Ion Focusing Effect Research Articles

Racetrack FAIMS for High-Resolution and High-Sensitivity Characterization of Peptide Conformers.

A racetrack field asymmetric waveform ion mobility spectrometry (r-FAIMS) device, which consists of both cylindrical FAIMS (c-FAIMS) and planar FAIMS (p-FAIMS) sections with a 1 mm gap width, was developed and applied for high-resolution and high-sensitivity exploration of conformational diversity for peptides. The optimal operating conditions of r-FAIMS were systemically studied, and the performance of the fully optimized r-FAIMS was compared to a previously developed p-FAIMS in detail by using pure nitrogen as the FAIMS carrier gas. Relying on the ion focusing effect in the c-FAIMS section, the intensity of the FAIMS spectrum for doubly charged bradykinin ions acquired by using r-FAIMS is ∼8.5-fold higher than that acquired by using p-FAIMS under the same resolving power/resolution condition, implying about an order of magnitude better sensitivity of r-FAIMS. In addition, the peak separation resolution of r-FAIMS was ∼1.70-fold higher than p-FAIMS under a similar sensitivity condition for doubly charged bradykinin ions. Due to a reduced gap width of the newly designed r-FAIMS (1 mm) as compared to the previously developed p-FAIMS (1.88 mm), r-FAIMS can operate at a much higher separation field with a similar FAIMS dispersion voltage (DV) to gain significantly higher resolving power. For triply charged syntide 2 ions, the resolving power of r-FAIMS can easily exceed 120 at -3.5 kV DV by using pure nitrogen as the FAIMS carrier gas as compared to 44.2 resolving power obtained by using p-FAIMS at -4.0 kV DV. All of the experimental results have confirmed that r-FAIMS can perform structural characterization of biomolecules with both high resolution and high sensitivity.

Simulation study of a new racetrack FAIMS analyzer to achieve both high-resolution and high-sensitivity

A new racetrack field-asymmetric waveform ion mobility spectrometry (r-FAIMS) analyzer was developed in this study by combining the existing planar FAIMS (p-FAIMS) and cylindrical FAIMS (c-FAIMS). The ion inlet and outlet regions of r-FAIMS were consisted of a half of c-FAIMS, respectively, and these c-FAIMS were further connected by two p-FAIMS to form a racetrack shaped FAIMS. With such FAIMS working electrode configuration, the ions entering the r-FAIMS can be focused and separated in the first c-FAIMS section, be further separated in the p-FAIMS section with high-resolution, be focused and separated again in the final c-FAIMS section and eventually enter the mass spectrometer or other analyzers for analysis. Detailed simulation by using SIMION software with the default FAIMS user program showed that the ion focusing effect in the first c-FAIMS section ensures the ions entering the following p-FAIMS section as a compact ion packet. This effectively decreases the ion loss caused by Coulomb repulsion and thermal diffusion in p-FAIMS section as compared to the ions being introduced into the p-FAIMS gap randomly in the conventional design. As a result, the ion transmission efficiency of r-FAIMS is at least 3.3-fold higher than the single p-FAIMS under the operating conditions used in this study. The ion trajectory simulation results also showed that the resolving power of r-FAIMS is about the sum of the resolving powers for its c-FAIMS and p-FAIMS sections. The resolving power of r-FAIMS is at least 3.6-fold higher than the single c-FAIMS under the operation conditions used in this study. Therefore, the r-FAIMS can realize both high-resolution and high-sensitive ion mobility separation.

Evaluation of a New DC-Ion Funnel Drift Tube for Use in Proton Transfer Reaction Mass Spectrometry.

