Simulation of rarefied gas flows in atmospheric pressure interfaces for mass spectrometry systems

A study of the transient gas flow affected ion transmission in atmospheric pressure interfaces based on large eddy simulation for electrospray ionization mass spectrometry

Modern atmosphere pressure interface (API) enables high-efficiency coupling between mass analyzers in high vacuum and atmosphere ionization sources such as electrospray ionization (ESI) source. The transient gas flow entering API possesses strong compressibility and turbulent characteristics, which exerts a huge impact on ion transmission. However, the instantaneous nature and vortical morphology of the turbulence in API and its affection in ion transmission were hardly covered in the reported research. Here we conduct a transient turbulent flow-affected ion transmission evaluation for two typical APIs, the ion funnel and the S-lens, based on scale-resolving large eddy simulation and electro-hydrodynamical ion tracing simulation. In our simulation, the transient properties of the gas flow in the two APIs are illustrated and analyzed in-depth. After experimentally validated on a homemade ESI-TOF-MS platform, the results suggest that the ion funnel can achieve a higher droplet desolvation rate by introducing a unique droplet recirculation mechanism. Meanwhile, the less-dispersed gas flow in S-lens is beneficial in actuating ions axially. In conclusion, the application of the scale-resolving turbulence model helps us to understand the complicated fluid-ion interaction mechanism in APIs and is promising in the development of mass spectrometry instruments of higher performance.

Development and application of a miniature mass spectrometer with continuous sub-atmospheric pressure interface and integrated ionization source

For the miniature mass spectrometer (MS) with a continuous atmospheric pressure interface (CAPI), the gas in the multi-stage chambers directly affects the performance of the instrument. In this study, a sealed ionization chamber is designed to couple with a conventional mini CAPI-MS. In this configuration, the gas environment in the first ionization chamber can be flexibly changed to regulate the gas conditions throughout the entire instrument. By studying the effect of gas pressure on the performance of the instrument, we found that the instrument shows some unique advantages when the first ionization chamber is under sub-atmospheric pressure (SAP) conditions, such as reducing the load of the vacuum pump by 40%, achieving pump-free injection for gas and liquid samples, and improving the resolution by a factor of 2 without loss of detection sensitivity. Therefore, we propose a new integrated interface called continuous sub-atmospheric pressure interface (CSAPI) for building a miniature ion trap mass spectrometer. The CSAPI specially integrates sample introduction, gas/ions interface, and ionizations, including electrospray ionization (ESI) and secondary electrospray ionization (SESI), making this system more convenient for non-professional handlers to rapidly identify or monitor target analytes in gaseous- and solution-phase samples. We also use this system to study gas composition to further improve performance, being able to achieve a 5-fold sensitivity and 2-fold resolution improvement. At last, some custom applications of the current CSAPI-MS platform are explored and demonstrated, including real-time monitoring of chemical reactions in solution and long-distance sampling and analysis of dried Chinese herbs. In conclusion, this study provides a new approach to constructing a complete, versatile and practical miniature MS instrument.

SimELIT: A Novel GUI-Based Comprehensive Ion Trajectory Simulation Software for Mass Spectrometry

Ion trajectory simulation in mass spectrometry systems from injection to detection is technically challenging but very important for better understanding the ion dynamics in instrument development. Here, we present SimELIT (Simulator of Eulerian and Lagrangian Ion Trajectories), a novel ion trajectory simulation platform. SimELIT is built upon a suite of multiphysics solvers compiled into OpenFOAM (an open-source numerical solver library particularly used for computational mechanics), with a simple web-based graphical user interface (GUI) allowing users to define the details of OpenFOAM cases and run simulations. SimELIT is a modular program and can provide extensions of physics (e.g., gas flows, electrodynamic fields) and thus enable ion trajectory simulations from the ion source to detector. The current version (SimELIT) provides two numerical solvers for ion trajectory simulations─(1) a Lagrangian particle tracker in vacuum and (2) a Eulerian ion density solver in background gas in the presence of electric fields. Here, we describe the architecture of SimELIT, including its use of Docker and the React Framework, and demonstrate the computation of ion trajectories of multiple m/z values in a static/linear voltage drop in vacuum (across a 1 m long flight tube). Further, the drift motion of ions under 1 Torr pressure conditions in a static background (N₂) gas through a 20 V/cm static electric field is shown. The results produced from SimELIT were compared with SIMION and theoretical estimates. In addition, we report the computation of ion trajectories in electrodynamic fields within a planar FAIMS device operating at atmospheric pressure.

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.

