Significance of Competitive Reactions in an Atmospheric Pressure Chemical Ionization Ion Source: Effect of Solvent

Ionization behaviors of nitrotoluenes and dinitrotoluenes by reactions with acetone-related reactant ion

Dinitrotoluenes (DNTs) and nitrotoluenes (NTs) are found in the environment as metabolites of trinitrotoluene (TNT). When acetone is used as the solvent/eluent in atmospheric pressure chemical ionization-mass spectrometry (APCI-MS), the reactant ion is [2Acetone + O2 ]•- for the negative ion mode. The reactant ion reacts with an analyte to produce M•- and/or [M - H]- under atmospheric pressure. In this study, ionization behaviors of NT (2-, 3-, and 4-NTs) and DNT isomers (2,3-, 2,4-, and 2,6-DNTs) by reactions with [2Acetone + O2 ]•- were investigated. The energy-minimized structures of the product ions and their energies were calculated to explain the differences in the ionization behaviors. Typical NT- and DNT-related ions were produced by reactions with [2Acetone + O2 ]•- ; NT•- , [NT - H]- , DNT•- , [DNT - H]- , and [DNT - NO]- ions. The ionization efficiencies of NT- and DNT-related ions increased by increasing the source fragmentor voltage, and those of DNT-related ions were higher than those of the NT-related ions owing to the presence of an additional nitro group. The ionization efficiency of 3-NT•- was higher than that of [NT - H]- , while that of [DNT - H]- was higher than those of DNT•- and [DNT - NO]- . The ionization efficiency order of NT•- was 3-NT > 4-NT > 2-NT, while that of [DNT - H]- was 2,4-DNT > 2,6-DNT > 2,3-DNT. The [NT - H]- and [DNT - H]- ions were stabilized by resonance structures containing nitro groups. The [DNT - NO]- ions were formed through the transition state.

Formic Acid as a Dopant for Atmospheric Pressure Chemical Ionization for Negative Polarity of Ion Mobility Spectrometry and Mass Spectrometry

Formic acid (FA) is introduced as a potent dopant for atmospheric pressure chemical ionization (APCI) for ion mobility spectrometry (IMS) and mass spectrometry (MS). The mechanism of chemical ionization with the FA dopant was studied in the negative polarity using a corona discharge (CD)-IMS-MS technique in air. Standard reactant ions of the negative polarity present in air are O2-·(CO2)n·(H2O)m (m = 0, 1 and n = 1, 2) clusters. Introduction of the FA dopant resulted in the production of HCOO-·FA reactant ions. The effect of the FA dopant on the APCI of different classes of compounds was investigated, including plant hormones, pesticides, acidic drugs, and explosives. FA dopant APCI resulted in deprotonation and/or adduct ion formation, [M - H]- and [M + HCOO]-, respectively. Supporting density functional theory (DFT) calculations showed that the ionization mechanism depended on the gas-phase acidity of the compounds. FA dopant APCI led to the improvement of detection sensitivity, suppression of fragmentation, and changes in the ion mobilities of the analyte ions for analytes with suitable molecular structures and gas acidity.

Mechanism of formation and ion mobility separation of protomers and deprotomers of diaminobenzoic acids and aminophthalic acids

Aminobenzoic acids are well-established candidates for understanding the formation of isomeric ions in positive mode electrospray ionization as they yield both N- and O-protomers (prototropic isomers) at the amine and carbonyl sites, respectively. In the present work, a combination of ion mobility-mass spectrometry and density functional theory calculations to determine the protonation and deprotonation behaviour of four diamino benzoic acid and four aminophthalic acid isomers is presented. The additional COOH group on the ring of aminophthalic acids provides experimental evidence regarding the mechanism of intramolecular NH₃⁺ → O proton transfer, which has been the subject of debate in recent years. To determine the proton acceptor O atom, ion mobility spectra of the fragments of protomers were used as a new method for the confidential assignment of the O-protomer structure, confirming only short-distance intramolecular NH₃⁺ → O proton transfer. Additionally, the substitution pattern both influences the basicity of the protonation sites and enables these molecules to form internal hydrogen bonds with the protonated or deprotonated sites. The formation of the hydrogen bonds in the deprotonated aminophthalic acids changed the charge distribution and subsequently their ion mobility-derived collision cross sections in nitrogen (CCS_N2) leading to separation of the four isomers studied. Finally, an interesting effect of the substitution pattern was observed as a synergistic electron-donating effect of the amine groups of 3,5-diaminobenzoic acid on enhancing the basicity of the carbon atom C2 of the ring and previously unreported formation of a C-protomer within aminobenzoic acid systems.

Monitoring intramolecular proton transfer with ion mobility-mass spectrometry and in-source ion activation

Here, we show how intramolecular proton transfer can be induced and monitored with the example of polycyclic aromatic amines using in-source ion-activation and ion mobility-mass spectrometry. Experiment and DFT calculations reveal that the protonation rate of C-atoms in aromatic rings is controlled by the energy barrier of intramolecular NH₃⁺ → C proton transfer.