Oxidative Ionization Under Certain Negative-Ion Mass Spectrometric Conditions

Photo-activated Carbon dots (CDs) as Catalysts in the Knoevenagel Condensation: A Mechanistic Study by Dual-Mode Monitoring via ESI-MS

Carbon dots (CDs) obtained from 5-(hydroxymethyl)furfural (5-HMF) were activated by a 365 nm-UV irradiation source and employed in the Knoevenagel condensation to investigate their photocatalytic mechanism. To this end, electrospray ionization mass spectrometry (ESI-MS) was used to monitor the time progress of the condensation and follow the formation of the final product in positive and negative ion modes at once. The intervention of the superoxide radical anion in the photocatalytic mechanism of CDs was highlighted.

Collisional alignment and molecular rotation control the chemi-ionization of individual conformers of hydroquinone with metastable neon

The relationship between the shape of a molecule and its chemical reactivity is a central tenet in chemistry. However, the influence of the molecular geometry on reactivity can be subtle and result from several opposing effects. Here, using a crossed-molecular-beam experiment in which individual rotational quantum states of specific conformers of a molecule are separated, we study the chemi-ionization reaction of hydroquinone with metastable neon atoms. We show that collision-induced alignment of the reaction partners caused by geometry-dependent long-range forces influences reaction pathways, which is, however, countered by molecular rotation. The present work provides insights into the conformation-specific stereodynamics of complex polyatomic systems and illustrates the capability of advanced molecule-control techniques to unravel these effects.

Miniaturized flexible micro-tube plasma ionization source for the effective ionization of non-easily ionizable pesticides in food with liquid chromatography/mass spectrometry

In this article, we have studied the potential of flexible microtube plasma (FμTP) as ionization source for the liquid chromatography high-resolution mass spectrometry detection of non-easily ionizable pesticides (viz. nonpolar and non-ionizable by acid/basic moieties). Phthalimide-related compounds such as dicofol, dinocap, o-phenylphenol, captan, captafol, folpet and their metabolites were studied. Dielectric barrier discharge ionization (DBDI) was examined using two electrode configurations, including the miniaturized one based on a single high-voltage (HV) electrode and a virtual ground electrode configuration (FμTP), and also the two-ring electrode DBDI configuration. Different ionization pathways were observed to ionize these challenging, non-easily ionizable nonpolar compounds, involving nucleophilic substitutions and proton abstraction, with subtle differences in the spectra obtained compared with APCI. An average sensitivity increase of 5-fold was attained compared with the standard APCI source. In addition, more tolerance with matrix effects was observed in both DBDI sources. The importance of the data reported is not just limited to the sensitivity enhancement compared to APCI, but, more notably, to the ability to effectively ionize nonpolar, late-eluting (in reverse-phase chromatography) non-ionizable compounds. Besides o-phenylphenol ([M - H]-), all the parent species were efficiently ionized through different mechanisms involving bond cleavages through the effect of plasma reagent species or its combination with thermal degradation and subsequent ionization. This tool can be used to figure out overlooked nonpolar compounds in different environmental samples of societal interest through non-target screening (NTS) strategies.

Metabolization of Free Oxidized Aromatic Amino Acids by Saccharomyces cerevisiae

The aromatic amino acids tryptophan, phenylalanine, and tyrosine are targets for oxidation during food processing. We investigated whether S. cerevisiae can use nonproteinogenic aromatic amino acids as substrates for degradation via the Ehrlich pathway. The metabolic fate of seven amino acids (p-, o-, m-tyrosine, 3,4-dihydroxyphenylalanine (DOPA), 3-nitrotyrosine, 3-chlorotyrosine, and dityrosine) in the presence of S. cerevisiae was assessed. All investigated amino acids except dityrosine were metabolized by yeast. The amino acids 3-nitrotyrosine and o-tyrosine were removed from the medium as fast as p-tyrosine, and m-tyrosine, 3-chlorotyrosine, and DOPA more slowly. In summary, 11 metabolites were identified by high-performance liquid chromatography-mass spectrometry (HPLC-MS/MS). DOPA, 3-nitrotyrosine, and p-tyrosine were metabolized predominantly to the Ehrlich alcohols, whereas o-tyrosine and m-tyrosine were metabolized predominantly to α-hydroxy acids. Our results indicate that nonproteinogenic aromatic amino acids can be taken up and transaminated by S. cerevisiae quite effectively but that decarboxylation and reduction to Ehrlich alcohols as the final metabolites is hampered by hydroxyl groups in the o- or m-positions of the phenyl ring. The data on amino acid metabolism were substantiated by the analysis of five commercial beer samples, which revealed the presence of hydroxytyrosol (ca. 0.01-0.1 mg/L) in beer for the first time.