We have developed and characterized a novel drift tube called the direct current-ion funnel (DC-ion funnel) drift tube, consisting of 20 traditional ring electrodes and 5 new DC-focusing electrodes (DC-FEs) for use in proton transfer reaction mass spectrometry (PTR-MS). Ion trajectory simulations demonstrate the ion focusing effect of the DC-FE and DC-ion funnel drift tube. Further comparative experiments show that the PTR-MS with the novel DC-ion funnel drift tube has a higher sensitivity (3.8-7.3 times for the volatile organic compounds considered in this work) than the PTR-MS with a traditional drift tube. Different from conventional radiofrequency (rf) focusing methods, the DC-ion funnel drift tube can realize ion focusing with only a DC electric field and no additional rf power supply, which makes it especially suitable for instruments requiring miniaturization and low power consumption to improve detection sensitivity. In addition, the DC-ion funnel drift tube can easily be coupled to other types of mass spectrometers to increase their detection sensitivity.

Rapid determination of plutonium isotopes in small samples using single anion exchange separation and ICP-MS/MS measurement in NH3-He mode for sediment dating.

To accurately determine ultra-trace Pu isotopes in small environmental samples, we explored ICP-MS/MS in NH3-He mode, and investigated mechanism of 238U interference removal and measurement sensitivity improvement for plutonium isotopes. The interference of uranium and uranium hydrides was effectively eliminated using 0.4mL/min NH3 as reaction gas by shifting them to U(NHm)n+ and UH(NHm)n+. The overall interference of uranium was reduced to <2.4×10-7, while remaining excellent 239Pu sensitivity (13,900 Mcps/(mg/L)) mainly due to ion focusing effect of Pu by helium gas. On this basis, the purification of plutonium using a single AG1-×4 column was proved to be sufficient for accurate determination of plutonium isotopes by the developed detection method, and the detection limits for the method were estimated to be 0.16fg (0.4 μBq) for 239Pu, 0.046fg (0.4 μBq) for 240Pu and 0.039fg (0.15mBq) for 241Pu. The method was validated by analyzing plutonium isotopes in certificated reference materials and reported environmental samples of only 1-2g. The analytical results of ultra-trace Pu isotopes in small amounts (∼1g) of lake sediments obtained by the developed method were successfully applied to sediment dating.

Numerical Simulation of Flow Field and Ion Transport for Different Ion Source Sampling Interfaces of a Mass Spectrometer.

Understanding ion transport mechanisms in the flow expansion section of the first vacuum region of a mass spectrometer (MS) with an atmospheric pressure ionization source is essential for optimizing the MS sampling interface design. In this study, numerical simulations of three types of ions in two different MS interface designs have been carried out. In contrast to previously reported numerical studies, nonequilibrium gas dynamics due to rarefied gas effects has been considered in modeling the flow expansion and a realistic space charge effect has been considered in a continuous ion injection mode. Numerical simulations reveal that a flat plate interface has a higher peak buffer gas velocity but a narrower zone of silence compared to the conical interface. Shock wave structures are clearly captured, and the Knudsen number distribution is displayed. Simulation results show that in the axial direction the buffer gas effect is much stronger than the electric force effect in the current configuration. The conical interface leads to both a strong ion acceleration in the zone of silence and a strong ion deceleration downstream. In the radial direction, both the electric force and buffer gas drag force play an important role. The conical interface introduces a relatively stronger ion focusing effect from the radial buffer gas effect and a stronger ion dispersion from the radial electric force than the flat plate interface. The net effect for the current configuration is an increase in ion losses for the conical interface. Nanoelectrospray ionization experiments were carried out to validate the ion transmission efficiency.

Anomalous Coefficient of Ion Reflection from a Crystal under Surface-Channeling Conditions

A significant difference in behavior between the coefficients of deuterium-atom reflection upon scattering at crystal and amorphous tungsten is shown. The region where the reflection coefficient is close to 100% is determined by scattering at the first and second crystal layers and depends on the orientation. The effect of ion focusing on second-layer atoms and also various orientation effects are observed. It is shown that, at small grazing angles, the dependence of the reflection coefficient on the energy of transverse particle motion is universal. The dependences on the azimuthal exit angle ϕ exhibit the rainbow scattering effect caused by particle motion in the surface semichannel, while the dependences on the polar angle demonstrate the effect of angular distribution narrowing (ion focusing).

