Reagent and analyte ion hydrates in secondary electrospray ionization mass spectrometry (SESI-MS), their equilibrium distributions and dehydration in an ion transfer capillary: Modelling and experiments

Rationale Secondary electrospray ionization (SESI) in a water spray environment at atmospheric pressure involves the reactions of hydrated hydronium reagent ions, H3O+(H2O)(n), with trace analyte compounds in air samples. Understanding the formation and dehydration of reagent and analyte ions is the foundation for meaningful quantification of trace compounds by SESI-mass spectrometry (MS). Methods A numerical model based on gas-phase ion thermochemistry is developed that describes equilibria in H3O+(H2O)(n) reagent cluster ion distributions and ligand switching reactions with polar NH3 molecules leading to equilibrated hydrated ammonium ions NH4+(H2O)(m). The model predictions are compared with experimental results obtained using a cylindrical SESI source coupled to an ion-trap mass spectrometer via a heated ion transfer capillary. Non-polar isoprene, C5H8, was used to further probe the nature of the reagent ions. Results Equilibrium distributions of H3O+(H2O)(n) ions and their reactions with NH3 molecules have been characterized by the model in the near-atmospheric pressure SESI source. NH3 analyte molecules displace H2O ligands from the H3O+(H2O)(n) ions at the collisional rate forming NH4+(H2O)(m) ions, which travel through the heated ion transfer capillary losing H2O molecules. The data for variable NH3 concentrations match the model predictions and the C5H8 test substantiates the notion of dehydration in the heated capillary. Conclusions Large cluster ions formed in the SESI region are dehydrated to H3O+(H2O)(1,2,3) and NH4+(H2O)(1,2) while passing through the heated capillary, and considerable diffusion losses also occur. This phenomenon is also predicted for other polar analyte molecules, A, that can undergo similar switching reactions, thus forming AH(+) and AH(+)(H2O)(m) analyte ions.

Automated summary

Secondary electrospray ionization (SESI) in a water spray environment at atmospheric pressure involves reactions of hydrated hydronium reagent ions with trace analyte compounds in air samples. Understanding the formation and dehydration of reagent and analyte ions is crucial for quantification of trace compounds by SESI-mass spectrometry (MS).