Numerical modeling of ion transport in an ESI-MS system

Numerical Analysis and Quantification of Transfer Efficiency Coupled with Capillary and Quadrupole Ion Guide in an API-MS System

Ion trajectory simulation is a significant and useful tool for understanding ion transfer mechanisms within the first vacuum region of the atmospheric pressure ionization mass spectrometer (API-MS). However, the complex dynamic gas field and wide pressure range lead to inaccurate simulation and huge computational costs. In this work, a novel electrohydrodynamic simulation called the statistical diffusion-hard-sphere (SDHS) mixed collision model was developed for characterizing the ion trajectories. For the first time, the influence of the dynamic pressure on the ion trajectory is considered for simulation, which helps to avoid an intolerable computational cost. Comparing with the conventional Monte Carlo collision model, the SDHS method helps to improve the calculation accuracy of ion trajectories under the first vacuum region and reduce the computational cost for at least 12-folds. Simulation results showed that the maximum ion loss came from the gap of the electrodes. The distance of the capillary-quadrupole ion guide was also a non-negligible factor. The trend of quantitative experimental results matches the SDHS simulation results. The maximum ion transfer efficiencies of quantitative experiment and simulation were 55% and 52%, respectively. Moreover, three ions, caffeine, reserpine, and Ultramark 1621, were measured for evaluating the applicability of SDHS in real API-MS. The trend of experimental results showed good agreement with that of computation. And the results of caffeine further illustrated the reason that the small mass ion transfer efficiency decreased with increasing radio frequency voltage. SDHS method is expected to be useful in the design of ion guides for further improvement of the sensitivity of API-MS.

Ion transmission in an electrospray ionization-mass spectrometry interface using an S-lens

We present the design and performance of an in-house built electrospray ionization-mass spectrometry (ESI-MS) interface equipped with an S-lens ion guide. The ion source was designed specifically for our ion beam experiments to investigate the chemical reactivity and deposition of the clusters and nanoparticles. It includes standard ESI-MS interface components, such as nanoelectrospray, ion transfer capillary, and the S-lens. A custom design enables systematic optimization of all relevant factors influencing ion formation and transfer through the interface. By varying the ESI voltage and flow rate, we determined the optimal operating conditions for selected silica emitters. A comparison of the pulled silica emitters with different tip inner diameters reveals that the total ion current is highest for the largest tip, whereas a tip with the smallest diameter exhibited the highest transmission efficiency through the ESI-MS interface. Ion transmission through the transfer capillary is strongly limited by its length, but the loss of ions can be reduced by increasing the capillary voltage and temperature. The S-lens was characterized over a wide range of RF frequencies and amplitudes. Maximum ion current was detected at RF amplitudes greater than 50 V peak-to-peak (p/p) and frequencies above 750 kHz, with a stable ion transmission region of about 20%. A factor of 2.6 increase in total ion current is observed for 650 kHz as RF amplitudes reach 400 V p/p. Higher RF amplitudes also focus the ions into a narrow beam, which mitigates their losses when passing through the ion guide.

Charging Effects in Inlet Capillaries

Glass or metal inlet capillaries are commonly used for flow restriction in atmospheric pressure ionization mass spectrometers. They exhibit a high ion transmission rate and stability at most operating conditions. However, transferring unipolar currents of ions through inlet capillaries can lead to sudden signal dropouts or drifts of the signal intensity, particularly when materials of different conductivity are in contact with the capillary duct. Molecular layers of water and other gases such as liquid chromatography solvents always form on the surfaces of inlet capillaries at atmospheric pressure ionization conditions. These surface layers play a major role in ion transmission and the occurrence of charging effects, as ions adsorb on the capillary walls as well, charging the walls to electric potentials of up to kilovolts and eventually leading to a hindrance of ion transport into or through the capillary duct. In this work, surface charging effects are reported in dependence on the capillary material, i.e., borosilicate glass, (reduced) lead silicate, quartz, and metal. Low electrical conductance materials show a more pronounced long-term signal drift (e.g., quartz), while higher electrical conductance materials lead to stable long-term behavior.

Metal Ionization in Sub-atmospheric Pressure MALDI Interface: A New Tool for Mass Spectrometry of Volatile Organic Compounds

A novel approach for the analysis of volatile organic compounds (VOCs) based on chemical ionization by Au⁺ ions has been proposed. The ionization is carried out in a commercially available dual sub-atmospheric pressure MALDI/ESI interface without any modifications. The Au⁺ ions are generated by laser ablation of a gold nanolayer with the MALDI laser, and VOCs are infused via the ESI capillary. The ultrahigh resolving power and sub-ppm mass accuracy of the employed mass spectrometer allow straightforward identification of the formed ion-molecule complexes and other products of Au⁺ interactions with VOCs in the gas phase. The performance of the technique is demonstrated on the analysis of various classes of organic molecules, namely, alkanes, alkenes, alcohols, aldehydes, ketones, aromatic compounds, carboxylic acids, ethers, or organosulfur compounds, expanding the portfolio of currently available methods for the analysis of VOCs such as secondary electrospray ionization, proton-transfer reaction, and selected ion flow tube mass spectrometry.