A Caged In-Source Laser-Cleavable MALDI Matrix with High Vacuum Stability for Extended MALDI-MS Imaging

Insufficient vacuum stability of matrix chemicals is a major limitation in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) of large tissue sample cohorts. Here, we designed and synthesized the photo-cleavable caged molecule 4,5-dimethoxy-2-nitrobenzyl-2,5-dihydroxyacetophenone (DMNB-2,5-DHAP) and employed it for lipid MALDI-MSI of mouse brain tissue sections. DMNB-2,5-DHAP is vacuum-stable in a high vacuum MALDI ion source for at least 72 h. Investigation of the uncaging process suggested that the built-in laser (355 nm) in the MALDI ion source promoted the in situ generation of 2,5-DHAP. A caging group is used for the first time in designing a MALDI matrix that is vacuum-stable, uncaged upon laser irradiation during the measurement process, and that boosts lipid ion intensity with MALDI-2 laser-induced postionization.

Evidence of Gas-Phase Attachment of Molecular Oxygen to Deprotonated Hydroquinone During Ion-Mobility Mass Spectrometry

Gas-phase addition of dioxygen to certain ions is a well-known phenomenon in mass spectrometry. For this reaction to occur, the presence of a distonic radical site on the precursor ion is thought to be a prerequisite. Herein, we report that oxygen adduct formation can take place also with deprotonated hydroquinone, which in fact is an even-electron species without a radical site. When the product-ion spectrum of the m/z 109 ion, generated by electrospray ionization from a solution of hydroquinone in acetonitrile, was recorded under ion-mobility conditions, a new peak was observed at m/z 141. However, an analogous peak was not visible in the spectrum acquired under nonmobility conditions (i.e., without any gas introduced to the mobility cell). Presumably, traces of oxygen present in the collision gas instigate an ion-molecule reaction to produce an adduct of m/z 141, which upon activation results in CO and H₂O loss to form a product ion of m/z 95. Isotope-labeling studies confirmed that one of the hydrogen atoms from the hydroxy group and another from the aromatic ring contribute to the water loss instigated from the m/z 141 adduct. Furthermore, computational methods indicated the three-dimensional structure of the ground-state deprotonated hydroquinone to be distinctly different from those of its 1,2- and 1,3-isomers. Calculations predicted that all atoms in the two m/z 109 ions generated from catechol and resorcinol lie on one plane. In contrast, the structure of the m/z 109 ion from hydroquinone was significantly different. Computations predicted that the hydrogen atom on the intact hydroxyl group of deprotonated hydroquinone protrudes out of plane from rest of the atoms. Consequently, the exposed OH group can interact with an incoming dioxygen molecule. Computations conducted at the CAM-B3LYP/6-311++g(2d,2p) level of theory detected a minimum energy crossing point (MECP) at -4.3 kJ mol^-1 below the separated O₂ + deprotonated hydroquinone triplet threshold. In contrast, similar calculations conducted for catechol and resorcinol yielded MECPs of +116.9 and +69.1 kJ mol^-1, respectively, above the associated triplet thresholds. These results indicated that the curve crossing required to form singlet products upon reaction with triplet O₂ is favorable in the case of hydroquinone and unfavorable in the cases of catechol and resorcinol. In practical terms, the selective oxygen addition appears to be a diagnostically useful reaction to differentiate hydroquinone from its ring isomers.

Frozen Hydrogen Peroxide and Nitrite Solution: The Acceleration of Benzoic Acid Oxidation via the Decreased pH in Ice

We investigated benzoic acid oxidation via the reaction of hydrogen peroxide (H₂O₂) and nitrite (NO₂^-). The oxidation of benzoic acid by reactive nitrous acid (HONO) was negligible, and the reactivity of the H₂O₂/NO₂^- system decreased with a decrease in temperature under aqueous conditions. However, freezing markedly accelerated the chemical reaction. Based on Raman microscope measurements, concentrated species were confirmed in certain regions of the ice. We proposed that the change in nitrite speciation (accordingly, a decrease in the pH below pK_a), derived from the freezing concentration effect, was the reason for the accelerated reactions. The oxidation characteristics of the system were monitored under varying conditions, such as initial pH, dosage ratio, benzoic acid concentration, and reaction with various benzene derivatives. The ultrahigh-performance liquid chromatography/electrospray ionization/mass spectrometry (UHPLC/ESI/MS) measurement showed that peroxynitrous acid (HOONO)-mediated oxidation generated hydroxylated and nitrated byproducts. Additionally, decarboxylated products were detected, indicating direct electron transfer from the organic compounds to HOONO. As freezing is a global phenomenon, and H₂O₂ and NO₂^- are ubiquitous in the environment, the transformation of aromatic compounds with H₂O₂/NO₂^- in cold environments must be considered in environmental chemistry.