Discontinuous Subatmospheric Pressure Interface Reduces the Gas Flow Effects on Miniature CAPI Mass Spectrometer

In the range of miniature mass spectrometers, the miniature ion trap mass spectrometer with continuous atmospheric pressure interface (CAPI) shows good performance potential and advantages due to its excellent sensitivity and analysis speed. However, in previous cases, placing the ion trap directly near the skimmer aperture means it will suffer high gas shock, which may affect performance. In this study, an improved miniature CAPI ion trap mass spectrometer was developed by gas flow optimization. According to the experimental results, excessive gas flow affects stability and resolution. The impact of the gas flow can be effectively reduced by reducing the inner diameter of the skimmer and adding an additional lens element to move the ion trap away from the skimmer aperture. However, this method will affect the sensitivity of the instrument to some extent, so a discontinuous subatmospheric pressure interface (DSPI) was developed to reduce the gas flow effects and improve the comprehensive performance. When using the DSPI system with a 0.4 mm skimmer and entrance lens, the resolution for roxithromycin was up to 2800 at a scanning speed of 1015 Th/s, which was 3.4-fold higher that without DSPI. The dynamic range of concentration reached 4 orders of magnitude and the detection limit for repaglinide was as low as 1 ng/mL. This study offers a new approach to develop better miniature ion trap mass spectrometers and to extend their practical application.

Direct Analysis of Organic Compounds in Liquid Using a Miniature Photoionization Ion Trap Mass Spectrometer with Pulsed Carrier-Gas Capillary Inlet

A miniature ion trap mass spectrometer with capillary direct sampling and vacuum ultraviolet photoionization source was developed to conduct trace analysis of organic compounds in liquids. Self-aspiration sampling is available where the samples are drawn into the vacuum chamber through a capillary with an extremely low flow rate (less than 1 μL/min), which minimizes sample consumption in each analysis to tens of micrograms. A pulsed gas-assisted inlet was designed and optimized to promote sample transmission in the tube and facilitate the cooling of ions, thereby improving instrument sensitivity. A limit of detection of 2 ppb could be achieved for 2,4-dimethylaniline in a methanol solution. The sampling system described in the present study is specifically suitable for a miniature photoionization ion trap mass spectrometer that can perform rapid and online analysis for liquid samples. Graphical Abstract ᅟ.

Ion transfer between ion source and mass spectrometer inlet: electro-hydrodynamic simulation and experimental validation

RATIONALE

The ion transfer from the atmospheric pressure ion source to mass spectrometer inlet is directly related to the sensitivity of the mass spectrometry (MS) analysis. Electric field and dynamic gas flow are typically used to facilitate the ionization process and ion transfer. While sophisticated methods have been developed for ion trajectory simulation with a pure electric field, the influence of the dynamic gas flow could not be easily incorporated for the study.

METHODS

A nanoESI (electrospray ionization) source was set off-axis in front of an MS inlet to study the ion transfer under the influence of both electric field and gas flows. Electro-hydrodynamic simulation (EHS) was performed to predict the ion transfer, which was subsequently validated with the experimental characterization.

RESULTS

The EHS results based on the gas dynamics were found to match well with the experimental results and therefore can be used to guide the instrumentation design. The relative intensities of different ion species could be modified by adjusting the gas flow rate, and a differential transfer of the selected ion species was achieved.

CONCLUSIONS

EHS is a powerful tool for the design of ion optics operating at atmospheric pressure. As a rapid and convenient method, proper combination of an air dynamic field and an electric field enabled a gas-phase ion separation in the source region without using sophisticated ion mobility devices. Copyright © 2016 John Wiley & Sons, Ltd.

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.

A pulsed pinhole atmospheric pressure interface for simplified mass spectrometry instrumentation with enhanced sensitivity

RATIONALE

A proof-of-concept pulsed pinhole atmospheric pressure interface, namely PP-API, was developed and characterized for mass spectrometry instrumentation.

METHODS

The PP-API was analyzed and optimized theoretically with respect to gas flow rate, opening force and dimensions. A PP-API interfaced mass spectrometry system was then constructed and tested.

RESULTS

As a discontinuous pressure interface, the PP-API allows efficient ion transfer from atmosphere pressure ionization sources to ion traps directly. Both nano-ESI and APCI ionization sources have been successfully integrated with the PP-API interface for the detection of volatile, organic, peptide and polymer samples. The use of multiple ionization sources has also been demonstrated to enhance signal intensity, as well as avoid charge competitions in ionization sources.

CONCLUSIONS

With simplified geometry and high ion transfer efficiency, this PP-API would enable miniaturized mass spectrometry systems with high sensitivity.

