A Radical-Mediated Pathway for the Formation of [M + H]⁺ in Dielectric Barrier Discharge Ionization

Abstract

Abstract. Active capillary plasma ionization is a highly efficient ambient ionization method. Its general principle of ion formation is closely related to atmospheric pressure chemical ionization (APCI). The method is based on dielectric barrier discharge ionization (DBDI), and can be constructed in the form of a direct flowthrough interface to a mass spectrometer. Protonated species ([M + H] + ) are predominantly formed, although in some cases radical cations are also observed. We investigated the underlying ionization mechanisms and reaction pathways for the formation of protonated analyte ([M + H] + ). We found that ionization occurs in the presence and in the absence of water vapor. Therefore, the mechanism cannot exclusively rely on hydronium clusters, as generally accepted for APCI. Based on isotope labeling experiments, protons were shown to originate from various solvents (other than water) and, to a minor extent, from gaseous impurities and/or self-protonation. By using CO 2 instead of air or N 2 as plasma gas, additional species like [M + OH] + and [M − H] + were observed. These gas-phase reaction products of CO 2 with the analyte (tertiary amines) indicate the presence of a radical-mediated ionization pathway, which proceeds by direct reaction of the ionized plasma gas with the analyte. The proposed reaction pathway is supported with density functional theory (DFT) calculations. These findings add a new ionization pathway leading to the protonated species to those currently known for APCI.

Mechanistic Understanding Leads to Increased Ionization Efficiency and Selectivity in Dielectric Barrier Discharge Ionization Mass Spectrometry: A Case Study with Perfluorinated Compounds

“…Additionally, the linear perfluorinated alkanes were measured using nitrogen which was humidified with isotope labeled water (H 2 18 O, 3.5 ppm g/g) as discharge gas. It is well known that water plays important roles in the ionization mechanism in positive polarity, such as acting as a proton source and in decreasing of fragmentation 11 . However, the mass spectrum (Figure 5b ions, implying that water is also a source for oxygen in the substitution reaction.…”

“…The processes leading to analyte ions are complicated and affected by many parameters such as the proton affinity and ionization energy of the analyte and also the polarity of the discharge gas. In positive ion mode, protonated analyte [M+H] + and/or the radical cation [M] + are predominantly formed, either through water clusters 10 or via a radical mediated pathway 11 . The selectivity for the generation of these two ions as well as the overall ionization efficiency can be influenced by tuning the plasma operating parameters, as shown by Badal et al 12 for the case of flowing atmospheric pressure afterglow (FAPA) ionization.…”

High-throughput screening of PAHs and polar trace contaminants in water matrices by direct solid-phase microextraction coupled to a dielectric barrier discharge ionization source

“…The current limited applicability to non-polar analytes, is mainly due to poor ionization efficiency, caused by the intrinsic mechanisms governing these techniques: the ionization in most spraybased sources is a solution process based on charged droplets, forming [M+H] + ions, and the poor ionization efficiency for non-polar compounds is not surprising. In plasma based techniques (e.g., APCI, DART, and the low temperature plasma probe, LTP) direct ionization by charge transfer is possible, but the main ionization pathway nonetheless is believed to rely on hydronium clusters formed by interactions with excited or ionized gas molecules [8,27,28]. The ionization efficiency thus also largely depends on proton transfer reactions and tends to favor semi-polar to polar compounds.…”

“…Novel understanding gained from an active capillary plasma ionization source, however, suggests that a radical pathway exists (and depending on experimental conditions could be dominant) in addition to the traditionally accepted hydronium cluster ionization mechanism [27], a significant difference to spray-based techniques. This opens up the possibility to modify certain conditions, such as for example the addition of solvents as dopants, to boost this alternative ionization pathway and subsequently increase the efficiency for nonpolar analytes.…”

Solid-Phase Microextraction Coupled to Capillary Atmospheric Pressure Photoionization-Mass Spectrometry for Direct Analysis of Polar and Nonpolar Compounds

“…In our setup, the softness of the ionization was also due to the very short residence time of the compounds inside the reactive plasma. In our experiments, nitrogen was used to ignite the plasma, although regular air or CO2 have also been shown to be suitable gases for this source 41 .…”