Solvent Jet Desorption Capillary Photoionization-Mass Spectrometry

New Processes for Ionizing Nonvolatile Compounds in Mass Spectrometry: The Road of Discovery to Current State-of-the-Art

This Perspective covers discovery and mechanistic aspects as well as initial applications of novel ionization processes for use in mass spectrometry that guided us in a series of subsequent discoveries, instrument developments, and commercialization. Vacuum matrix-assisted ionization on an intermediate pressure matrix-assisted laser desorption/ionization source without the use of a laser, high voltages, or any other added energy was simply unbelievable, at first. Individually and as a whole, the various discoveries and inventions started to paint, inter alia, an exciting new picture and outlook in mass spectrometry from which key developments grew that were at the time unimaginable, and continue to surprise us in its simplistic preeminence. We, and others, have demonstrated exceptional analytical utility. Our current research is focused on how best to understand, improve, and use these novel ionization processes through dedicated platforms and source developments. These ionization processes convert volatile and nonvolatile compounds from solid or liquid matrixes into gas-phase ions for analysis by mass spectrometry using, e.g., mass-selected fragmentation and ion mobility spectrometry to provide accurate, and sometimes improved, mass and drift time resolution. The combination of research and discoveries demonstrated multiple advantages of the new ionization processes and established the basis of the successes that lead to the Biemann Medal and this Perspective. How the new ionization processes relate to traditional ionization is also presented, as well as how these technologies can be utilized in tandem through instrument modification and implementation to increase coverage of complex materials through complementary strengths.

Direct sub-atmospheric pressure ionization mass spectrometry: Evaporation/sublimation-driven ionization is amazing, fundamentally, and practically

This paper covers direct sub-atmospheric pressure ionization mass spectrometry (MS). The discovery, applications, and mechanistic aspects of novel ionization processes for use in MS that are not based on the high-energy input from voltage, laser, and/or high temperature but on sublimation/evaporation within a region linking a higher to lower pressure and modulated by heat and collisions, are discussed, including how this new reality has guided a series of discoveries, instrument developments, and commercialization. A research focus, inter alia, is on how best to understand, improve, and use these novel ionization processes, which convert volatile and nonvolatile compounds from solids (sublimation) or liquids (evaporation) into gas-phase ions for analysis by MS providing reproducible, accurate, sensitive, and prompt results. Our perception on how these unprecedented versus traditional ionization processes/methods relate to each other, how they can be made to coexist on the same mass spectrometer, and an outlook on new and expanded applications (e.g., clinical, portable, fast, safe, and autonomous) is presented, and is based on ST's Opening lecture presentation at the Nordic Mass spectrometry Conference, Geilo, Norway, January 2023. Focus will be on matrix-assisted ionization (MAI) and solvent-assisted ionization (SAI) MS covering the period from 2010 to 2023; a potential paradigm shift in the making.

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%.

In-Line Dopant Generation for Atmospheric Pressure Ionization Mass Spectrometry

A concentric trace gas permeation tube that diffuses chemical reagents to a central carrier gas stream is used to drive chemical reaction pathways and influence gas-phase chemistry for a variety of atmospheric pressure ionization sources for mass spectrometry. Tunable permeation through the reservoir-jacketed polymer membrane is triggered by the heated gas moving through the tube, evaporating the dopant into a sheath dry gas or into a sample stream in room air without diluting the analyte concentration. The permeator is used to add dopants to an electrospray plume for analyte ion charge reduction and to perform hydrogen-deuterium exchange on biomolecules in different spray conditions. Dopants are also added to atmospheric pressure chemical ionization to favor the ionization of select components of diesel fuel. Atmospheric pressure photoionization is performed with the permeation tube in line with tubing transporting sample headspace to an enclosed discharge lamp. Toluene dopant from the permeator increases the proton transfer and charge exchange signal from clove oil and mothballs many times without exposing the laboratory to reagent fumes. Water permeation is also used to humidify the sample gas stream.

