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.

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.