscopy can deliver a consistent resolution as the TSurface is varied, which is a great advantage because many catalytic reactions require elevated temperatures.

Focused deposition of levitated nanoscale Au droplets

We describe a method for depositing nanoscale liquid Au droplets, initially levitated in an ion trap in high vacuum, onto a remote substrate. A levitated Au nanosphere is melted, expelled from the trap, and maintained in the molten state, with a laser directed along the droplet trajectory, until it reaches the substrate and rapidly solidifies. During transit, the charged droplets are focused to a small region of the substrate with an electrostatic lens. After deposition, the substrate is removed from the vacuum chamber and imaged and analyzed by techniques such as electron microscopy and energy dispersive spectroscopy. Over 90% of launched particles are deposited on the substrate, and when the lens is focused, particles land in a region of diameter 120 μm after traversing a distance of 236 mm. Our technique is of value for analysis of materials prepared or processed while levitated that can be melted. Also, Au droplets may be useful as tracers for future experiments involving smaller projectiles or oriented solids.

Design and performance of a magnetic bottle electron spectrometer for high-energy photoelectron spectroscopy.

We describe the design and performance of a magnetic bottle electron spectrometer (MBES) for high-energy electron spectroscopy. Our design features a 2m long electron drift tube and electrostatic retardation lens, achieving sub-electronvolt (eV) electron kinetic energy resolution for high energy (several hundred eV) electrons with a close to 4π collection solid angle. A segmented anode electron detector enables the simultaneous collection of photoelectron spectra in high resolution and high collection efficiency modes. This versatile instrument is installed at the time-resolved molecular and optical sciences instrument at the Linac Coherent Light Source x-ray free-electron laser (XFEL). In this paper, we demonstrate its high resolution, collection efficiency, and spatial selectivity in measurements where it is coupled to an XFEL source. These combined characteristics are designed to enable high-resolution time-resolved measurements using x-ray photoelectron, absorption, and Auger-Meitner spectroscopy. We also describe the pervasive artifact in MBES time-of-flight spectra that arises from a periodic modulation in electron collection efficiency and present a robust analysis procedure for its removal.

A versatile setup for hydrogen isotope permeation studies.

The Testbed for Analysis of Permeation of Atoms in Samples (TAPAS) is an experimental setup for ion-driven permeation studies with a focus on investigating wall materials for nuclear fusion devices. A monoenergetic, mass-filtered high-intensity keV ion beam is focused and directed onto the permeation sample by electrostatic ion optics and decelerated to the desired ion energy by a dedicated set of apertures close to the sample. We were able to obtain ion energies as low as 170 eV/D with a D3+ ion beam with an ion flux density of the order of 1020 D/m2s on a beam-wetted area of ∼33mm2. These conditions avoid sputtering of W targets by the ion beam and are representative of the particle flux and energy spectrum impinging on the first wall of a prospective nuclear fusion power reactor. Permeation samples can be heated up to 1000K in an ultra-high vacuum. The design of the deceleration system, together with a high pumping speed in the loading chamber, ensures a low pressure of recycling hydrogen isotope molecules in front of the sample. In addition to ion-driven permeation, TAPAS provides a limited capability for gas-driven permeation at low pressures up to nearly 1mbar. Permeating hydrogen isotopes are detected with a quadrupole mass spectrometer in the downstream ultra-high vacuum chamber. After a detailed description of the setup and calibration procedures for implanted particle flux, mass spectrometer, and neutral gas pressure, benchmark experiments on recrystallized, 50μm thick tungsten foils are shown, demonstrating that diffusion-limited boundary conditions for permeation were reached.

Velocity-map imaging with counter-propagating laser pulses.

Velocity-map imaging (VMI) is a key tool for studying outgoing electrons or ions following optical strong-field interactions of atoms and molecules and provides good momentum resolution even if the source volume of the fragments extends along a laser beam path. Here, we demonstrate within an enhancement cavity how, independently of the focal Rayleigh length, counter-propagating pulses longitudinally compress the ionization volume down to few tens of micrometers. We observe nonlinear above-threshold ionization (ATI) processes confined to the spatial overlap of femtosecond pulses, whereas the shortened ionization volume makes an electrostatic lens unnecessary for VMI.

