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

JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY (2009)

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JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY

卷 20, 期 1, 页码 42-50

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ELSEVIER SCIENCE INC

DOI: 10.1016/j.jasms.2008.09.016

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Biochemical Research Methods Chemistry, Analytical Chemistry, Physical Spectroscopy

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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 (EST). 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. (J Am Soc Mass Spectrom 2009, 20, 42-50) (C) 2009 Published by Elsevier Inc. oil behalf of American Society for Mass Spectrometry

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Manipulation of Ion Conversion in Dichloromethane-Enhanced Vacuum Ultraviolet Photoionization Mass Spectrometry of Oxygenated Volatile Organic Compounds

Yi Yu, Jichun Jiang, Lei Hua, Yiqian Xu, Chuang Chen, Yi Chen, Haiyang Li

Summary: The ion conversion processes in CH2Cl2-enhanced vacuum ultraviolet photoionization of oxygenated volatile organic compounds (OVOCs) were studied by regulating pressure, humidity, and reaction time in the ionization source. The main characteristic ions changed with varying pressure, and interconversion between ions through ion-molecule reactions was observed with increasing humidity. The intensity of protonated methanol/ethanol ion was linearly correlated with the intensity of hydrated hydronium ions, indicating a proton transfer reaction mechanism. The reaction time was regulated by the electric field strength, and step-by-step ion-molecule reactions were observed. This study provides insights into the formation mechanism of intermediate product ions and hydrated hydronium ions in dopant-enhanced VUV photoionization and helps optimize experimental conditions for enhancing the sensitivity of OVOCs.

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