Helium-Plasma-Ionization Mass Spectrometry of Metallocenes and Their Derivatives

Ferrocene and its derivatives and nickelocene undergo facile ionization when exposed directly to the ionizing plasma of a helium-plasma ionization (HePI) source. Mass spectra recorded from such samples under ambient positive-ion-generating conditions show intense peaks for the respective molecular ions [M^+•] and protonated species [(M + H)⁺]. The protonation process occurs most efficiently when traces of water are present in the heated nitrogen used as the "heating gas." In fact, the relative population of the two categories of ions generated in this way can be manipulated by regulating the heating-gas flow. Moreover, rapid and highly efficient gas-phase hydrogen-deuterium exchange (HDX) reactions can be performed in the ion source by passing the heating gas through a vial with D₂O before it reaches the HePI source. Moreover, the ionized species generated in this way can be subjected to in-source CID fragmentation in the QDa-HePI source very efficiently by varying the sampling-cone voltage. By this procedure, ions generated from ferrocene and nickelocene could be stripped so far as to ultimately generate the bare-metal cation. Other typical fragment-ions produced from protonated metallocenes included the M(cp)₁⁺ ions (M = Fe or Ni), by elimination of a cyclopentadiene molecule, or the molecular cation, by loss of a H^• radical. Moreover, H/D exchanges and subsequent tandem mass spectrometric analysis indicated that the central metal core participates in the initial protonation process of ferrocene under HePI conditions. However, in compounds such as ferrocene carboxaldehyde and ferrocene boronic acid, the protonation takes place at the peripheral functional group.

Atmospheric pressure chemical ionization mass spectrometry at low flow rates: Importance of ion source housing

RATIONALE

Hyphenation of atmospheric pressure chemical ionization (APCI) mass spectrometry with capillary and micro high-performance liquid chromatography (HPLC) is attractive for many applications, but reliable ion sources dedicated to these conditions are still missing. There are a number of aspects to consider when designing such an ion source, including the susceptibility of the ionization processes to ambient conditions. Here we discuss the importance of ion source housing for APCI at low flow rates.

METHODS

Selected compounds dissolved in various solvents were used to study ionization reactions at 10 μL/min flow rate. APCI spectra were generated using the Ion Max-S source (Thermo Fisher Scientific) operated with or without the ion source housing.

RESULTS

The APCI spectra of most compounds measured in the open and enclosed ion sources were markedly different. The differences were explained by water and oxygen molecules that entered the plasma region of the open ion source. Water tended to suppress charge transfer processes while oxygen diminished electron capture reactions and prevented the formation of acetonitrile-related radical cations useful for localizing double bonds in lipids. The effects associated with the ion source housing were significantly less important for compounds that are easy to protonate or deprotonate.

CONCLUSIONS

The use of ion source housing prevented alternative ionization channels leading to unwanted or unexpected ions. Compared with the conventional flow rate mode (1 mL/min), the effects of ambient air components were significantly higher at 10 μL/min, emphasizing the need for ion source housing in APCI sources dedicated to low flow rates.

Screening freshness of seafood by measuring trimethylamine (TMA) levels using helium-plasma ionization mass spectrometry (HePI-MS)

Background

Trimethylamine (TMA) is a marker used for monitoring the quality of seafood because it is the primary component of the “fishy” odor.

Methods

The levels of TMA in seafood samples were directly measured by helium-plasma ionization mass spectrometry (HePI-MS). Each sample was directly exposed to the HePI source, and the intensity of the m/z 60 signal for protonated TMA was monitored by a selected-ion-recording (SIR) protocol. Using a set of TMA-spiked water standards, the TMA levels in seafood samples were quantified.

Results

The signal intensity of the m/z 60 ion from shrimp samples maintained at room temperature for 2 days can be attenuated to baseline levels by adding lime juice. The amounts of TMA in samples of salmon and shrimp recovered from some sushi preparations, and in squid samples, were found to be 0.24 μg, 0.16 μg, and 17.2 μg per gram, respectively.

Conclusions

HePI-MS is an efficient technique to screen and monitor the TMA content and assess the quality of seafood.

