Sensitivity of secondary electrospray ionization mass spectrometry to a range of volatile organic compounds: Ligand switching ion chemistry and the influence of Zspray™ guiding electric fields

RATIONALE Secondary electrospray ionization (SESI) is currently only semi-quantitative. In the Zspray™ arrangement of SESI-MS, the transfer of ions from near atmospheric pressure to a triple quadrupole is achieved by guiding E-fields that partially desolvate both reagent and analyte ions which must be understood. Also, to make SESI-MS more quantitative, the mechanisms and the kinetics of the reaction processes, especially ligand switching reactions of the hydrated hydronium reagent ions, H3 O+ (H2 O)n , with VOC molecules, need to be understood. METHODS A modified Zspray™ ESI ions source operating at sub-atmospheric pressure with analyte sample gas introduced via an inlet coaxial with the spray is used. Variation of the ion guiding E-fields was used to reveal the degree of desolvation of both reagent and analyte ions. The instrument sensitivity was determined for several classes of VOCs by introducing bag samples of suitably varying concentrations as quantified on-line using SIFT-MS. RESULTS E-field desolvation resulted in largely the protonated VOCs, MH+ , and their monohydrates, MH+ H2 O, and for some VOCs proton-bound dimer ions, MH+ M, were formed. There was a highly linear response of the ion signal to the measured VOC sample concentration, which provided the instrument sensitivities, S, for 25 VOCs. The startling results show very wide variations in S from near-zero to 1 for hydrocarbons, and up to 100, on a relative scale, for polar compounds such as monoketones and unsaturated aldehydes. CONCLUSIONS The complex ion chemistry occurring in the SESI ion source, largely involving gas-phase ligand switching, results in widely variable sensitivities for different classes of VOCs. The sensitivity is observed to depend on the dipole moment and proton affinity of the analyte VOC molecule, M, and to decrease with the observed fraction of MH+ H2 O, but other yet unrecognised factors must play a significant role.