Ion optics simulation of an ion beam setup coupled to an electrospray ionization source, strengths, and limitations

A unified approach to achieve a start-to-end ion optics simulation of an ion beam apparatus coupled to an electrospray ionization source is presented. We demonstrate that simulations enable reliable information on the behavior and operation of the apparatus to be obtained, but due to the collisions with the buffer gas in the initial stages of the setup, the results concerning the kinetic energy of the ion beam must be treated with care.

The FUNPET-a New Hybrid Ion Funnel-Ion Carpet Atmospheric Pressure Interface for the Simultaneous Transmission of a Broad Mass Range

An atmospheric pressure interface transports ions from ambient pressure to the low-pressure environment of a mass spectrometer. A capillary coupled to an ion funnel is widely used. However, conventional ion funnels do little to negate the large amount of energy picked up by high-mass ions from the gas flow through the capillary. There has been little work done on the effects of gas flow on ion transmission, and the previous studies have all been limited to low-mass, low-charge ions. In this work, we account for the effects of gas flow, diffusion, and electric fields (static and oscillating) on ion trajectories and use simulations to design a new hybrid ion funnel-ion carpet (FUNPET) interface that transmits a broad mass range with a single set of instrument conditions. The design incorporates a virtual jet disruptor where pressure buildup and counter flow dissipate the supersonic jet that results from gas flow into the interface. This, and the small exit aperture that can be used with the FUNPET, reduces the gas flow into the next stage of differential pumping. The virtual jet disruptor thermalizes ions with a broad range of masses (1 kDa to 1 GDa), and once thermalized, they are transmitted into next region of the mass spectrometer with low excess kinetic energy. The FUNPET interface is easy to fabricate from flexible printed circuit board and a support frame made by 3D printing. The performance of the interface was evaluated using charge detection mass spectrometry. Graphical Abstract ᅟ.

Gas Flow and Ion Transfer in Heated ESI Capillary Interfaces

Transfer capillaries are the preferred means to transport ions, generated by electrospray ionization, from ambient conditions to vacuum. During the transfer of ions through the narrow, long tubes into vacuum, substantial losses are typical. However, recently it was demonstrated that these losses can be avoided altogether. To understand the experimental observation and provide a general model for the ion transport, here, we investigate the ion transport through capillaries by numerical simulation of interacting ions. The simulation encompasses all relevant factors, such as space charge, diffusion, gas flow, and heating. Special attention is paid to the influence of the gas flow on the transmission and especially the change imposed by heating. The gas flow is modeled by a one-dimensional gas dynamics description. A large number of ions are treated as point particles in this gas flow. This allows to investigate the influence of the capillary heating on the gas flow and by this on the ion transport. The results are compared with experimental findings. Graphical Abstract ᅟ.

Numerical Simulation of Ion Transport in a Nano-Electrospray Ion Source at Atmospheric Pressure

Understanding ion transport properties from the ion source to the mass spectrometer (MS) is essential for optimizing device performance. Numerical simulation helps in understanding of ion transport properties and, furthermore, facilitates instrument design. In contrast to previously reported numerical studies, ion transport simulations in a continuous injection mode whilst considering realistic space-charge effects have been carried out. The flow field was solved using Reynolds-averaged Navier-Stokes (RANS) equations, and a particle-in-cell (PIC) method was applied to solve a time-dependent electric field with local charge density. A series of ion transport simulations were carried out at different cone gas flow rates, ion source currents, and capillary voltages. A force evaluation analysis reveals that the electric force, the drag force, and the Brownian force are the three dominant forces acting on the ions. Both the experimental and simulation results indicate that cone gas flow rates of ≤250 slph (standard liter per hour) are important for high ion transmission efficiency, as higher cone gas flow rates reduce the ion signal significantly. The simulation results also show that the ion transmission efficiency reduces exponentially with an increased ion source current. Additionally, the ion loss due to space-charge effects has been found to be predominant at a higher ion source current, a lower capillary voltage, and a stronger cone gas counterflow. The interaction of the ion driving force, ion opposing force, and ion dispersion is discussed to illustrate ion transport mechanism in the ion source at atmospheric pressure. Graphical Abstract.