1,4-Benzoquinone antimicrobial agents against Staphylococcus aureus and Mycobacterium tuberculosis derived from scorpion venom

Two 1,4-benzoquinone derivatives, found in the venom of the scorpion Diplocentrus melici following exposure to air, have been isolated, characterized, synthesized, and assessed for antimicrobial activities. Initially a white, viscous liquid, the extracted venom colors within minutes under ambient conditions. From this colored mixture, two compounds, one red, the other blue, were isolated and purified using chromatography. After a variety of NMR and mass spectrometry experiments, the red compound was determined to be 3,5- dimethoxy-2-(methylthio)cyclohexa-2,5-diene-1,4-dione, and the blue compound was determined to be 5-methoxy-2,3- bis(methylthio)cyclohexa-2,5-diene-1,4-dione. Because extremely small amounts of these compounds were isolated from the scorpion venom, we developed laboratory syntheses from commercially available precursors, allowing us to produce sufficient quantities for crystallization and biological assays. The red benzoquinone is effective against Staphylococcus aureus [minimum inhibitory concentration (MIC) = 4 µg/mL], while the blue benzoquinone is active against Mycobacterium tuberculosis (MIC = 4 µg/mL) and even against a multidrug-resistant (MDR) strain with nearly equal effectiveness. The bactericidal effects of both benzoquinones show comparable activity to commercially available antibiotics used against these pathogens and were cytotoxic to neoplastic cell lines, suggesting their potential as lead compounds for the development of novel antimicrobial and anticancer drugs. Importantly, the blue benzoquinone was also effective in vivo with mouse models of MDR tuberculosis infection. After treatment for 2 mo, four mice with late-stage active MDR tuberculosis had a significant decrease in pulmonary bacillary loads and tissue damage. Healthy mice served as negative controls and tolerated treatment well, without adverse side effects.

Fast Discrimination of Chocolate Quality Based on Average-Mass-Spectra Fingerprints of Cocoa Polyphenols

This work aims to sort cocoa beans according to chocolate sensory quality and phenolic composition. Prior to the study, cocoa samples were processed into chocolate in a standard manner, and then the chocolate was characterized by sensory analysis, allowing sorting of the samples into four sensory groups. Two objectives were set: first to use average mass spectra as quick cocoa-polyphenol-extract fingerprints and second to use those fingerprints and chemometrics to select the molecules that discriminate chocolate sensory groups. Sixteen cocoa polyphenol extracts were analyzed by liquid chromatography-low-resolution mass spectrometry. Averaging each mass spectrum provided polyphenolic fingerprints, which were combined into a matrix and processed with chemometrics to select the most meaningful molecules for discrimination of the chocolate sensory groups. Forty-four additional cocoa samples were used to validate the previous results. The fingerprinting method proved to be quick and efficient, and the chemometrics highlighted 29 m/ z signals of known and unknown molecules, mainly flavan-3-ols, enabling sensory-group discrimination.

Gas phase reaction between chromones and solvent in an electrospray ionization source

Chromones were measured by using electrospray ionization mass spectrometry in negative mode. Interestingly, in addition to the deprotonated ion ([M - H]^- ), unexpected [M + 17]^- and [M + 31]^- ions were observed in high intensity when water and methanol were used as the solvent. Chromones with different substitutes were tested. Compared with the deprotonated ion, [M + 17]^- and [M + 31]^- ions were observed with higher abundances when the C-3 site of chromones was substituted by electron withdrawing groups. Based on high performance liquid chromatography-mass spectrometry (LC-MS), deuterium-labeling and collisional-induced dissociation experiments, a covalent gas-phase nucleophilic addition reaction between chromone and water, and the formation of a noncovalent complex between chromone and methanol were proposed as the mechanism for the observed [M + 17]^- and [M + 31]^- ions, respectively. Understanding and using these unique gas phase reactions can avoid misannotation when analyzing chromones and their metabolites.

Unexpected Reduction of Iminoquinone and Quinone Derivatives in Positive Electrospray Ionization Mass Spectrometry and Possible Mechanism Exploration

Unexpected reduction of iminoquinone (IQ) and quinone derivatives was first reported during positive electrospray ionization mass spectrometry. Upon increasing spray voltage, the intensities of IQ and quinone derivatives decreased drastically, accompanying the increase of the intensities of the reduction products, amodiaquine (AQ) and phenol derivatives. To gain more insight into the mechanism of such reduction, we explored the experimental factors that are influential to corona discharge (CD). The results show that experimental parameters that favor severe CD, including metal spray emitter, using water as spray solvent, sheath gas with low dielectric strength (e.g., nitrogen), and shorter spray tip-to-mass spectrometer inlet distance, facilitated the reduction of IQ and quinone derivatives, implying that the reduction should be closely related to CD in the gas phase. Graphical Abstract ᅟ.

Corona discharge-induced reduction of quinones in negative electrospray ionization mass spectrometry

Quinone reduction during negative ESI MS was illustrated to be closely related to corona discharge (CD).