Schlieren visualization of fluid dynamics effects in direct analysis in real time mass spectrometry

High-Speed Schlieren Imaging of Vapor Formation in Electrospray Plume

Previous mechanistic descriptions of electrosprays mostly focused on the dynamics of Taylor cones, initial droplets, and progeny droplets. However, vapor formation during droplet desolvation in an electrospray plume has not been discussed to a great extent. Here, we implement a double-pass on-axis schlieren high-speed imaging system to observe generation and propagation of vapors in an offline electrospray source under different conditions. Switching between turbulent and laminar vapor flow was observed for all of the scanned conditions, which may be attributed to randomly occurring disturbances in the sample flow inside the electrospray emitter. Calculation of mean vapor flow velocity and analysis of vapor flow patterns were performed using in-house developed image processing programs. Experiments performed at different electrospray voltages (0-6 kV), solvent flow rates (100-600 μL min-1), and methanol concentrations (50-100%), indicate only a weak dependency between electrospray voltage and mean vapor velocity, implying that the vapor is mostly neutral; thus, the vapor is not accelerated by electric field. On the other hand, electrospraying solutions of analytes (with mass 151 Da or 12 kDa) did not remarkably increase the overall vapor flow velocity. The source of vapor's velocity is attributed to the inertia of the electrospray droplets. Although there are some differences between a modern electrospray ionization (ESI) setup and the setup used in our experiment (e.g., using a higher flow rate and larger emitter), we believe the findings of our study can be projected to a modern ESI setup.

Flow-Optimized Model for Gas Jet Desorption Sampling Mass Spectrometry

Thermal gas jet probes, including post-plasma desorption/ionization sources, have not been studied using computational fluid dynamics (CFD) models, as have other ambient mass spectrometry sampling techniques. Two systems were constructed: a heated nitrogen jet probe to establish practical bounds for a sampling/transmission experiment and a CFD model to study trajectories of particles desorbed from a surface through optimization of streamlines and temperatures. The physical model configuration as tested using CFD revealed large losses, transmitting less than 10% of desorbed particles. Different distances between the desorption probe and the transport tube and from the sample surface were studied. The transmission improved when the system was very close to the sample, because the gas jet otherwise creates a region of low pressure that guides the streamlines below the inlet. A baffle positioned to increase pressure in the sample region improves collection efficiency. A Lagrangian particle tracking approach confirms the optimal design leading to a transmission of almost 100%.

Online coupling of matrix solid-phase dispersion to direct analysis in real time mass spectrometry for high-throughput analysis of regulated chemicals in consumer products

The current work is the first study on online coupling of matrix solid-phase dispersion (MSPD) to direct analysis in real time mass spectrometry (DART-MS) bridging with solid-phase analytical derivatization (SPAD) based on a graphene oxide nanosheets (GONs)-coated cotton swab. Proof-of-concept demonstrations were explored for high-throughput analysis of a diversity of regulated chemicals in consumer products such as textiles, toys, and cosmetics. On-demand sorbent combinations were blended with samples, packed into MSPD columns, and mounted on a homemade 3D-printed rack module for automated sample feeding. To achieve good synergy between MSPD and DART-MS, a cotton swab with a conical tip deposited with GONs was attached to the bottom of the MSPD column. The swabs serve as a solid-phase microextraction probe for convenient enrichment of the eluted analytes from MSPD, thermal desorption of the enriched analytes by DART, and sensitive detection by a hybrid quadrupole-Orbitrap mass spectrometer. Furthermore, the utility of an on-swab SPAD strategy was demonstrated for the detection of formaldehyde by use of the derivatizing reagent of dansyl hydrazine, contributing to improved ionization efficiency without compromising the overall coherence of the analytical workflow. The MSPD-DART-MS methodology was systematically optimized and validated, obtaining acceptable recovery (71.7-110.3%), repeatability (11.8-19.3%), and sensitivity (limits of detection and quantitation in the ranges of 6.2-19.5 and 23.7-75.9 μg/kg) for 32 target analytes. The developed protocol streamlined sample extraction, clean-up, desorption, ionization, and detection, highlighting the appealing potential for high-throughput analysis of samples with complex matrices.

Machine Learning Approaches to Identify Discriminative Signatures of Volatile Organic Compounds (VOCs) from Bacteria and Fungi Using SPME-DART-MS

Point-of-care screening tools are essential to expedite patient care and decrease reliance on slow diagnostic tools (e.g., microbial cultures) to identify pathogens and their associated antibiotic resistance. Analysis of volatile organic compounds (VOC) emitted from biological media has seen increased attention in recent years as a potential non-invasive diagnostic procedure. This work explores the use of solid phase micro-extraction (SPME) and ambient plasma ionization mass spectrometry (MS) to rapidly acquire VOC signatures of bacteria and fungi. The MS spectrum of each pathogen goes through a preprocessing and feature extraction pipeline. Various supervised and unsupervised machine learning (ML) classification algorithms are trained and evaluated on the extracted feature set. These are able to classify the type of pathogen as bacteria or fungi with high accuracy, while marked progress is also made in identifying specific strains of bacteria. This study presents a new approach for the identification of pathogens from VOC signatures collected using SPME and ambient ionization MS by training classifiers on just a few samples of data. This ambient plasma ionization and ML approach is robust, rapid, precise, and can potentially be used as a non-invasive clinical diagnostic tool for point-of-care applications.