Effects of translation misalignment on ion optics with slit apertures

For ion thrusters, the deflection of the ion beam caused by the relative translation of the grids is one of the primary factors limiting the erosion lifetime of the ion optical system. A two-dimensional (2D) simulation package of the ion optic system is developed to investigate the ion sputtering corrosion due to grid translation misalignment. For benchmark cases, the 2D simulation package shows a reasonable consistency compared to the experimental method in ion beam deflection angle and drain-to-beam current ratio, indicating the effectiveness of the simulation package. The deviation between the simulated deflection angle and the experimental value is within 8.86%. Furthermore, this 2D package is employed to analyze the variation patterns of ion beam, ion collection, and the distribution of ion sputtering rates caused by grid translation. The simulation results indicate that the ion beam deflection occurs in the direction opposite to grid translation. The number of ions collected at different positions on the acceleration grid shows different tendencies. Only the upstream surface Sy− and the aperture surface Sx+ will be subjected to energetic ion impingement. The sputtering of energetic ions on these two surfaces becomes the dominant factor limiting the grid’s ion corrosion lifetime when the grid translation is significant. The sputtering rate of energetic ions can exceed that of charge exchange (CEX) ions by more than 10 times. The proportion of the region where energetic ions contribute to sputtering expands with increasing grid misalignment, reaching up to 52.5% on surface Sx+. Additionally, the downstream surface Sy+ and the aperture surface Sx− are only subjected to CEX ion sputtering regardless of grid translation. Moreover, the CEX ion sputtering regions on surfaces Sx− and Sx+ expand as the grid misalignment distance increases. The peak in the ion sputtering rate distribution profile on surface Sy+ becomes more prominent due to grid translation, with the sputtering rate at the peak position reaching approximately 1.65 times that of the surrounding lower-rate regions. The analysis of the electric field indicates that local electric field variations caused by grid misalignment are the underlying reason for the ion erosion characteristics.

Electrode length effects on the performance of the electrostatic quadrupole triplet lens

The present work aims to better understand the relationship between the electrode length of the electrostatic quadrupole triplet lens (EQTL) and its optical properties, as well as the possibility of increasing efficiency by selecting the electrode length that produces the best results, as this topic has not been discussed in previous studies. The effects of electrode length on the performance of the quadrupole triplet lens (QTL) were studied. The quadrupole triplet lens (QTL) electrode voltage combinations were found to produce the stigmatic image, and the effects of electrode length on the image’s characteristics were examined. Two lenses with electrode lengths L of 80 mm and 90 mm were designed, and the comparisons were made to show the effects on the electrode voltages, electron beam trajectories, and aberration figures. The findings indicated that a 12.5% increase in electrode length resulted in a 60% increase in aberration figures for Δy in point object-to-point image focusing mode and an 80% increase for Δz in parallel object-to-point image focusing mode.

A novel design of the electrostatic storage ring using quadrupole deflector

A novel electrostatic storage ring (ESR) design was created using an electrostatic quadrupole deflector (EQD) and quadrupole doublet lens (QDL). EQD was used to turn the electron beam 90 degrees in order to create an innovative ESR design. The impact of EQD designs on ESR performance was investigated, and the results showed that the performance of ESR was directly affected by the EQD design. The calculations showed that increasing the EQD diameter by 20% led to an enhancement in the electron beam dimensions by 88% for ∆x and 44% for ∆y for the initial angle of the electron beam α = 0.2°. Also, the design achieved its goal of keeping kinetic energy constant, where for an initial kinetic energy of 1000 eV, the difference between the energy of the injected electron beam and the emerging electron beam from the ESR is approximately 1–5 eV, i.e., despite changing the diameter of the EQD, the dimensions of the electron beam improve and the kinetic energy remains constant.

Ion confinement and separation using asymmetric electrodynamic fields in structures for lossless ion manipulations.

