Investigation of reaction mechanisms by electrospray ionization mass spectrometry: characterization of intermediates in the degradation of phenol by a novel iron/magnetite/hydrogen peroxide heterogeneous oxidation system

Organic Reactions in Microdroplets: Reaction Acceleration Revealed by Mass Spectrometry

The striking finding that reaction acceleration occurs in confined-volume solutions sets up an apparent conundrum: Microdroplets formed by spray ionization can be used to monitor the course of bulk-phase reactions and also to accelerate reactions between the reagents in such a reaction. This Minireview introduces droplet and thin-film acceleration phenomena and summarizes recent methods applied to study accelerated reactions in confined-volume, high-surface-area solutions. Conditions that dictate either simple monitoring or acceleration are reconciled in the occurrence of discontinuous and complete desolvation as the endpoint of droplet evolution. The contrasting features of microdroplet and bulk-solution reactions are described together with possible mechanisms that drive reaction acceleration in microdroplets. Current applications of droplet microreactors are noted as is reaction acceleration in confined volumes and possible future scale-up.

Reactive porous composites for chromium(vi) reduction applications based on Fe/carbon obtained from post-consumer PET and iron oxide

By using post-consumer PET waste and iron oxide (Fe₂O₃), novel porous magnetic Fe/carbon composites for environmental applications have been obtained by a very simple process.

Electrospray ionization mass spectrometry (ESI-MS) monitoring of the photolysis of diazinon in aqueous solution: degradation route and toxicity of by-products against Artemia salina

The photolytic degradation of diazinon, an organophosphorus pesticide, in aqueous medium under assorted pH values was continuously monitored by direct infusion electrospray ionization mass spectrometry (ESI-MS). The results indicated that the UV radiation was quite efficient in promoting the pesticide degradation at the three pH levels evaluated (5, 7 and 8). The m/z of the most abundant ions observed in the mass spectra (MS), in conjunction with the fragmentation patterns of such ionic species (MS/MS data), made possible the proposition of chemical structures for the main by-products formed. As a result, routes for the photodegradation of diazinon in aqueous solution could thus be suggested. In the assays using Artemia salina (brine shrimp) it was verified that the photodegradation products exhibited much lower toxicity than the primary substrate. Aiming at mimicking the conditions ordinarily found in water treatment plants, an additional series of tests was conducted with a solution containing sodium hypochlorite and diazinon. This solution, when not exposed to UV radiation, exhibited high toxicity against the microorganisms. Under the influence of UV radiation, however, the toxicity rates decreased dramatically. This result is relevant because it points toward the confident application of UV radiation to neutralize the deleterious effects caused by diazinon (and perhaps other organophosphorus pesticides) as well as sodium hypochlorite to the environment.

The expanding role of electrospray ionization mass spectrometry for probing reactive intermediates in solution

Within the past decade, electrospray ionization mass spectrometry (ESI-MS) has rapidly occupied a prominent position for liquid-phase mechanistic studies due to its intrinsic advantages allowing for efficient "fishing" (rapid, sensitive, specific and simultaneous detection/identification) of multiple intermediates and products directly from a "real-world" solution. In this review we attempt to offer a comprehensive overview of the ESI-MS-based methodologies and strategies developed up to date to study reactive species in reaction solutions. A full description of general issues involved with probing reacting species from complex (bio)chemical reaction systems is briefly covered, including the potential sources of reactive intermediate (metabolite) generation, analytical aspects and challenges, basic rudiments of ESI-MS and the state-of-the-art technology. The main purpose of the present review is to highlight the utility of ESI-MS and its expanding role in probing reactive intermediates from various reactions in solution, with special focus on current progress in ESI-MS-based approaches for improving throughput, testing reality and real-time detection by using newly developed MS instruments and emerging ionization sources (such as ambient ESI techniques). In addition, the limitations of modern ESI-MS in detecting intermediates in organic reactions is also discussed.