Gas Flow in the Capillary of the Atmosphere-to-Vacuum Interface of Mass Spectrometers

Numerical simulations of a gas flow through a capillary being a part of mass spectrometer atmospheric interface were performed using a detailed laminar flow model. The simulated interface consisted of atmospheric and forevacuum volumes connected via a thin capillary. The pressure in the forevacuum volume where the gas was expanding after passing through the capillary was varied in the wide range from 10 to 900 mbar in order to study the volume flow rate as well as the other flow parameters as functions of the pressure drop between the atmospheric and forevacuum volumes. The capillary wall temperature was varied in the range from 24 to 150 °C. Numerical integration of the complete system of Navier-Stokes equations for a viscous compressible gas taking into account the heat transfer was performed using the standard gas dynamic simulation software package ANSYS CFX. The simulation results were compared with experimental measurements of gas flow parameters both performed using our experimental setup and taken from the literature. The simulated volume flow rates through the capillary differed no more than by 10% from the measured ones over the entire pressure and temperatures ranges. A conclusion was drawn that the detailed digital laminar model is able to quantitatively describe the measured gas flow rates through the capillaries under conditions considered. Graphical Abstract ᅟ.

Gas Flow Dynamics in Inlet Capillaries: Evidence for non Laminar Conditions

In this work, the characteristics of gas flow in inlet capillaries are examined. Such inlet capillaries are widely used as a first flow restriction stage in commercial atmospheric pressure ionization mass spectrometers. Contrary to the common assumption, we consider the gas flow in typical glass inlet capillaries with 0.5 to 0.6 mm inner diameters and lengths about 20 cm as transitional or turbulent. The measured volume flow of the choked turbulent gas stream in such capillaries is 0.8 L·min(-1) to 1.6 L·min(-1) under typical operation conditions, which is in good agreement to theoretically calculated values. Likewise, the change of the volume flow in dependence of the pressure difference along the capillary agrees well with a theoretical model for turbulent conditions as well as with exemplary measurements of the static pressure inside the capillary channel. However, the results for the volume flow of heated glass and metal inlet capillaries are neither in agreement with turbulent nor with laminar models. The velocity profile of the neutral gas in a quartz capillary with an inner diameter similar to commercial inlet capillaries was experimentally determined with spatially resolved ion transfer time measurements. The determined gas velocity profiles do not contradict the turbulent character of the flow. Finally, inducing disturbances of the gas flow by placing obstacles in the capillary channel is found to not change the flow characteristics significantly. In combination the findings suggest that laminar conditions inside inlet capillaries are not a valid primary explanation for the observed high ion transparency of inlet capillaries under common operation conditions. Graphical Abstract ᅟ.

An aerodynamic assisted miniature mass spectrometer for enhanced volatile sample analysis

Low ppb-level VOC detection sensitivity was achieved by integrating an in-vacuum plasma ionization source into the continuous atmospheric pressure interfaced miniature mass spectrometer.

Numerical analysis of ion-funnel transmission efficiency in an API-MS system with a continuum/microscopic approach

A multi-step numerical approach is used to analyze the efficiency of an ion-funnel to transport ions over a wide range of m/z. A continuum approach based on the solution of the Navier-Stokes equations is applied to model the gas flow through a capillary connecting the atmospheric and subatmospheric sections of a mass spectrometer. A microscopic, fully kinetic approach based on the solution of the Boltzmann equation is used to examine the ion and gas transport through an ion-funnel kept at a 0.1-3 Torr pressure to the quadrupole section kept at a 0.01 Torr pressure. In addition to aerodynamic drag, the developed approach takes into account the combined effect of the DC field driving the ions downstream toward the funnel exit, the rf field confining the ions in radial direction, and the space charge causing ion repulsion. The sensitivity of the ion transmission to the gas pressure in the ion-funnel, the rf, and the total ion current injected to the funnel from capillary nozzle is shown. Graphical Abstract ᅟ.

Ghost peaks observed after atmospheric pressure matrix-assisted laser desorption/ionization experiments may disclose new ionization mechanism of matrix-assisted hypersonic velocity impact ionization

RATIONALE

Understanding the mechanisms of matrix-assisted laser desorption/ionization (MALDI) promises improvements in the sensitivity and specificity of many established applications in the field of mass spectrometry. This paper reports a serendipitous observation of a significant ion yield in a post-ionization experiment conducted after the sample had been removed from a standard atmospheric pressure (AP)-MALDI source. This post-ionization is interpreted in terms of collisions of microparticles moving with a hypersonic velocity into a solid surface. Calculations show that the thermal energy released during such collisions is close to that absorbed by the top matrix layer in traditional MALDI. The microparticles, containing both the matrix and analytes, could be detached from a film produced inside the inlet capillary during the sample ablation and accelerated by the flow rushing through the capillary. These observations contribute some new perspective to ion formation in both laser and laser-less matrix-assisted ionization.