Analysis of retarding field energy analyzer transmission by simulation of ion trajectories

Retarding field energy analyzers (RFEAs) are used routinely for the measurement of ion energy distribution functions. By contrast, their ability to measure ion flux densities has been considered unreliable because of lack of knowledge about the effective transmission of the RFEA grids. In this work, we simulate the ion trajectories through a three-gridded RFEA using the simulation software SIMION. Using idealized test cases, it is shown that at high ion energy (i.e., >100 eV) the transmission is equal to the optical transmission rather than the product of the individual grid transparencies. Below 20 eV, ion trajectories are strongly influenced by the electric fields in between the grids. In this region, grid alignment and ion focusing effects contribute to fluctuations in transmission with ion energy. Subsequently the model has been used to simulate the transmission and energy resolution of an experimental RFEA probe. Grid misalignments reduce the transmission fluctuations at low energy. The model predicts the minimum energy resolution, which has been confirmed experimentally by irradiating the probe with a beam of ions with a small energy bandwidth.

Ion Distribution Profiling in an Ion Mobility Spectrometer by Laser-Induced Fluorescence

Measuring the ion distribution pattern in a drift tube under atmospheric pressure is very useful for studies of ion motion and design of ion mobility spectrometers (IMS); however, no mature method is available for conducting such measurements at present. We propose a simple and low-cost technique for profiling the two-dimensional ion distribution in any cross section of a drift tube. Similar to particle-image velocimetry, we first send sample ions with fluorescence properties into the drift tube and use a receiving plate to collect and accumulate them. Then, the receiving plate is illuminated by exciting light, and the ion distribution appears as a fluorescence image. In this study, Rhodamine 6G was selected as a typical fluorescence-tracer particle. Electrospray ionization (ESI) was chosen as an ionization source to keep the fluorophore undamaged. A plasma-cleaned coverslip was placed at the detection position as a receiving plate. When a layer of ions was collected, the slide was placed under the exciting light with a wavelength of 473 nm. A camera with a 490 nm high-pass light filter was used to capture the fluorescence image representing the ion distribution. The measured-ion detection efficiency of the method was 156 ion/dN, which is equivalent to the level of IonCCD. In addition, we studied the ion-passing characteristics of a Bradbury-Nielsen (BN) ion shutter and the ion-focusing effect in the drift tube using this method. The two-dimensional ion-distribution images behind the ion shutter and the images of the focused ion spot were first observed experimentally. Further theoretical analysis yielded the same conclusions as the experimental results, proving the feasibility of this method and producing a deeper understanding of ion motion in the IMS. This method has promising prospective application to the design, debugging, and optimization of IMS instruments and hyphenated systems.

Theory of Heating of a Nonreciprocal System of Particles

A simple theory, which describes a source of additional kinetic energy and its redistribution in systems of many particles with a nonreciprocal interaction (when Newton’s third law is not valid), is presented. The theory yields the satisfactory results for a qualitative analysis of the features of dust dynamics in an anisotropic complex plasma. Unlike the theoretical models proposed earlier to explain the anomalous heating in the systems of particles with a nonreciprocal interaction, the model presented here allows to predict not only the kinetic energy growth for grains in laboratory RF discharges, but also an increase of their kinetic temperature in the direction of an ion flow as well as a difference in the energy distribution between the degrees of freedom. To verify the theory, we carried out numerical simulations of vertically aligned chains and extended-layered structures with a nonreciprocal model of interaction that is similar to interaction due to an effect of the ion focusing in a laboratory complex plasma.

Energy exchange in systems of particles with nonreciprocal interaction

A model is proposed to describe the sources of additional kinetic energy and its redistribution in systems of particles with a nonreciprocal interaction. The proposed model is shown to explain the qualitative specific features of the dust particle dynamics in the sheath region of an RF discharge. Prominence is given to the systems of particles with a quasi-dipole–dipole interaction, which is similar to the interaction induced by the ion focusing effects that occur in experiments on a laboratory dusty plasma, and with the shadow interaction caused by thermophoretic forces and Le Sage’s forces.