Ozonation of the food dye Brilliant Blue in aqueous medium: monitoring and characterization of products by direct infusion electrospray ionization coupled to high-resolution mass spectrometry

RATIONALE

Dyes have been widely used to accentuate or to provide different colors to foods. However, the high concentrations of dyes in effluents from the food industries can cause serious and unpredictable damages to aquatic life in general. Furthermore, since conventional biological treatments have been shown to be ineffective, the use of advanced oxidation processes to promote the depletion of such dyes in water bodies has turned out to be mandatory.

METHODS

The degradation of the food dye Brilliant Blue by ozone in aqueous solution is reported herein. The overall process was monitored in real time by using direct infusion electrospray ionization high-resolution mass spectrometry in the negative ion mode, ESI(-)-HRMS.

RESULTS

Preliminary results (visual inspection and UV-vis spectra) showed the high efficiency of ozonation in causing the decoloration of an aqueous solution of the dye whereas TOC (total organic carbon) measurements revealed that such an oxidation process was unable to promote its complete mineralization. ESI(-)-HRMS data showed that the substrate consumption occurred concomitantly with the appearance of four by-products, all of them produced by an initial attack of hydroxyl radicals (generated via the decomposition of ozone) on the two imino moieties of the dye molecule. Structures were proposed for all the by-products based mainly on the high-resolution mass measurements and on the characteristic reactivity of typical functional groups towards hydroxyl radicals. An unprecedented degradation route of Brilliant Blue by ozone in aqueous solution could thus be proposed.

CONCLUSIONS

A greater ecotoxicity against Artemia salina was observed for the by-products than for the original dye. This indicates that the identification of by-products arising from oxidation treatments is of primary importance since such compounds can be more hazardous than the precursor itself.

Probing the mechanism and dynamic reversibility of trianglimine formation using real-time electrospray ionization time-of-flight mass spectrometry

RATIONALE

The [3+3]-cyclocondensation reactions of chiral (1R,2R)-1,2-diaminocyclohexane with aromatic or aliphatic bis-aldehydes to form trianglimine macrocycles were reported a decade ago and were believed to proceed through a stepwise mechanistic pathway; however, no intermediates were ever isolated or detected and characterized.

METHODS

We investigated the mechanism of the [3+3]-cyclocondensation reaction using a selection of dialdehyde starting materials using real-time electrospray ionization time-of-flight mass spectrometry.

RESULTS

We observed up to a maximum of 16 reaction intermediates along the reaction pathway, more than for any other multistep reaction reported. We also probed the dynamic reversibility of trianglimines using selected small dynamic combinatorial libraries and showed that trianglimine formation is indeed fully reversible.

CONCLUSIONS

This study represents a significant contribution towards understanding the mechanism of trianglimine formation and its potential applicability can be extended to include other cascade reactions.

Potential of modified iron-rich foundry waste for environmental applications: Fenton reaction and Cr(VI) reduction

A magnetic fraction (15%) from a waste of foundry sand (WFS), composed of sand, carbon, bentonite clay and iron (10%) was modified by thermal treatment at 400, 600 and 800°C under inert atmosphere. Mössbauer analyses showed that the thermal treatment increased the amount of Fe(3)O(4) from 25 to 55% by reduction of Fe(2)O(3) and highly dispersed Fe(3+) by the carbon present in the waste. The Fe(3)O(4) caused a significant increase on the activity of two important reactions with application in environmental remediation: the Fenton oxidation of indigo carmine dye with H(2)O(2) and the reduction of Cr(VI) to Cr(III). The magnetic fraction of WFS was also mixed with hematite (Fe(2)O(3)) and thermally treated at 400, 600 and 800°C. This treatment produced large amounts of surface Fe(3)O(4) and increased substantially the rate of Fenton reaction as well as Cr(VI) reduction. This reactivity combined with the presence of carbon (an adsorbent for organic contaminants), bentonite clay (an adsorbent for metallic contaminants) and the granulometry/packing/hydrodynamic features make WFS a promising material for use in reactive permeable barriers.

