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The Maillard reaction kinetics in the confined volume of the thin film produced by ESI microdroplet deposition was studied by mass spectrometry. The almost exclusive formation of the Amadori product from the reaction of D-xylose and D-glucose toward L-glycine and L-lysine was demonstrated. The thin film Maillard reaction occurred at a mild synthetic condition under which the same process in solution was not observed. The comparison of the thin film kinetics with that of the reaction performed in solution showed strong thin film rate acceleration factors.

The peroxymonocarbonate anion, HCO4−, the covalent adduct between the carbon dioxide and hydrogen peroxide anion, effectively reacts with SO2 in the gas phase following three oxidative routes. Mass spectrometric and electronic structure calculations show that sulphur dioxide is oxidised through a common intermediate to the hydrogen sulphate anion, sulphur trioxide, and sulphur trioxide anion as primary products through formal HO2−, oxygen atom, and oxygen ion transfers. The hydrogen sulphite anion is also formed as a secondary product from the oxygen atom transfer path. The uncommon nucleophilic behaviour of HCO4− is disclosed by the Lewis acidic properties of SO2, an amphiphilic molecule that forms intermediates with characteristic and diagnostic geometries with peroxymonocarbonate.

The reactivity of [NaL·ClO2]− cluster anions (L = ClOx−; x = 0–3) with sulphur dioxide has been investigated in the gas phase by ion–molecule reaction experiments (IMR) performed in an in-house modified Ion Trap mass spectrometer (IT-MS). The kinetic analysis revealed that SO2 is efficiently oxidised by oxygen-atom (OAT), oxygen-ion (OIT) and double oxygen transfer (DOT) reactions. The main difference from the previously investigated free reactive ClO2− is the occurrence of intracluster OIT and DOT processes, which are mediated by the different ligands of the chlorite anion. This gas-phase study highlights the importance of studying the intrinsic properties of simple reacting species, with the aim of elucidating the elementary steps of complex processes occurring in solution, such as the oxidation of sulphur dioxide.

The capability to deposit intact highly pure ionic species makes ion soft landing an advantageous mass spectrometric method able to create molecular layers on solid surfaces. Recent advances in this technique gave the possibility to manipulate solvent-free ions in open-air conditions. The high ionic current produced by the new sources as well as the developments of accelerated systems dramatically increase the efficiency of the soft landing process, opening new opportunities for surface modification with self-organized polyatomic structures. This review reports the developments of the ambient ion soft landing, highlights its most recent applications and discusses future perspectives.

The gas-phase ion chemistry of protonated 5-hydroxymethyl-2-furaldehyde (5-HMF), the main product of hexose sugar dehydration reaction, was investigated by means of mass spectrometric and theoretical calculations. Calculations at the B3LYP-D3/6-311++G** and the G3 (MP2, CCSD(T)) levels of theory indicated the carbonyl oxygen as the 5-HMF preferred protonation site. The Cooks’ “extended” kinetic method was used to measure the unknown proton affinity (PA) and gas-phase basicity (GB) of 5-HMF. The experimentally measured values, PA= 207.3 kcal mol−1 and GB = 200.0 kcal mol−1, were found to be in good agreement with those calculated by means of G3 (MP2, CCSD(T)) composite methods. Among the seven different 5-HMF optimized protomers, the theoretically predicted potential energy surface of [5-HMF]H+ ions indicated that the carbonyl oxygen protonated most stable species, as well as each one of the four ring-protonated less stable isomers, were trapped in deep potential wells and, at least in principle, could be formed from the gas phase acid-catalyzed hexose sugar decomposition.

In this study, the ability of an unmodified ESI Z-spray source to carry out ambient ion soft landing (AISL) and microdroplet reactions (MR) was tested. To this aim, heme proteins were ionized and landed onto the working area of multi-walled carbon nanotubes electrodes. The voltammetric and spectroscopic analyses of the modified electrodes evidenced a strong interaction of unfolded structures of the heme proteins with the carbon nanotubes surface. Moreover, the possibility to use the ESI Z-spray source to study microdroplet reactions was attempted by exploring the accelerated acid-catalyzed decomposition of carbohydrates. The study demonstrated that a commercial ESI Z-spray source can provide successful modifications of surfaces by AISL, and represents a suitable platform for the study of accelerated microdroplet reactions.

The reactivity of transition metal dioxide anions of cobalt, nickel, copper and zinc with sulfur dioxide has been investigated in the gas phase by mass spectrometric techniques and theoretical calculations. Dioxide anions of the late transition metals Cu and Zn promote an unprecedented fast and efficient oxidation to the sulfate radical anion through a double oxygen transfer, whereas the earlier transition metals Ni and Co only undergo the addition of two SO2 molecules. The ligation of a water molecule to ZnO2 − switches the reaction path to a single oxygen anion transfer, forming SO3 −.

Unprecedented redox reactions of chromium oxide anions have been observed in the gas phase and investigated by mass spectrometric techniques and electronic structure calculations. The effective oxidizing and reducing power of chromium oxide anions is unravelled by sulfur dioxide: CrO4 − and CrO2 − generate CrO3 − as the reduction and oxidation product, respectively, together with SO3 and SO as the oxidation and reduction products of SO2, respectively. Computations predict that the reactions of CrO4 − and CrO2 − proceed through rather stable ion-dipole complexes and that CrO4 - produces also a strongly bound [CrSO6] - species.

Furan derivatives, potentially carcinogenic to humans, can be formed, in addition to carbohydrates and other sources, from the degradation of ascorbic acid (AA). At present, the mechanisms involved in the ascorbic acid degradation are not yet fully understood. In this study, we reported a gas-phase investigation, performed using Triple Quadrupole (TQ/MS) and Ion Trap Mass Spectrometry (QIT/MS) together with quantum mechanical calculations at the B3LYP/6-31+G(d,p) level of theory, on the non-oxidative degradation mechanism of l-ascorbic acid (AA) to furan derivatives. Gaseous protonated ascorbic acid ions, the AAH+ ionic reagents, were generated by Electrospray Ionization (ESI) of an aqueous AA solution added with a mild protonation reagent. The Collisionally Induced Dissociation (CID) mass spectra of the AAH+ ions displayed gaseous fragment ions corresponding to the degradation intermediates and reaction products, which were structurally characterized and identified. Precise mechanistic insights were achieved by using l-[1-13C]-AA. On the basis of experimental and computational results, the gas phase non-oxidative acid catalyzed degradation mechanism of ascorbic acid is proposed, a benchmark point for the comprehension of the mechanism in the condensed phase.

The oxidation of SO2 by nonmetal oxoanions in the gas phase is investigated in an experimental and theoretical study of the structure of the species involved and the reaction kinetics and mechanism. SO3 , SO3(.-) and SO4(.-) are efficiently produced by reaction of On XO(-) anions (X=Cl, Br, and I; n=0 and 1) with SO2 ; XO(-) ions mainly react to give SO3 by oxygen-atom transfer, whereas OXO(-) ions mainly give SO3(.-) by oxygen-anion transfer. On descending the halogen group from chlorine to iodine, the SO3 /SO3(.-) ratio decreases and increases for reactions involving XO(-) and OXO(-) anions, respectively, whereas the formation of SO4(.-) is particularly significant with OIO(-). Kinetic factors play a major role in the reactions of On XO(-), depending on the halogen atom and its oxidation state.