Optimization of confined direct analysis in real time mass spectrometry (DART-MS)

Direct analysis in real time mass spectrometry (DART-MS) is seeing increased use in many fields, including forensic science, environmental monitoring, food safety, and healthcare. With increased use, novel configurations of the system have been created to either aid in detection of traditionally difficult compounds or surfaces, provide a more reproducible analysis, and/or chemically image surfaces. This work focuses on increasing the fundamental understanding of one configuration, where the DART ionization gas is confined in a junction, such as with thermal desorption (TD) DART-MS. Using five representative compounds and a suite of visualization tools, the role of the DART ionization gas, Vapur flow rate, gas back pressure, and exit grid voltage were examined to better understand both the chemical and physical processes occurring inside the confined configuration. The use of nitrogen as a DART ionization gas was found to be more beneficial than helium because of enhanced mixing with the analyte vapors, providing a more reproducible response. Lower Vapur flow rates were also found to be advantageous as they increased the analyte residence time in the junction, thus increasing the probability of its ionization. Operation at even lower Vapur flow rates was achieved by modifying the junction to restrict the DART gas flow. The DART exit grid voltage and gas back pressure had little observed impact on analyte response. These results provide the foundation to better understand and identify best practices for using a confined DART-MS configuration.

Systematic Ion Source Parameter Assessment by Automated Determination of the Distribution of Ion Acceptance (DIA) Using APLI

The determination of the spatially resolved ion signal with atmospheric pressure laser ionization (APLI), which was introduced as distribution of ion acceptance (DIA), serves as a valuable tool for the understanding of complex and highly dynamical conditions in modern atmospheric pressure (AP) ion sources. DIA provides information about fluid dynamics, ion transport, and ion transformation processes in such sources and is an ideal basis for the validation of numerical models of the dynamics in the ion source enclosure.We present a fully automated setup for DIA measurements, which enabled us to acquire a comprehensive dataset of over 700 individual DIA measurements in a commercial AP ion source (Bruker Multi Purpose Ion Source, MPIS). Ion source parameters as voltages, gas heater temperatures and gas flows, were varied, and the effect of those parameters on the DIA of a chemically inert analyte, pyrene, was systematically investigated. It is shown that the response of the DIA is nonlinear and that gas dynamics largely dominates the ion transport in the ion source. Particularly, the position of the heated nebulizer, which is used to introduce one of two gas flows and the analyte into the ion source chamber, had a profound effect on the DIA. This suggests that the gas dynamics in the source switches between different states. The now available comprehensive DIA dataset reveals such critical effects and will guide further numerical modeling efforts to understand the details of the dynamics of ions in the source chamber. Graphical Abstract.

Direct analysis in real time-Mass spectrometry (DART-MS) in forensic and security applications

Over the last decade, direct analysis in real time (DART) has emerged as a viable method for fast, easy, and reliable "ambient ionization" for forensic analysis. The ability of DART to generate ions from chemicals that might be present at the scene of a criminal activity, whether they are in the gas, liquid, or solid phase, with limited sample preparation has made the technology a useful analytical tool in numerous forensic applications. This review paper summarizes many of those applications, ranging from the analysis of trace evidence to security applications, with a focus on providing the forensic scientist with a resource for developing their own applications. The most common uses for DART in forensics are in studying seized drugs, drugs of abuse and their metabolites, bulk and detonated explosives, toxic chemicals, chemical warfare agents, inks and dyes, and commercial plant and animal products that have been adulterated for economic gain. This review is meant to complement recent reviews that have described the fundamentals of the ionization mechanism and the general use of DART. We describe a wide range of forensic applications beyond the field of analyzing drugs of abuse, which dominates the literature, including common experimental and data analysis methods. © 2016 Wiley Periodicals, Inc. Mass Spec Rev 37:171-187, 2018.

Use of Interrupted Helium Flow in the Analysis of Vapor Samples with Flowing Atmospheric-Pressure Afterglow-Mass Spectrometry

The flowing atmospheric-pressure afterglow (FAPA) source was used for the mass-spectrometric analysis of vapor samples introduced between the source and mass spectrometer inlet. Through interrupted operation of the plasma-supporting helium flow, helium consumption is greatly reduced and dynamic gas behavior occurs that was characterized by schlieren imaging. Moreover, mass spectra acquired immediately after the onset of helium flow exhibit a signal spike before declining and ultimately reaching a steady level. This initial signal appears to be due to greater interaction of sample vapor with the afterglow of the source when helium flow resumes. In part, the initial spike in signal can be attributed to a pooling of analyte vapor in the absence of helium flow from the source. Time-resolved schlieren imaging of the helium flow during on and off cycles provided insight into gas-flow patterns between the FAPA source and the MS inlet that were correlated with mass-spectral data. Graphical Abstract ᅟ.

Multimodal Vacuum-Assisted Plasma Ion (VaPI) Source with Transmission Mode and Laser Ablation Sampling Capabilities

We have developed a multimodal ion source design that can be configured on the fly for various analysis modes, designed for more efficient and reproducible sampling at the mass spectrometer atmospheric pressure (AP) interface in a number of different applications. This vacuum-assisted plasma ionization (VaPI) source features interchangeable transmission mode and laser ablation sampling geometries. Operating in both AC and DC power regimes with similar results, the ion source was optimized for parameters including helium flow rate and gas temperature using transmission mode to analyze volatile standards and drug tablets. Using laser ablation, matrix effects were studied, and the source was used to monitor the products of model prebiotic synthetic reactions.