TW-SLIM ion mobility separations have demonstrated exceptional resolution by leveraging long paths with minimal loss. All previously reported experiments have used electrode surfaces which are mirrored to generate symmetrically opposing electric fields for ion confinement. However, work with other planar ion optics indicates this may be unnecessary. This study explores conditions under which separations may be obtained using a SLIM with asymmetric electric fields. The asymmetric field configuration was defined by applying a uniform DC potential to all electrodes of the top PCB of a standard TW-SLIM board pair, with no electrode placement modifications. This configuration was simulated in SIMION to assess transmission through the SLIM. A benchtop TW-SLIM instrument outfitted with a Faraday plate detector was modified likewise, so the top PCB had a uniform DC potential applied to all electrodes, while the bottom board was operated normally. Simulations show full ion transmission for four different m/z ion populations over a range of DC biases applied to the "pusher" board. Likewise, the modified benchtop instrument is capable of transmitting, separating, and cycling ions with minimal losses. The effect of pusher strength on separation quality is explored, and comparisons between the standard and modified SLIM are made with respect to resolving the +2 and +3 charge states of neurotensin ions. A functional IMS instrument using asymmetric confining fields demonstrates additional field modifications may be a means to achieve additional functionality with limited interruption of the analysis. A TW-SLIM PCB specifically designed as a pusher board would benefit from minimized manufacturing cost, simplifying assembly, reducing drive electronics, and improved field consistency.

Design of electrostatic lenses through genetic algorithm and particle swarm optimisation methods integrated with differential algebra

Genetic algorithm (GA) and particle swarm optimisation (PSO) techniques have been integrated with the differential algebra (DA) method in charged particle optics to optimise an Einzel lens. The DA method is a robust and efficient tool for the calculation of aberration coefficients of electrostatic lenses, which makes use of nonstandard analysis for ray tracing a particle as it is subjected to the field generated by a lens. In this study, initial populations of lenses with random geometrical configurations are generated. These initial populations are then subjected to GA and PSO algorithms to alter the geometry of each lens for a set number of iterations. The lens performance is evaluated by calculating the spot size using the aberrations coefficients up to third-order generated by the DA method. Moreover, a focusing column comprising two lenses and a Wien filter was optimised using GA method.

Design and characterization of a compact Einzel lens for portable quadrupole mass spectrometry (QMS)

Quadrupole mass spectrometry (QMS) offers better resolution, selectivity and sensitivity than the metal-oxide, field effect transistors and thermal-based gas sensors. However, existing QMS systems are unsuitable for field applications. The miniaturized QMS requires dimensionally scaled components with comparable performance to the conventional bulky systems. For a portable quadrupole mass spectrometry, a compact focusing ion system is required to transfer the maximum number of ions from the ion source to the filter. The present paper discusses the design of a miniature focusing Einzel lens and the effect of design parameters on its performance. The three-cylindrical axially symmetric compact Einzel lens assembly was designed using commercially available software. The relationship between the design parameters is explored for compact and low-powered ion optics. The beam focal length was tuned by varying the bias voltage. The dimensions, as well as voltages across the Einzel lens, were reduced significantly to open a new way for portable point-of-care applications. At an applied voltage of 70 V, the focal length of the ions is 11 mm without being scattered, which is favorable for a portable quadrupole mass filter. The designed Einzel lens was fabricated using micromachining techniques and characterized at a vacuum of 3.05 × 10−4 mbar. The developed lens system shows a current output of 0.114 µA at 72 V lens voltage. A current of 94.8 µA is observed with a background noise of 0.01 µA for the system with nitrogen (28 amu) as the sample.

Soft Electrostatic RAFA Lens’s Electron Beam Imaging and Diagnosis of Individual Atoms In 3D Specimen – A Proposal

Soft Electrostatic RAFA Lens’s Electron Beam Imaging and Diagnosis of Individual Atoms In 3D Specimen – A Proposal

Performance Evaluation of an EI&CI Dual Ionization TOFMS Hyphenated with a Flow Modulated GC×GC System.

The use and compatibility of a dual-ionization TOFMS operating an EI source and a CI source in parallel using a single TOF mass analyzer with flow modulated two-dimensional GC (GC×GC) is described. Important figures of merit of the mass spectrometer that are required for two-dimensional GC hyphenation such as acquisition speed, ion source response, EI/CI switching, the GC transfer, and data alignment are carefully investigated and addressed. Improved fast switching ion optics allow switching in a 100 Hz frequency between EI and CI spectra sampled from the same GC×GC effluent. The spectra quality also influenced by the preseparation, especially of the EI source, is compared to a standard setup operating a single quadrupole MS coupled to the same GC system. Further, two setups including and excluding an additional flame ionization detector are presented. High increments in CI sensitivities are achieved by utilizing the high pumping efficiencies of the CI stage of the used mass spectrometer. By leading high flow ratios of the GC×GC modulation flows toward the CI source, the intensity can be increased by factors of up to 37 while maintaining the pressure balance of the less robust EI source. Finally, thermal desorption GC×GC-EI&CI-TOFMS analyses of traffic emission samples from a federal highway in Germany are executed with the presented setup.

