Ionization Step Research Articles

Postionization Mass Spectrometry Imaging: Past, Present, and Future.

Mass spectrometry imaging (MSI) technologies are widely used today to study the in situ spatial distributions for a variety of analytes. As these technologies advance, the pursuit of higher resolution in MSI has intensified. The limitation of direct desorption/ionization is its insufficient ionization, posing a constraint on the advancement of high-resolution MSI technologies. The introduction of postionization process compensates the low ionization efficiency caused by sacrificing the desorption area while pursuing high spatial resolution, resolving the conflict between high spatial resolution and high sensitivity in direct desorption/ionization method. Here, we discuss the sampling and ionization steps of MSI separately, and review the postionization methods in MSI according to three different sampling modes: laser sampling, probe sampling, and ion beam sampling. Postionization technology excels in enhancing ionization efficiency, boosting sensitivity, mitigating discrimination effect, simplifying sample preparation, and expanding the scope of applicability. These advantages position postionization technology as a promising tool for biomedical sciences, materials sciences, forensic analysis and other fields.

Ion Source Complementarity for Characterization of Complex Organic Mixtures Using Fourier Transform Mass Spectrometry: A Review.

Complex organic mixtures are found in many areas of research, such as energy, environment, health, planetology, and cultural heritage, to name but a few. However, due to their complex chemical composition, which holds an extensive potential of information at the molecular level, their molecular characterization is challenging. In mass spectrometry, the ionization step is the key step, as it determines which species will be detected. This review presents an overview of the main ionization sources employed to characterize these kinds of samples in Fourier transform mass spectrometry (FT-MS), namely electrospray (ESI), atmospheric pressure photoionization (APPI), atmospheric pressure chemical ionization (APCI), atmospheric pressure laser ionization (APLI), and (matrix-assisted) laser desorption ionization ((MA)LDI), and their complementarity in the characterization of complex organic mixtures. First, the ionization techniques are examined in the common direct introduction (DI) usage. Second, these approaches are discussed in the context of coupling chromatographic techniques such as gas chromatography, liquid chromatography, and supercritical fluid chromatography.

Enzymatic Synthesis of Isotopically Labeled Hydrogen Peroxide for Mass Spectrometry-Based Applications.

Methionine oxidation is involved in multiple biological processes including protein misfolding and enzyme regulation. However, it is often challenging to measure levels of methionine oxidation by mass spectrometry, in part due to the prevalence of artifactual oxidation that occurs during the sample preparation and ionization steps of typical proteomic workflows. Isotopically labeled hydrogen peroxide (H218O2) can be used to block unoxidized methionines and enables accurate measurement of in vivo levels of methionine oxidation. However, H218O2 is an expensive reagent that can be difficult to obtain from commercial sources. Here, we report a method for synthesizing H218O2 in-house. Glucose oxidase catalyzes the oxidation of β-d-glucose and produces hydrogen peroxide in the process. We took advantage of this reaction to enzymatically synthesize H218O2 from 18O2 and assessed its concentration, purity, and utility in measuring methionine oxidation levels by mass spectrometry.

Investigation of the laser fluence and wavelength dependence in surface-assisted laser desorption/ionization mass spectrometry using gold nanoparticles.

Surface-assisted laser desorption/ionization (SALDI) mass spectrometry (MS) builds on the use of nanostructured surfaces (e.g., coatings of colloidal nanoparticles) to promote analyte desorption and ionization. The SALDI process is believed to occur mainly through thermal processes, resulting from heating of the nanosubstrate upon absorption of the photon energy, and by assisting ionization steps. Mostly due to the accessibility of the respective hardware, the majority of SALDI-MS studies use standard laser wavelengths for MALDI (i.e., 337 or 355 nm), even though peak absorption of the SALDI nanosubstrate might completely differ from these values. Here, we investigated the wavelength dependence in SALDI-MS to determine if wavelength adjustment would be beneficial, and to provide new experimental data for a better understanding of the SALDI mechanism. To this end, gold nanoparticles (AuNPs) sprayed onto microscope glass slides were employed as SALDI nanosubstrates and L-arginine as a model analyte. In addition, we used 2,5-dihydroxyacetophenone (2,5-DHAP) for classical MALDI-MS using the same experimental setup. Arginine ion signals were recorded as a function of laser wavelength and laser fluence. Mass spectra were acquired in the wavelength range between 310 and 630 nm, including the absorption maximum of the sprayed AuNPs around 550 nm and that of 2,5-DHAP around 380 nm. Laser fluence thresholds for the generation of arginine ions were found to be dependent on the laser wavelength and to inversely correlate with the absorbance profiles of the deposited AuNPs and 2,5-DHAP, respectively. Very differently to MALDI, in SALDI ionization efficiency was found to strictly linearly decrease with increasing laser wavelength. Our results, therefore, corroborate the general assumption that material ejection in SALDI-MS is mainly driven by thermal processes in the low laser fluence range and add new evidence that the ionization process is directly influenced by photon energy when AuNPs are employed as nanosubstrates.