Kinetic energy in a system of particles with a nonreciprocal interaction

We present a theoretical model for the description of the heating mechanism and the redistribution of kinetic energy in a system of particles with nonreciprocal interactions that occur in disperse systems of different nature. To verify the theory, we carried out the numerical simulations of two-particle systems with a nonreciprocal “quasi-dipole–dipole” interaction that is similar to the interaction due to the effect of ion focusing in a laboratory complex plasma under experimental conditions. The proposed model of heating can explain various features of dust dynamics in anisotropic complex plasma.

Vertically aligned dust particles under the influence of crossed electric and magnetic fields in the sheath of a radio frequency discharge

We present experiments on two dust particles with a size of a few microns that are levitated in the sheath region of an rf discharge in the presence of an external magnetic field transverse to the sheath electric field. The two particles are vertically aligned due to the ion focusing effect. First, it is observed that the magnetic field causes a displacement of the dust particles either in the E→×B→- or in the opposite direction. Second, at a sufficiently large neutral gas pressure, the vertical alignment breaks up when the magnetic field strength is increased. The occurrence of this dissociation is described by the horizontal force balance on the two particles.

Time-Resolved Imaging of the MALDI Linear-TOF Ion Cloud: Direct Visualization and Exploitation of Ion Optical Phenomena Using a Position- and Time-Sensitive Detector

In this study, we describe the implementation of a position- and time-sensitive detection system (Timepix detector) to directly visualize the spatial distributions of the matrix-assisted laser desorption ionization ion cloud in a linear-time-of-flight (MALDI linear-ToF) as it is projected onto the detector surface. These time-resolved images allow direct visualization of m/z-dependent ion focusing effects that occur within the ion source of the instrument. The influence of key parameters, namely extraction voltage (E(V)), pulsed-ion extraction (PIE) delay, and even the matrix-dependent initial ion velocity was investigated and were found to alter the focusing properties of the ion-optical system. Under certain conditions where the spatial focal plane coincides with the detector plane, so-called x-y space focusing could be observed (i.e., the focusing of the ion cloud to a small, well-defined spot on the detector). Such conditions allow for the stigmatic ion imaging of intact proteins for the first time on a commercial linear ToF-MS system. In combination with the ion-optical magnification of the system (~100×), a spatial resolving power of 11–16 μm with a pixel size of 550 nm was recorded within a laser spot diameter of ~125 μm. This study demonstrates both the diagnostic and analytical advantages offered by the Timepix detector in ToF-MS.

Suppression of unimolecular decay of laser desorbed peptide and protein ions by entrainment in rarefied supersonic gas jets under weak electric fields

Unimolecular decay of sample ions imposes a limit on the usable laser fluence in matrix-assisted laser desorption/ionization (MALDI) ion sources. Traditionally, some modest degree of collisional sample ion cooling has been achieved by connecting MALDI ion sources directly to gas-filled radio frequency (RF) multipoles. It was also discovered in the early 1990s that gas-filled RF multipoles exhibit increased ion transmission efficiency due to collisional ion focusing effects. This unexpected experimental finding was later supported by elementary Monte Carlo simulations. Both experiments and simulations assumed a resting background gas with typical pressures of the order of 1 Pa. However, considerable additional improvements can be achieved if laser desorbed sample ions are introduced immediately after desorption, still within the ion source, in an axisymmetric rarefied supersonic gas jet with peak pressure of the order of 100 Pa and flow velocities >300 m/s, and under weak electric fields. We describe here the design principle and report performance data of an ion source coined "MALDI-2," which incorporates elements of both rarefied aerodynamics and particle optics. Such a design allows superb suppression of metastable fragmentation due to rapid collisional cooling in <10 μs and nearly perfect injection efficiency into the attached RF ion guide, as numerous experiments have confirmed.

