Generation of ion-bound solvent clusters as reactant ions in dopant-assisted APPI and APLI

Abstract

We provide experimental and theoretical evidence that the primary ionization process in the dopant-assisted varieties of the atmospheric pressure ionization methods atmospheric pressure photoionization and atmospheric pressure laser ionization in typical liquid chromatography-mass spectrometry settings is--as suggested in the literature--dopant radical cation formation. However, instead of direct dopant radical cation-analyte interaction--the broadly accepted subsequent step in the reaction cascade leading to protonated analyte molecules--rapid thermal equilibration with ion source background water or liquid chromatography solvents through dopant ion-molecule cluster formation occurs. Fast intracluster chemistry then leads to almost instantaneous proton-bound water/solvent cluster generation. These clusters interact either directly with analytes by ligand switching or association reactions, respectively, or further downstream in the intermediate-pressure regions in the ion transfer stages of the mass spectrometer via electrical-field-driven collisional decomposition reactions finally leading to the predominantly observed bare protonated analyte molecules [M + H](+).

Charge Exchange Reaction in Dopant-Assisted Atmospheric Pressure Chemical Ionization and Atmospheric Pressure Photoionization

“…As discussed earlier in literature [31,32], water and solvent clusters must thus take part in the ionization process. Water and methanol clusters have sufficiently low IEs [33,34] to be ionized by the 10.0 and 10.6 eV photons, and subsequent self-protonation can lead to generation of protonated solvent clusters.…”

“…Water and methanol clusters have sufficiently low IEs [33,34] to be ionized by the 10.0 and 10.6 eV photons, and subsequent self-protonation can lead to generation of protonated solvent clusters. Alternatively, neutral solvent molecules can associate with dopant radical cations, and this also leads to generation of protonated solvent clusters [31]. The protonated solvent clusters can thereby protonate the analytes (Scheme 2, Reaction 4), possibly by ligand-switching as suggested by Klee et al [31].…”

“…Alternatively, neutral solvent molecules can associate with dopant radical cations, and this also leads to generation of protonated solvent clusters [31]. The protonated solvent clusters can thereby protonate the analytes (Scheme 2, Reaction 4), possibly by ligand-switching as suggested by Klee et al [31].…”

“…Since the experiments were conducted on a MS instrument with an atmospheric pressure interface that was not specifically designed for measuring clusters, we were not able to measure the true solvent cluster ion distribution in the source [31]. This is because the clusters dissociate due to high electric fields in the ion optics before detection.…”

“…ion, which is the main charge exchange reagent in the solvent system. Also, self-protonation of chlorobenzene or generation of protonated solvent clusters via interaction between chlorobenzene radical cation and neutral solvent clusters [31] are not able to efficiently generate protonated reagents, and proton transfer between chlorobenzene radical cation and analytes is expected to be negligible. On the other hand, in DA-APCI the generation of both protonated solvent clusters and dopant radical cations is feasible.…”