Ionization Mechanism of Negative Ion-Direct Analysis in Real Time: A Comparative Study with Negative Ion-Atmospheric Pressure Photoionization

• Published in: Journal of the American Society for Mass Spectrometry Vol. 20; no. 1; pp. 42 - 50
• Main Authors: Song, Liguo, Dykstra, Andrew B., Yao, Huifang, Bartmess, John E.
• Format: Journal Article
• Language: English
• Published: New York Elsevier Inc 2009; Springer-Verlag; Elsevier; Springer Nature B.V
• Subjects: Acetone; Acetonitrile; Analytical Chemistry; Anisole; Atmospheric pressure; Beta decay; Bioinformatics; Biotechnology; Carboxylic acids; Chemistry; Chemistry and Materials Science; Chloroform; Cyclohexane; Dichloromethane; Dopants; Electron capture; Exact sciences and technology; Heptanes; Hexanes; Ionization; Ions; Mass spectrometry; Organic Chemistry; Perfluorocarbons; Phenols; Photoionization; Polarity; Proteomics; Reactivity and mechanisms; Real time; Solvents; Tetrahydrofuran; Toluene; Fullerene; Dissociative Electron Capture; Ionization Mechanism; Liquid Chromatography Solvent; Nitroaromatic Explosive; Solvent effect; Nitro compound; Atmospheric pressure; Organic perhalocompound; Fragmentation pattern; Aniline derivatives; Derivatization; Mechanism; Benzoic acid derivatives; Organic solvent; Photoionization; Carboxylic acid; Direct Analysis in Real Time; Fullerenes; Phenols; Explosives; Negative ion; Fluorine Organic compounds; Mass spectrometry; Comparative study; Aromatic amine
• ISSN: 1044-0305; 1879-1123
• DOI: 10.1016/j.jasms.2008.09.016

The ionization mechanism of negative ion-direct analysis in real time (NI-DART) has been investigated using over 42 compounds, including fullerenes, perfluorocarbons (PFC), organic explosives, phenols, pentafluorobenzyl (PFB) derivatized phenols, anilines, and carboxylic acids, which were previously studied by negative ion-atmospheric pressure photoionization (NI-APPI). NI-DART generated ionization products similar to NI-APPI, which led to four ionization mechanisms, including electron capture (EC), dissociative EC, proton transfer, and anion attachment. These four ionization mechanisms make both NI-DART and NI-APPI capable of ionizing a wider range of compounds than negative ion-atmospheric pressure chemical ionization (APCI) or negative ion-electrospray ionization (ESI). As the operation of NI-DART is much easier than that of NI-APPI and the gas-phase ion chemistry of NI-DART is more easily manipulated than that of NI-APPI, NI-DART can be therefore used to study in detail the ionization mechanism of LC/NI-APPI-MS, which would be a powerful methodology for the quantification of low-polarity compounds. Herein, one such application has been further demonstrated in the detection and identification of background ions from LC solvents and APPI dopants, including water, acetonitrile, chloroform, methylene chloride, methanol, 2-propanol, hexanes, heptane, cyclohexane, acetone, tetrahydrofuran (THF), 1,4-dioxane, toluene, and anisole. Possible reaction pathways leading to the formation of these background ions were further inferred. One of the conclusions from these experiments is that THF and 1,4-dioxane are inappropriate to be used as solvents and/or dopants for LC/NI-APPI-MS due to their high reactivity with source basic ions, leading to many reactant ions in the background. Similar to NI-APPI, EC, dissociative EC, proton transfer, and anion attachment were identified as being responsible for the ionization of over 42 compounds by NI-DART.