Manipulating a beam of barium fluoride molecules using an electrostatic hexapole

An electrostatic hexapole lens is used to manipulate the transverse properties of a beam of barium fluoride molecules from a cryogenic buffer gas source. The spatial distribution of the beam is measured by recording state-selective laser-induced fluorescence on an emccd camera, providing insight into the intensity and transverse position spread of the molecular beam. Although the high mass and unfavorable Stark shift of barium fluoride pose a considerable challenge, the number of molecules in the low-field seeking component of the N = 1 state that pass a 4 mm diameter aperture 712 mm behind the source is increased by a factor of 12. Furthermore, it is demonstrated that the molecular beam can be displaced by up to ±5 mm by moving the hexapole lens. Our measurements agree well with numerical trajectory simulations. We discuss how electrostatic lenses may be used to increase the sensitivity of beam experiments such as the search for the electric dipole moment of the electron.

A Hybrid Orbitrap‐Nanoelectromechanical Systems Approach for the Analysis of Individual, Intact Proteins in Real Time

AbstractNanoelectromechanical systems (NEMS)‐based mass spectrometry (MS) is an emerging technique that enables determination of the mass of individual adsorbed particles by driving nanomechanical devices at resonance and monitoring the real‐time changes in their resonance frequencies induced by each single molecule adsorption event. We incorporate NEMS into an Orbitrap mass spectrometer and report our progress towards leveraging the single‐molecule capabilities of the NEMS to enhance the dynamic range of conventional MS instrumentation and to offer new capabilities for performing deep proteomic analysis of clinically relevant samples. We use the hybrid instrument to deliver E. coli GroEL molecules (801 kDa) to the NEMS devices in their native, intact state. Custom ion optics are used to focus the beam down to 40 μm diameter with a maximum flux of 25 molecules/second. The mass spectrum obtained with NEMS‐MS shows good agreement with the known mass of GroEL.

A Hybrid Orbitrap-Nanoelectromechanical Systems Approach for the Analysis of Individual, Intact Proteins in Real Time.

Nanoelectromechanical systems (NEMS)-based mass spectrometry (MS) is an emerging technique that enables determination of the mass of individual adsorbed particles by driving nanomechanical devices at resonance and monitoring the real-time changes in their resonance frequencies induced by each single molecule adsorption event. We incorporate NEMS into an Orbitrap mass spectrometer and report our progress towards leveraging the single-molecule capabilities of the NEMS to enhance the dynamic range of conventional MS instrumentation and to offer new capabilities for performing deep proteomic analysis of clinically relevant samples. We use the hybrid instrument to deliver E. coli GroEL molecules (801 kDa) to the NEMS devices in their native, intact state. Custom ion optics are used to focus the beam down to 40 μm diameter with a maximum flux of 25 molecules/second. The mass spectrum obtained with NEMS-MS shows good agreement with the known mass of GroEL.

Maximizing Glycoproteomics Results through an Integrated Parallel Accumulation Serial Fragmentation Workflow.

Glycoproteins play important roles in numerous physiological processes and are often implicated in disease. Analysis of site-specific protein glycobiology through glycoproteomics has evolved rapidly in recent years thanks to hardware and software innovations. Particularly, the introduction of parallel accumulation serial fragmentation (PASEF) on hybrid trapped ion mobility time-of-flight mass spectrometry instruments combined deep proteome sequencing with separation of (near-)isobaric precursor ions or converging isotope envelopes through ion mobility separation. However, the reported use of PASEF in integrated glycoproteomics workflows to comprehensively capture the glycoproteome is still limited. To this end, we developed an integrated methodology using timsTOF Pro 2 to enhance N-glycopeptide identifications in complex mixtures. We systematically optimized the ion optics tuning, collision energies, mobility isolation width, and the use of dopant-enriched nitrogen gas (DEN). Thus, we obtained a marked increase in unique glycopeptide identification rates compared to standard proteomics settings, showcasing our results on a large set of glycopeptides. With short liquid chromatography gradients of 30 min, we increased the number of unique N-glycopeptide identifications in human plasma samples from around 100 identifications under standard proteomics conditions to up to 1500 with our optimized glycoproteomics approach, highlighting the need for tailored optimizations to obtain comprehensive data.