Radiolytic degradation mechanism of gallic acid and its end-products

Gallic acid is one of the most representative biorecalcitrant phenolic compounds present in cork processing wastewater. In this communication, chemical oxidation of gallic acid was studied by gamma irradiation as an advanced oxidation process. This technology turns out to be an advantageous tool for the degradation of gallic acid. The results obtained by UV-Vis and electrospray ionization mass spectrometry (ESI-MS) techniques are in agreement concerning the suitability of this technique to degrade gallic acid. ESI-MS and ESI-MS(2) monitoring of the non-irradiated and irradiated gallic acid solutions leads to the identification of the main intermediate products. Based on the overall results obtained a consistent mechanism of radiolytic degradation of gallic acid is proposed. The application of radiation as a tool to increase the biodegradability of wastewaters is an important issue from the perspective of Green Chemistry.

Controlled reduction of red mud waste to produce active systems for environmental applications: heterogeneous Fenton reaction and reduction of Cr(VI)

In this work, controlled reduction of red mud with H(2) was used to produce active systems for two different environmental applications, i.e. the heterogeneous Fenton reaction and the reduction of Cr(VI). Mössbauer, powder X-ray diffraction, thermal analyses and scanning electron microscopy analyses showed that at different temperatures, i.e. 300, 400, 500 and 600 degrees C, H(2) reduces red mud to different phases, mainly Fe(3)O(4), Fe(0)/Fe(3)O(4) and Fe(0). These Fe phases are dispersed on Al, Si and Ti oxides present in the red mud and show high reactivity towards two environmental applications, i.e. the heterogeneous Fenton reaction and the reduction of Cr(VI). Reduction with H(2) at 400 degrees C showed the best results for the oxidation of the model dye methylene blue with H(2)O(2) at neutral pH due to the presence of the composite Fe(0)/Fe(3)O(4). The reduced red mud at 500-600 degrees C produced Fe(0) highly active for the reduction of Cr(VI) in aqueous medium. Another feature of these red mud based system is that after deactivation due to extensive use they can be completely regenerated by simple treatment with H(2).

Degradation of the insecticides thiamethoxam and imidacloprid by zero-valent metals exposed to ultrasonic irradiation in water medium: electrospray ionization mass spectrometry monitoring

The degradation of thiamethoxam (1) and imidacloprid (2), prototype neonicotinoid insecticides bearing a characteristic N-NO2 moiety in their structures, promoted by a number of zero-valent metals (Fe, Sn, Zn) upon ultrasonic irradiation in acidic aqueous solution (pH 2) was investigated. It was verified that thiamethoxam (1) and imidacloprid (2) are quickly and almost completely consumed under these experimental conditions (degradation >90% after a reaction time of 30 min) and that ultrasonic irradiation strongly enhances the degradation rate for both insecticides, especially when zinc and tin are employed. Based on the results from electrospray ionization mass (and tandem mass) spectrometry in the positive ion mode, degradation routes for both insecticides, comprising an initial NO2 --> NH2 reduction, were proposed. In addition, products from the dehydrochlorination of imidacloprid were also found to be formed under these conditions.

Potential application of highly reactive Fe(0)/Fe3O4 composites for the reduction of Cr(VI) environmental contaminants

We describe the use of highly reactive Fe(0)/Fe3O4 composites for the reduction of Cr(VI) species in aqueous medium. The composites were prepared by simple mechanical alloying of metallic iron and magnetite in different proportions, i.e. Fe(0) 25, 50, 75 and 90wt%. While after 3h of reaction pure Fe(0) and pure Fe3O4 showed only a low reduction efficiency of 15% and 25% Cr(VI) conversion, respectively, the composites, in particular Fe(0)(25wt%)/Fe3O4, showed a remarkable activity with ca. 65% Cr(VI) conversion. Kinetic experiments showed a high reaction rate during the first 3h, which subsequently decreased strongly, probably due to a pH increase from 6 to 8. Experiments with composites based on Fe(0)/alpha-Fe2O3, Fe(0)/gamma-Fe2O3 and Fe(0)/FeOOH showed very low activities, suggesting that Fe(oct)2+ in the magnetite structure plays an important role in the reaction. Scanning and high resolution electron microscopies and Mössbauer spectra (transmission and conversion electron Mössbauer spectroscopy) indicated that the mechanical alloying process promotes a strong interaction and interface between the metallic and oxide phases, with the Fe(0) particles completely covered by Fe3O4 particles. The high efficiency of the composite Fe(0)/Fe3O4 for Cr(VI) reduction is discussed in terms of a special mechanism where an electron is transferred from Fe(0) to magnetite to reduce Fe(oct)3+ to Fe(oct)2+, which is active for Cr(VI) reduction.