Extractive Atmospheric Pressure Photoionization (EAPPI) Mass Spectrometry: Rapid Analysis of Chemicals in Complex Matrices

Extractive atmospheric pressure photoionization (EAPPI) mass spectrometry was designed for rapid qualitative and quantitative analysis of chemicals in complex matrices. In this method, an ultrasonic nebulization system was applied to sample extraction, nebulization, and vaporization. Mixed with a gaseous dopant, vaporized analytes were ionized through ambient photon-induced ion-molecule reactions, and were mass-analyzed by a high resolution time-of-flight mass spectrometer (TOF-MS). After careful optimization and testing with pure sample solution, EAPPI was successfully applied to the fast screening of capsules, soil, natural products, and viscous compounds. Analysis was completed within a few seconds without the need for preseparation. Moreover, the quantification capability of EAPPI for matrices was evaluated by analyzing six polycyclic aromatic hydrocarbons (PAHs) in soil. The correlation coefficients (R (2) ) for standard curves of all six PAHs were above 0.99, and the detection limits were in the range of 0.16-0.34 ng/mg. In addition, EAPPI could also be used to monitor organic chemical reactions in real time. Graphical Abstract ᅟ.

Improved desorption/ionization and ion transmission in surface scanning by direct analysis in real time mass spectrometry

RATIONALE

Modifications to the Direct Analysis in Real Time mass spectrometry (DART-MS) interface, its source cap and transfer tube were necessary to obtain highest efficiency in desorption and ionization from the sampling surface and in ion transmission into the MS system. These issues are crucial for the trace analysis of any surface and the hyphenation of high-performance thin-layer chromatography (HPTLC) with DART-MS.

METHODS

The ion source mounting was modified to enable short source caps to be utilized in combination with a short transfer tube. The grid voltage contact section was readjusted to increase the intensity of the metastable gas stream towards the substrate. Eighteen different cap and two transfer tube geometries (including gas-stream focusing), along with the influence of their distance from the mass spectrometer glass capillary, were investigated for best signal intensity.

RESULTS

Using shortened source caps with staged inner bore, a transfer tube with gas-stream focusing and an optimized mounting geometry for DART-MS scanning along five identical deposited bands (600 ng each) of butyl 4-hydroxybenzoate, an average signal precision of 3.6% was obtained and the signal intensity was increased by a factor of 34. The width of the gas impact area did not exceed 1.5 mm and the smallest FWHM was determined to be 0.9 mm.

CONCLUSIONS

The desorption strength, ionization efficacy and ion transmission were improved significantly giving increased detectability using this further modified DART-MS interface with reduced cap length and optimum transfer tube geometry. The resolution was comparable with state-of-the-art densitometry. With this setup, reliable HPTLC surface scanning is possible, even for substance amounts in the low-nanogram range.

"Magic" Ionization Mass Spectrometry

The systematic study of the temperature and pressure dependence of matrix-assisted ionization (MAI) led us to the discovery of the seemingly impossible, initially explained by some reviewers as either sleight of hand or the misinterpretation by an overzealous young scientist of results reported many years before and having little utility. The “magic” that we were attempting to report was that with matrix assistance, molecules, at least as large as bovine serum albumin (66 kDa), are lifted into the gas phase as multiply charged ions simply by exposure of the matrix:analyte sample to the vacuum of a mass spectrometer. Applied heat, a laser, or voltages are not necessary to achieve charge states and ion abundances only previously observed with electrospray ionization (ESI). The fundamentals of how solid phase volatile or nonvolatile compounds are converted to gas-phase ions without added energy currently involves speculation providing a great opportunity to rethink mechanistic understanding of ionization processes used in mass spectrometry. Improved understanding of the mechanism(s) of these processes and their connection to ESI and matrix-assisted laser desorption/ionization may provide opportunities to further develop new ionization strategies for traditional and yet unforeseen applications of mass spectrometry. This Critical Insights article covers developments leading to the discovery of a seemingly magic ionization process that is simple to use, fast, sensitive, robust, and can be directly applied to surface characterization using portable or high performance mass spectrometers.