Time-Resolved Ion Mobility Spectrometry with a Stop Flow Confined Volume Reaction Region.

An ion source concept is described where the sample flow is stopped in a confined volume of an ion mobility spectrometer creating time-dependent patterns of ion patterns of signal intensities for ions from mixtures of volatile organic compounds and improved signal-to-noise rate compared to conventional unidirectional drift gas flow. Hydrated protons from a corona discharge were introduced continuously into the confined volume with the sample in air at ambient pressure, and product ions were extracted continuously using an electric field for subsequent mobility analysis. Ion signal intensities for protonated monomers and proton bound dimers were measured and computationally extracted using mobilities from mobility spectra and exhibited distinct times of appearance over 30 s or more after sample injection. Models, and experimental findings with a ternary mixture, suggest that the separation of vapors as ions over time was consistent with differences in the reaction rate for reactions between primary ions from hydrated protons and constituents and from cross-reactions that follow the initial step of ionization. The findings suggest that the concept of stopped flow, introduced here for the first time, may provide a method for the temporal separation of atmospheric pressure ions. This separation relies on ion kinetics and does not require chromatographic technology.

Analysis of Anodic Linear Sweep Voltammograms Considering Insufficient Lability of Cu|Cu(II), Glycine System

Anodic LS voltammograms of the alkaline Cu|Cu(II), glycine system were converted into mass-transport corrected Tafel plots considering that the glycinate anion L− takes part in the first stage of copper ionization. The anodic current density was normalized relative to the surface concentration of L− species, which was determined using the mass transfer model of chemically interacting species. The lability of the system was assessed based on available kinetic characteristics and specific experimental data. To linearize the Tafel plots, corrections were made to account for the insufficient mobility of the proton attached to glycine amino group. Obtained Tafel constants indicate that the second step of copper ionization, involving the oxidation of the intermediate copper(I)-glycine complex, is the rate-determining step.

Desorption electrospray ionization mass spectrometry imaging of latent fingerprints revealed by Oil Red O.

To optimize the ionization step independently from surface extraction, the ORO reagent and its mixture with model compounds (triolein and linoleic acid) were first studied in solution using high-resolution electrospray ionization tandem mass spectrometry (ESI-MS/MS). Then, DESI-MSI experiments were performed in both polarity modes for ORO-processed fingermarks deposited on pieces of paper used as porous substrates. ESI-MS of ORO reveals a complex mixture of azo dyes. Two main impurities detected beside the targeted species were characterized using MS/MS and then were usefully employed to produce DESI-MS images of fingermarks, decreasing the scanning rate to get sufficient ion abundance from natural fingerprints. ORO did not prevent chemical profiling, and one major added value of this pink dye was to produce MS images with contrast that cannot be obtained optically for some colored substrates. DESI-MS has demonstrated imaging compatibility with the application of ORO used to enhance latent fingerprints on paper and could also enable chemical profiling in natural fingermarks. In addition, MS images of ORO impurities were of higher quality than optical ones for fingerprints revealed on colored paper.

Tracking Charge Migration with Frequency-Matched Strobo-Spectroscopy.