Photolytic degradation of the insecticide thiamethoxam in aqueous medium monitored by direct infusion electrospray ionization mass spectrometry

Photodegradation of the insecticide thiamethoxam (1), 3-[(2-chloro-5-thiazolyl)methyl]tetrahydro-5-methyl-N-nitro-4H-1,3,5-oxadiazin-4-imine, in an aqueous medium was monitored by electrospray ionization mass spectrometry in the positive ion mode, ESI(+)-MS. An aqueous solution of (1) was incessantly exposed to a UV radiation source and aliquots were taken after reaction times of 1, 2, 3, and 4 h. Analysis by GC/NCI-MS revealed that (1) was continuously degraded under these experimental conditions. However, the total organic carbon (TOC) content remained practically constant during the exposition period, thereby indicating that 1 was not mineralized but continuously converted into other compounds. ESI(+)-MS monitoring revealed that whereas the intensity of the ions of m/z 292/294 ([1 + H](+)) constantly decreased, there was the emergence of other ions of m/z 247/249, 197, 168, and 116 whose intensities simultaneously increased. Their structures were proposed on the basis of: (1) the data of their ESI(+)-MS/MS; (2) their high resolution m/z values; and (3) a plausible reactivity of the thiamethoxam molecule exposed to UV radiation in aqueous solution. Finally, these data allowed us to suggest a reaction route for the photodegradation of 1 in an aqueous medium.

Monitoring the degradation of tetracycline by ozone in aqueous medium via atmospheric pressure ionization mass spectrometry

The degradation of tetracycline (1) by ozone in aqueous solution was investigated. High performance liquid chromatography (HPLC), UV-visible spectroscopy (UV-Vis), and total organic carbon (TOC) analyses revealed that although tetracycline was quickly consumed under this oxidative condition, it did not mineralize at all. Continuous monitoring by electrospray ionization mass spectrometry in the positive ion mode, ESI(+)-MS, revealed that tetracycline (1), detected in its protonated form ([1 + H]+) of m/z 445, reacted to yield almost exclusively two unprecedented oxidation products (2 and 3) via a net insertion of one and two oxygen atoms, respectively. Compound 2, suggested to be formed via an initial 1,3-dipolar cycloaddition of ozone at the C11a-C12 double-bond of 1, and Compound 3, proposed to be produced via a subsequent ozone attack at the C2-C3 double-bond of 2, were detected in their protonated forms in the ESI(+)-MS, i.e., [2 + H]+ of m/z 461 and [3 + H]+ of m/z 477, and were further characterized by ESI(+)-MS(n). LC-APCI(+)-MS (liquid chromatography coupled with atmospheric pressure chemical ionization mass spectrometry in the positive ion mode) experiments corroborated the results.

Electrospray ionization mass spectrometry: a major tool to investigate reaction mechanisms in both solution and the gas phase

Electrospray ionization mass spectrometry (ESI-MS), in conjunction with its tandem version ESI-MS/MS, is now established as a major tool to study reaction mechanisms in solution. This suitability results mainly from the ability of ESI to "fish" ions directly from solution to the gas phase environment of mass spectrometers. In this review, we summarize recent studies from our laboratory on the use of on-line monitoring by ESI-MS ion fishing of several types of reactions that permitted us to follow how these reactions progress as a function of both time and conditions using the ultra-high sensitivity and speed of ESI-MS to detect and even characterize transient reaction intermediates. We also show that the intrinsic reactivity of each key gaseous species fished by ESI can be further investigated via ESI-tandem mass spectrometry experiments, searching for the most active species via gas-phase ion/molecule reactions. In the gas-phase, solvent and counter-ion effects are absent. These studies often permit a detailed overview of major steps via the interception, characterization and reactivity investigation of key reaction players.