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

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.

Evaluation of Inlet Temperature with Three Sprayer Designs for Desorption Electrospray Ionization Mass Spectrometry Tissue Analysis

Mass spectrometry imaging (MSI) allows for the spatially resolved detection of endogenous and exogenous molecules and atoms in biological samples, typically prepared as thin tissue sections. Desorption electrospray ionization (DESI) is one of the most commonly utilized MSI modalities in preclinical research. DESI ion source technology is still rapidly evolving, with new sprayer designs and heated inlet capillaries having recently been incorporated in commercially available systems. In this study, three iterations of DESI sprayer designs are evaluated: (1) the first, and until recently only, commercially available Waters sprayer; (2) a developmental desorption electro-flow focusing ionization (DEFFI)-type sprayer; and (3) a prototype of the newly released Waters commercial sprayer. A heated inlet capillary is also employed, allowing for controlled inlet temperatures up to 500 °C. These three sprayers are evaluated by comparative tissue imaging analyses of murine testes across this temperature range. Single ion intensity versus temperature trends are evaluated as exemplar cases for putatively identified species of interest, such as lactate and glutamine. A range of trends are observed, where intensities follow either increasing, decreasing, bell-shaped, or other trends with temperature. Data for all sprayers show approximately similar trends for the ions studied, with the commercial prototype sprayer (sprayer version 3) matching or outperforming the other sprayers for the ions investigated. Finally, the mass spectra acquired using sprayer version 3 are evaluated by uniform manifold approximation and projection (UMAP) and k-means clustering. This approach is shown to provide valuable insight that is complementary to the presented univariate evaluation for reviewing the parameter space in this study. Full spectral temperature optimization data are provided as supporting data to enable other researchers to design experiments that are optimal for specific ions.

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.

Few-cycle Yb laser source at 20 kHz using multidimensional solitary states in hollow-core fibers

We demonstrate ultrashort pulse compression from 300 fs down to 17 fs at a repetition rate of 20 kHz and 160-µJ output pulse energy (3.2 W of average power) using multidimensional solitary states (MDSS) in a 1-meter hollow-core fiber (HCF) filled with N₂O. Under static pressure, thermal limitations at this repetition rate annihilate the MDSS with suppression of spectral broadening. The results obtained in differential pressure configuration mitigate thermal effects and significantly increase the range of repetition rate over which MDSS can be used to compress sub-picosecond laser pulses.

Insight into the Ozone-Assisted Low-Temperature Combustion of Dimethyl Ether by Means of Stabilized Cool Flames

The low-temperature combustion kinetics of dimethyl ether (DME) were studied by means of stabilized cool flames in a heated stagnation plate burner configuration using ozone-seeded premixed flows of DME/O₂. Direct imaging of CH₂O* chemiluminescence and laser-induced fluorescence of CH₂O were used to determine the flame front positions in a wide range of lean and ultra-lean equivalence ratios and ozone concentrations for two strain rates. The temperature and species mole fraction profiles along the flame were measured by coupling thermocouples, gas chromatography, micro-chromatography, and quadrupole mass spectrometry analysis. A new kinetic model was built on the basis of the Aramco 1.3 model, coupled with a validated submechanism of O₃ chemistry, and was updated to improve the agreement with the obtained experimental results and experimental data available in the literature. The main results show the efficiency of the tested model to predict the flame front position and temperature in every tested condition, as well as the importance of reactions typical of atmospheric chemistry in the prediction of cool flame occurrence. The agreement on the fuel and major products is overall good, except for methanol, highlighting some missing kinetic pathways for the DME/O₂/O₃ system, possibly linked to the direct addition of atomic oxygen on the fuel radical, modifying the product distribution after the cool flame.

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.

Investigation of spatiotemporal output beam profile instabilities from differentially pumped capillaries

Differentially pumped capillaries, i.e., capillaries operated in a pressure gradient environment, are widely used for nonlinear pulse compression. In this work, we show that strong pressure gradients and high gas throughputs can cause spatiotemporal instabilities of the output beam profile. The instabilities occur with a sudden onset as the flow evolves from laminar to turbulent. Based on the experimental and numerical results, we derive guidelines to predict the onset of those instabilities and discuss possible applications in the context of nonlinear flow dynamics.

Sub-ambient pressure IR-MALDI ion mobility spectrometer for the determination of low and high field mobilities

A new ion mobility (IM) spectrometer, enabling mobility measurements in the pressure range between 5 and 500 mbar and in the reduced field strength range E/N of 5-90 Td, was developed and characterized. Reduced mobility (K₀) values were studied under low E/N (constant value) as well as high E/N (deviation from low field K₀) for a series of molecular ions in nitrogen. Infrared matrix-assisted laser desorption ionization (IR-MALDI) was used in two configurations: a source working at atmospheric pressure (AP) and, for the first time, an IR-MALDI source working with a liquid (aqueous) matrix at sub-ambient/reduced pressure (RP). The influence of RP on IR-MALDI was examined and new insights into the dispersion process were gained. This enabled the optimization of the IM spectrometer for best analytical performance. While ion desolvation is less efficient at RP, the transport of ions is more efficient, leading to intensity enhancement and an increased number of oligomer ions. When deciding between AP and RP IR-MALDI, a trade-off between intensity and resolving power has to be considered. Here, the low field mobility of peptide ions was first measured and compared with reference values from ESI-IM spectrometry (at AP) as well as collision cross sections obtained from molecular dynamics simulations. The second application was the determination of the reduced mobility of various substituted ammonium ions as a function of E/N in nitrogen. The mobility is constant up to a threshold at high E/N. Beyond this threshold, mobility increases were observed. This behavior can be explained by the loss of hydrated water molecules.

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.

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.