We present frequency-matched strobo-spectroscopy (FMSS) of charge migration (CM) in bromobutadiyne, simulated with time-dependent density functional theory. CM + FMSS is a pump-probe scheme that uses a frequency-matched high harmonic generation (HHG)-driving laser as an independent probe step, following the creation of a localized hole on the bromine atom that induces CM dynamics. We show that the delay-dependent harmonic yield tracks the phase of the CM dynamics through its sensitivity to the amount of electron density on the bromine end of the molecule. FMSS takes advantage of the intrinsic attosecond time resolution of the HHG process in which different harmonics are emitted at different times and thus probe different locations of the electron hole. Finally, we show that the CM-induced modulation of the HHG signal is dominated by the recombination step of the HHG process, with a negligible contribution from the ionization step.

Alternative Pathway to Double-Core-Hole States.

Excited double-core-hole states of isolated water molecules resulting from the sequential absorption of two x-ray photons have been investigated. These states are formed through an alternative pathway, where the initial step of core ionization is accompanied by the shake-up of a valence electron, leading to the same final states as in the core-ionization followed by core-excitation pathway. The capability of the x-ray free-electron laser to deliver very intense, very short, and tunable light pulses is fully exploited to identify the two different pathways.

Desorption Electrospray Ionization Mass Spectrometry: 20 Years.

ConspectusMass spectrometry (MS) is one of the most widely used technologies in the chemical sciences. With applications spanning the monitoring of reaction products, the identification of disease biomarkers, and the measurement of thermodynamic parameters and aspects of structural biology, MS is well established as a universal analytical tool applicable to small compounds as well as large molecular complexes. Regardless of the application, the generation of gas-phase ions from neutral compounds is a key step in any MS experiment. However, this ionization step was for many years limited to high-energy approaches that required gas-phase analytes and thus it was restricted to volatile samples. Over the last few decades, new methodologies have been developed to address this limitation and facilitate ionization of biological molecules. Electrospray ionization (ESI) is the most broadly used of these methods, as it facilitates the ionization of intact polar compounds from solution.Twenty years ago, our group reported a new ionization method that uses a charged solvent spray to impact a surface, generating ions from objects rather than just solutions and doing so directly in the ambient environment with no vacuum requirements and little to no sample preparation. This method was termed desorption electrospray ionization (DESI), and it initiated a new field that would come to be known as ambient mass spectrometry. The simplicity and wide applicability of the DESI technology─and the tens of ambient ionization methods developed subsequently─revolutionized the MS analysis of complex materials for their organic components, especially for in situ applications.This Account describes the history of DESI, starting with the development of the technique from early electrosonic spray ionization (ESSI) experimental observations as well as the studies leading to the understanding of its mechanism as a "droplet pick-up" phenomenon involving sequential events (i.e., thin film formation, solid-liquid extraction, secondary droplet generation, and ESI-like ionization from these droplets). We also overview the developments and applications of the technology that have been demonstrated by our group during the last two decades. In particular, we describe (i) the use of DESI for tissue imaging, one of its more significant applications to date, and its extension to intraoperative clinical diagnosis; (ii) the integration of the technology with portable instrumentation for in situ analysis, especially when coupled with tandem mass spectrometry (MS/MS); (iii) the use of DESI microdroplets as microvessels to accelerate organic reactions by orders of magnitude compared to those in bulk solution; and (iv) the combination of all these capabilities for automated high-throughput experiments aimed at accelerating drug discovery.

Dynamic core-electron-polarization effect on the high-order harmonic generation process from a quantum-trajectory perspective

It is argued that the dynamic core-electron polarization (DCEP) in polar molecules primarily affects harmonic processes at the ionization step only. This manifestation can also be understood in view of the strong-field assumption that the parent ion's potential becomes irrelevant when the electron is accelerated in the continuum-energy region. However, the scenario becomes vastly different, especially in the case of long-wavelength lasers as shown in this paper, where we demonstrate a complete physical picture of the DCEP on the harmonic process from asymmetric carbon monoxide (CO) molecules comprising the propagation step. To do so, we develop a visualization method for the harmonic process with and without DCEP based on Bohmian mechanics. As tracer particles evolving along quantum trajectories, Bohmian trajectories provide an intuitive picture of the ``harmonic process from the CO molecules. Remarkably, when the change of the harmonic intensity with respect to the DCEP inclusion cannot be explained by the instantaneous ionization rate, the Bohmian trajectories can attribute this change to the difference in the number of returning events and returning time of the electron after the propagation stage. By analyzing the dynamics of individual Bohmian trajectories (the acceleration and the time-frequency profile), we show that the DCEP alters nonlocally the innermost trajectories, which encode all the dynamics of the harmonic process. This insight into the DCEP effect necessitates a careful reinvestigation into other strong-field physics theories on the role of the target over the propagation stage.