Ion focusing in enhanced glow discharge plasma immersion ion implantation of hydrogen and nitrogen into silicon

Ion focusing in enhanced glow discharge plasma immersion ion implantation (EGD-PIII) of hydrogen into silicon affects the lateral ion fluence uniformity. The phenomenon and its effects are investigated experimentally and theoretically under different conditions and compared to those in nitrogen EGD-PIII. Consistent results are obtained from experiments and numerical simulation disclosing that the lower the plasma density, the more severe is the ion focusing effect. The influence of the negative high voltage on the ion focusing effect is small compared to that of the plasma density.

One-step, non-contact pattern transfer by direct-current plasma immersion ion implantation

A one-step, non-contact pattern transfer method by direct-current plasma immersion ion implantation is demonstrated. Complex patterns with micrometre-size linewidths can be transferred onto a silicon wafer by placing the metal masks 4 mm away from the wafer. Scanning electron microscopy reveals that by negatively biasing the metal mask, ions coming from a hole with a diameter of 200 µm in the mask can be confined to a smaller region of 100 µm. The ion focusing effect is confirmed by two-dimensional multiple-grid particle-in-cell simulation.

Colloquium: Fundamentals of dust-plasma interactions

Dusty plasmas are ubiquitous in low-temperature laboratory discharges as well as in the near-earth environment, planetary rings, and interstellar spaces. In this paper, updated knowledge of fundamentals of collective dust-plasma interactions and several novel phenomena are presented that have been observed in laboratories and in space dusty plasmas. Mechanisms that are responsible for the charging of dust grains are discussed, and the fact that the dust charge perturbation is a new dynamical variable in a dusty plasma. The underlying physics of different forces that act on a charged dust grain is reviewed. In dusty plasmas, there are new attractive forces (e.g., due to wakefield and ion focusing effects and dipole-dipole interactions between unevenly charged dust rods). Furthermore, in the presence of an ensemble of charged dust grains, there are collective dust-plasma interactions featuring new waves (e.g., the dust acoustic wave, the dust ion-acoustic wave, the dust lattice wave, etc.), new instabilities, and coherent nonlinear structures (dust acoustic and dust ion-acoustic shocks, dust voids, and dust vortices), which are also discussed. Theoretical models for numerous collective dust-plasma interactions are compared with existing observations from laboratories and space environments.

One-step non-contact patterns transferring by plasma based ion implantation

A one-step non-contact patterns transferring method is developed. Clear non-identical images with well-defined boundaries are simultaneously transferred to a substrate by −15 kV plasma immersion ion implantation through a patterned metal mask. The metal mask is 6 cm away from the substrate and no lens system has to be fabricated beforehand to the substrate. The ion focusing effect is observed reducing the line width of the pattern from 6 to 2 mm. Particle-in-cell simulation confirms that the ion focusing effect is caused at the pattern opening of the 1 mm thick mask.

Highly localized ion focusing effects in PBIID

AbstractPlasma Based Ion Implantation and Deposition (PBIID) is characterized by a complex interaction of a supersonic plasma flux, a plasma sheath expanding and receding on a timescale of microseconds and a substrate which can be of an arbitrary complex shape. It is shown that ions impinging on the substrate in the wake region consist of two groups with strongly differing incidence angles. At the same time, localized ion focusing is observed only for very long pulses of 50 µs and intermediate voltages of 2.5 – 3.5 kV, independent of the cathode material – Ti or Al – and the Ar background pressure. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Two-dimensional particle-in-cell simulation of the ion sheath dynamics in plasma source ion implantation of a hemispherical bowl-shaped target

Plasma source ion implantation into a hemispherical bowl-shaped target is simulated by the two-dimensional particle-in-cell method. The numerical procedure is based on solving the Poisson's equation on a grid and tracing the movement of the ions through the grid. The potential and the ion density distributions in the sheath are studied in detail and the trajectories and dynamic states of ions are considered. The implantation dose and impact angle of the ions at different parts of the target surface are obtained. The ion focusing effect due to the nonuniformity of the sheath potential near the brim of the vessel is observed. The results presented here show that the ion focusing causes the nonuniformity of dose on the target surface.