METHODS

An AP-MALDI ion source hyphenated with a three-stage high-pressure ion funnel system was utilized for peptide mass analysis. After the laser had been turned off and the MALDI sample removed, ions were detected during a gradual reduction of the background pressure in the first funnel. The constant-rate pressure reduction led to the reproducible appearance of different singly and doubly charged peptide peaks in mass spectra taken a few seconds after the end of the MALDI analysis of a dried-droplet spot.

RESULTS

The ion yield as well as the mass range of ions observed with a significant delay after a completion of the primary MALDI analysis depended primarily on the background pressure inside the first funnel. The production of ions in this post-ionization step was exclusively observed during the pressure drop. A lower matrix background and significant increase in relative yield of double-protonated ions are reported.

CONCLUSIONS

The observations were partially consistent with a model of the supersonic jet from the inlet capillary accelerating detached particles to kinetic energies suitable for matrix-assisted hypersonic-velocity impact ionization.

Development of an ion activation stage for atmospheric pressure ionization sources

RATIONALE

The ion-molecule chemistry in typical atmospheric pressure ion sources is essentially thermodynamically controlled. Methods relying on gas-phase protonation reactions, e.g. atmospheric pressure chemical ionization (APCI), thus suffer from the low reactivity of the equilibrated reagent ion population, which is mostly [H + (H2O)n](+). Reagent ion activation to yield reactive species such as H3O(+) is largely uncontrolled in commercial API mass spectrometers.

METHODS

The ion activation stage (IAS) is realized as an ion 'tunnel' device. The 30-electrode geometry has an octagonal cross section and an inner diameter of 10 mm. The tunnel is mounted in a vacuum chamber, which directly attaches to the first pumping stage of API mass spectrometers. The effluent from a typical inlet capillary is expanding into the IAS. Reagent ions are generated at atmospheric pressure. Mass spectrometric analysis is performed with quadrupole and time-of-flight instruments.

RESULTS

The performance of the IAS is demonstrated by the controlled activation of the initially equilibrated proton-bound water cluster system. It is shown that a gradual increase in the RF voltage amplitude applied to the electrode structure leads to a reproducible shift of the cluster distribution along with clearly discernible protonation thresholds of selected analytes. Increasing the radiofrequency (RF) voltage from zero to maximum values does not change the average ion residence time within the IAS.

CONCLUSIONS

We have developed an IAS for operation in the intermediate (1-10 mbar) regime in the ion transfer region of API mass spectrometers. This stage is fully compatible with the recently introduced cAPCI method, which relies on the operation of a liquid point electrode generating very clean and stable thermal distributions of [H + (H2O)n] clusters. The IAS allows controlled ion activation by increasing the ion temperature, which is demonstrated by selective analyte protonation.

Flowing gas in mass spectrometer: method for characterization and impact on ion processing

Mass spectrometers are complex instrumentation systems where ions are transferred though different pressure regions and mass-analyzed under high vacuum. In this work, we have investigated the impact of the gas flows that exit almost universally in all pressure regions. We developed a method that incorporates the dynamic gas field with the electric field in the simulation of ion trajectories. The scope of the electro-hydrodynamic simulation (EHS) method was demonstrated for characterizing the ion optical systems at atmospheric pressure interfaces. With experimental validation, the trapping of the externally injected ions in a linear ion trap at low pressure was also studied. Further development of the EHS method and the knowledge acquired in this research are expected to be useful in the design of hybrid instruments and the study of ion energetics.

Simulation-based design of a microfabricated pneumatic electrospray nebulizer

A microfabricated pneumatic electrospray nebulizer has been developed and evaluated using computer simulations and experimental measurements of the MS signals. The microdevice under development is designed for electrospray MS interfacing without the need to fabricate an electrospray needle and can be used as a disposable or an integral part of a reusable system. The design of the chip layout was supported by computational fluid dynamics simulations. The tested microdevices were fabricated in glass using conventional photolithography, followed by wet chemical etching and thermal bonding. The performance of the microfabricated nebulizer was evaluated by means of TOF-MS with a peptide mixture. It was demonstrated that the nebulizer, operating at supersonic speed of the nebulizing gas, produced very stable nanospray (900 nL/min) as documented by less than 0.1% (SE) fluctuation in total mass spectrometric signal intensity.