Three-body recombination of ultracold barium plasma created by two-step photoionization of atoms through an excited 6s6p P11 level

The kinetics of ionization and recombination of an ultracold barium plasma created in a two-step process, taking into account the transfer of resonant radiation in 3D cylindrical geometry, is studied by numerical simulation. At the first step, a pump laser excites the upper level of the resonant transition 6s2 S10↔6s6p P11 (λ1=553.5 nm). At the second step, the laser with quantum energy exceeding the ionization potential from the level 6s6p P11 (λ2=417.79 nm) ionizes the atoms. A scheme is proposed for increasing the efficiency of electron yield: at the second ionization step, the laser radiation with frequency corresponding to the continuum from the metastable D32 is used. The electron temperature from the initial value 0.1 K during the action of the pump and ionizing lasers increases by more than 200 times due to superelastic processes. As a result, the time of three-body recombination of plasma increases significantly. The results of numerical simulation indirectly confirm the fact of Killian et al. [Phys. Rev. Lett. 83(23), 4776 (1999)] that the deceleration of recombination of ultracold xenon plasma can be explained by the heating of electrons in superelastic quenching collisions.

Electron Correlations in Sequential Two-Photon Double Ionization of an Ar Atom

Sequential two-photon ionization is a process that is experimentally accessible due to the use of new free-electron laser sources for excitation. For the prototypical rare Ar gas atoms, a photoelectron spectrum (PES) corresponding to the second step of the sequential two-photon double ionization (2PDIII) at a photon excitation energy of 65.3 eV was studied theoretically with a focus on the consequences of electron correlations in the considered process. The calculation predicts many intense lines at low photoelectron energies, which cannot be explained on the basis of a one-electron approximation. The processes that lead to the appearance of these lines include many-electron correlations, either in the first or second step of photoionization. A significant fraction of the intensity of the low-energy part of PES is associated with the Auger decay of the excited states formed at the second step of 2PDI. The shape of the low-energy part of the 2PDIII PES is expected to be dependent on both the energy of photon excitations and the flux of the exciting beam.

Three in one: Rational engineering of multifunctional MIL-101-based ionic catalysts for carbon dioxide-epoxide cycloaddition

CO2-epoxide cycloaddition represents one of most attractive approaches for chemical fixation and utilization of CO2, but highly efficient and fully heterogeneous green catalysis is still challenging. Ionic metal-organic frameworks (IMOFs) are nice porous matrices heterogenizing the essential free nucleophilic anions and meanwhile providing Lewis acids, but the catalytic efficiency has been limited for low anion content and lack of auxiliary catalytic sites. In this work, we report stepwise optimization of IMOF catalysts by integrating multiple catalytic sites into MIL-101. Various monocationic IMOFs with quaternary ammonium and bromide anions were prepared by facile alkyne-azide click chemistry. Different numbers of hydroxyl groups were introduced in this ionization step to optimize the activity. To introduce more anions, dicationic IMOFs were created by a second ionization step, N-quaternization of triazolyl to triazolium. By the stepwise ionization and functionalization, we demonstrated the synergic catalytic effects among different sites, the impacts of hydroxyl/bromide content, and the size effect of the cationic group. The optimal IMOFs are superior to previous ionic heterogeneous catalysts in absence of cocatalysts. In particular, the dicationic catalyst can quantitatively convert epichlorohydrin to the carbonate within 1 h at 1 MPa CO2 and 80 °C, the apparent turnover frequency being up to 478 h−1. The highly active and selective, fully heterogeneous and recyclable catalyst has the appeal for practical applications.

Increasing DESI-MS Ion Signal by Plasma Treatment.