Development of miniature mass spectrometry systems for bioanalysis outside the conventional laboratories

Mass spectrometry (MS) is known for highly specific and sensitive analysis. The general applicability of this technique makes it a good candidate for biological applications over a much broader range than is now the case. The limiting factors preventing MS from being applied at the biologist's bench or in a physician's office are identified as the large size of the systems, as well as the complicated analytical procedures required. An approach for developing miniature MS analysis systems with simplified operational procedures is described and the associated technical developments are discussed.

Development of a miniature mass spectrometer with continuous atmospheric pressure interface

The demand for on-the-spot analysis is met by a miniature mass spectrometer which is preferred to be robust, stable, as small as possible and capable of analyzing different samples by coupling with various ionization methods.

On an aerodynamic mechanism to enhance ion transmission and sensitivity of FAIMS for nano-electrospray ionization-mass spectrometry

Simulations show that significant ion losses occur within the commercial electrospray ionization-field asymmetric waveform ion mobility spectrometer (ESI-FAIMS) interface owing to an angular desolvation gas flow and because of the impact of the FAIMS carrier gas onto the inner rf (radio frequency) electrode. The angular desolvation gas flow diverts ions away from the entrance plate orifice while the carrier gas annihilates ions onto the inner rf electrode. A novel ESI-FAIMS interface is described that optimizes FAIMS gas flows resulting in large improvements in transmission. Simulations with the bromochloroacetate anion showed an improvement of ~9-fold to give ~70% overall transmission). Comparable transmission improvements were attained experimentally for six peptides (2+) in the range of m/z 404.2 to 653.4 at a chromatographic flow rate of 300 nL/min. Selected ion chromatograms (SIC) from nano-LC-FAIMS-MS analyses showed 71% (HLVDEPQNLIK, m/z 653.4, 2+) to 95% (LVNELTEFAK, m/z 582.3, 2+) of ion signal compared with ion signal in the SIC from LC-MS analysis. IGSEVYHNLK (580.3, 2+) showed 24% more ion signal compared with LC-MS and is explained by enhanced desolvation in FAIMS. A 3-10 times lower limits of quantitation (LOQ) (<15% RSD) was achieved for chemical noise limited peaks with FAIMS. Peaks limited by ion statistics showed subtle improvement in RSD and yielded comparable LOQ to that attained with nano-LC-MS (without FAIMS). These improvements were obtained using a reduced FAIMS separation gap (from 2.5 to 1.5 mm) that results in a shorter residence time (13.2 ms ± 3.9 ms) and enables the use of a helium free transport gas (100% nitrogen).

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.

Ion sponge: a 3-dimentional array of quadrupole ion traps for trapping and mass-selectively processing ions in gas phase

In this study, the concept of ion sponge has been explored for developing 3D arrays of large numbers of ion traps but with simple configurations. An ion sponge device with 484 trapping units in a volume of 10 × 10 × 3.2 cm has been constructed by simply stacking 9 meshes together. A single rf was used for trapping ions and mass-selective ion processing. The ion sponge provides a large trapping capacity and is highly transparent for transfer of ions, neutrals, and photons for gas phase ion processing. Multiple layers of quadrupole ion traps, with 121 trapping units in each layer, can operate as a single device for MS or MS/MS analysis, or as a series of mass-selective trapping devices with interlayer ion transfers facilitated by AC and DC voltages. Automatic sorting of ions to different trapping layers based on their mass-to-charge (m/z) ratios was achieved with traps of different sizes. Tandem-in-space MS/MS has also been demonstrated with precursor ions and fragment ions trapped in separate locations.

Numerical modeling of ion transport in an ESI-MS system

Gas and ion transport in the capillary-skimmer subatmospheric interface of a mass spectrometer, which is typically utilized to separate unevaporated micro-droplets from ions, was studied numerically using a two-step approach spanning multiple gas dynamic regimes. The gas flow in the heated capillary and in the interface was determined by solving numerically the Navier-Stokes equation. The capillary-to-skimmer gas/ion flow was modeled through the solution of the full Boltzmann equation with a force term. The force term, together with calculated aerodynamic drag, determined the ion motion in the gap between the capillary and skimmer. Three-dimensional modeling of the impact of the voltage applied to the Einzel lens on the transmission of doubly charged peptide ions through the skimmer orifice was compared with experimental data obtained in the companion study. Good agreement between measured and computed signals was observed. The numerical results indicate that as many as 75% of the ions that exit from the capillary are lost on the conical surface of the skimmer or capillary outer surface because of the electrostatic force and plume divergence.