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

Abstract

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 electric 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 hydrated hydronium reagent ions, H3O+(H2O)n, with volatile organic compound (VOC) molecules, need to be understood. Methods A modified Zspray™ ESI ion source operating at sub‐atmospheric pressure with analyte sample gas introduced via an inlet coaxial with the spray was used. Variation of the ion‐guiding electric 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 selected ion flow tube MS. Results Electric field desolvation resulted in largely protonated VOCs, MH+, and their monohydrates, MH+H2O, 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 0 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+H2O, but other yet unrecognized factors must play a significant role.

Kinetics of reactions of NH4+ with some biogenic organic molecules and monoterpenes in helium and nitrogen carrier gases: A potential reagent ion for selected ion flow tube mass spectrometry

“…The major objective of this study is to understand the ion chemistry of NH 4 + ions by studying the rate coefficients and product ion distributions for their reactions with a selected number of VOCs that include several biogenic molecules and multiple monoterpene structural isomers of significance in atmospheric chemistry 40 and in breath analysis 41 . Note that the corresponding H 3 O + reactions are well understood and extensively discussed in the literature 4,30,42 . In contrast, not much is known about the ion chemistry of NH 4 + and a better understanding is required if this ion is to be used as a reagent in SIFT‐MS analysis.…”

“… b Polarisability (α) and dipole moments ( D ) taken from references 30,43,45,46 and calculated for the monoterpenes using the B3LYP aug‐cc‐pVTZ level of theory.…”

“…The proton affinities (PAs) were then calculated as the difference between the total enthalpies of the neutral molecules and the corresponding lowest energy (most stable) protonated structures, accounting for the thermal enthalpy of a free proton of 6.2 kJ/mol at 298.15 K. 30,37 To test whether the used B3LYP aug-cc-pVTZ D4 level of theory provides appropriate energetics, the method was benchmarked on the known PA of ammonia and the biogenic compounds. 8 For ammonia, the calculated value agreed to within 1 kJ/mol (854 kJ/mol).…”

“…41 Note that the corresponding H 3 O + reactions are well understood and extensively discussed in the literature. 4,30,42 In contrast, not much is known about the ion chemistry of NH 4 + and a better understanding is required if this ion is to be used as a reagent in SIFT-MS analysis. At the very least, the product ions and the rate coefficients are needed.…”

“…The rationale for the present SIFT study of the reactions of NH 4 + is thus to gain an understanding of the reactivity of NH 4 + with VOC molecules. Included in this limited study are the reactions of seven VOCs of biogenic relevance 30 and 15 monoterpene structural isomers with a common molecular formula C 10 H 16 , in both He and N 2 carrier gases. As we will show, the ion chemistry of NH 4 + is unusual, in some ways unique in our experience, and the results reveal its potential value as a reagent ion in SIFT‐MS.…”