Many studies are focused on using plasma in mass spectrometry as an ionization source or postionization method. In this study, the effect of plasma treatment in the sample preparation step of desorption electrospray ionization (DESI) has been investigated. The plasma treatment of polar samples, including morphine, codeine, captopril, theophylline, fructose, and amphiphilic compounds such as phosphatidylethanolamine (PE) in E. coli bacteria, as well as nonpolar compounds, including thebaine, papaverine, and noscapine, has been followed for ionization efficiency in DESI technique. An atmospheric-pressure glow discharge plasma (GDP) along with the electrospray ionization technique is examined. Plasma treatment before ambient ionization has a dramatic effect on polar and nonpolar sample signals in DESI-TOF mass spectrometry. The intensity of the mass spectrum shows an increase of 1.9-3.4 times for polar compounds, 2.1-2.5 times for nonpolar compounds, and 3.0 times for PE in E. coli bacteria (N = 4). Plasma is a source of reactive atoms, molecules, ions, radicals, and ultraviolet radiation. Plasma surface treatment before DESI analysis by energetic species through momentum/energy transfer yields higher energy surface molecules, leading to more/easier desorption. Under optimal treatment conditions, an improved ion signal intensity is observed without any fragmentation, decomposition, or chemical changes. Ion signals are increased possibly by both increased ionization through protonation of molecules and enhanced subsequent desorption during DESI analysis.

Nondipole effects in tunneling ionization by intense laser pulses

The limit of decreasing laser frequency cannot be considered independently from nondipole effects due to the increase in the laser-induced continuum electron speed in this limit. Therefore, in this work, tunneling ionization in the adiabatic limit is considered for an effective field that includes effects beyond the electric-dipole term to first order in $1/c$, with $c$ being the speed of light. The nondipole term describes the interaction resulting from the electric dipole-induced velocity of the electron and the magnetic-field component of the laser. The impact of this term on the ionization rate, tunnel exit point, momentum at the tunnel exit, and electron dynamics is discussed. In the appropriate limit, the results of a nondipole strong-field approximation approach and those of the strict adiabatic limit, where time and field strength are parameters, are discussed. The nondipole strong-field approximation approach is used to identify nonadiabatic modifications of the initial conditions. The results open up an avenue to include nondipole effects in the initial tunneling ionization step in semiclassical models of strong-field and attosecond physics.

Two mechanisms of H+/OH− ion generation in anion-exchange membrane systems with polybasic acid salt solutions

Electrodialysis are increasingly used in hybrid membrane technologies for the recovery and separation of weak polybasic acids from municipal and industrial wastewater and other solutions. It is known that the rate of generation of H+ and OH− ions at ion-exchange membrane boundaries during the treatment of such solutions is greater than that in the case of strong electrolytes, such as NaCl. One of the mechanisms of this generation, water splitting involving catalytic participation of functional groups, is relatively well understood. However, the second possible mechanism, where H+ ions are generated during acid dissociation at the depleted solution/anion-exchange membrane interface, is not sufficiently clear. Current-voltage characteristics (CVCs) of a Neosepta AMX anion-exchange membrane in 0.02 M NaH2PO4, NaH2Cit, KHT, and NaCl solutions are recorded. The effective transport number of H+ ion in the depleted boundary solution is measured to quantify the rate of the H+ ion generation. While the “water splitting” mechanism takes place in any kind of electrolytes, the “acid dissociation” mechanism is characteristic only for weak polybasic acids and other ampholytes, the electric charge of which depends on the pH. With that, the first mechanism occurs only at overlimiting currents, and the second occurs both in the underlimiting and overlimiting current modes. The dissociation of acid species at an AEM interface is caused by Donnan exclusion of protons as co-ions from the membrane. The rate of this reaction increases in the order KHT < NaH2PO4<NaH2Cit. This order is determined by the values of equilibrium acid dissociation constants pKai for the first and second ionization steps as well as by the dissociation rate constants for acid anions. The contribution of each of the H+/OH− ion generation mechanisms depends on the current density, more precisely, on the interval between the 1st, 2nd or 3rd limiting currents, recorded on the I–V characteristic. At relatively low currents, the contribution of “water splitting” in the studied weakly acidic systems is insignificant. At i > 2.6 ilimLev, there is a competition between both mechanisms of H+ ion generation. The obtained results give new insight into the understanding of the kinetics of H+/OH− ion generation and the impact of this process on the efficiency of electrodialysis of solutions containing polybasic acid species.

A Small Footprint and Robust Interface for Solid Phase Microextraction and Mass Spectrometry Based on Vibrating Sharp-Edge Spray Ionization.

Combining solid phase microextraction (SPME) and mass spectrometry (MS) analysis has become increasingly important to many bioanalytical, environmental, and forensic applications due to its simplicity, rapid analysis, and capability of reducing matrix effects for complex samples. To further promote the adoption of SPME-MS based analysis and expand its application scope calls for efficient and convenient interfaces that couple the SPME sample handling with the efficient analyte ionization for MS. Here, we report a novel interface that integrates both the desorption and the ionization steps in one device based on the capillary vibrating sharp-edge spray ionization (cVSSI) method. We demonstrated that the cVSSI is capable of nebulizing liquid samples in a pulled-tip glass capillary with a battery powered function generator. The cVSSI device allows the insertion of a SPME probe into the spray capillary for desorption and then direct nebulization of the desorption solvent in situ. With the integrated interface, we have demonstrated rapid MS analysis of drug compounds from serum samples. Quantitative determination of various drug compounds including metoprolol, pindolol, acebutolol, oxprenolol, capecitabine, and irinotecan was achieved with good linearity (R2 = 0.97-0.99) and limit of detection ranging from 0.25 to 0.59 ng/mL without using a high voltage source. Only 3.5 μL of desorption solvent and 3 min desorption time were needed for the present method. Overall, we demonstrated a portable SPME-MS interface featuring high sensitivity, short analysis time, small footprint, and low cost, which makes it an attractive method for many applications requiring sample cleanup including drug compound monitoring, environmental sample analysis, and forensic sample analysis.

Photoionization of Ne and Xe atoms induced by extreme ultraviolet photons

The interaction of extreme ultraviolet (XUV) photon with matter is a meaningful way to understand the electronic structure of microscopic particles. In this paper, the electron angular distributions of single ionization and double ionization of Ne and Xe atoms interacting with XUV photons are investigated by utilizing a reaction microscope. The &lt;i&gt;β&lt;/i&gt;-asymmetric parameters of 2p electrons of Ne atom, and 5p, 5s electrons of Xe atom combined with the reported experimental data are compared with those from different theoretical models. The result shows that the electron correlation effect can be ignored in the ionization of 2p electron of Ne atom. While the ionization of 5p electron of Xe atom is strongly influenced by the electron correlation effect, but not by the relativistic effect. These two effects play an important role in ionizing the 5s shell of Xe atom. In addition, this study finds that both direct double ionization and indirect double ionization exist simultaneously during the ionization of Xe atom, and gives the photoelectron angular distributions and the &lt;i&gt;β&lt;/i&gt;-asymmetric parameters of the first step and the second step of indirect double ionization.

Divergence and efficiency optimization in polarization-controlled two-color high-harmonic generation

Improving the brightness of high-harmonic generation (HHG) sources is one of the major goals for next-generation ultrafast, imaging and metrology applications in the extreme-ultraviolet spectrum. Previous research efforts have demonstrated a plethora of techniques to increase the conversion efficiency of HHG. However, few studies so far have addressed how to simultaneously minimize the divergence and improve focusability, which all contribute to an increased brightness of the source. Here, we investigate how to improve both photon yield and divergence, which is directly linked to focusability, when adding the second harmonic to the fundamental driving field. We study the effects of the relative polarization in two-color HHG and compare the results to a one-color configuration. In a perpendicular two-color field, the relative phase between the two colors can be used to suppress or enhance recombination of either the long or the short trajectories. This allows to exert control over the divergence of the harmonics. In a parallel two-color field, the ionization rate is modified through the two-color phase, which selects trajectories during the ionization step. This enhances the total yield. We elaborate on the underlying mechanisms for parallel, perpendicular, and intermediate polarization angles, and confirm our experimental observations with simulations.