Electrospray ionization mass spectrometry (ESIMS) is widely used to characterize
neutral and ionic species in solvents. Typically, electrical, thermal, and pneumatic
potentials are applied to create electrosprays from which charged ionic species are ejected
for downstream analysis by mass spectrometry. Most recently, ESIMS has been exploited
to investigate ambient proton transfer reactions at air-water interfaces in real time. We
assessed the validity of these experiments via complementary laboratory experiments.
Specifically, we characterized the products of two reaction scenarios via ESIMS and
proton nuclear magnetic resonance (1H-NMR): (i) emulsions of pH-adjusted water and
isoprene (C5H8) that were mechanically agitated, and (ii) electrosprays of pH-adjusted
water that were collided with gas-phase isoprene. Our experiments unambiguously
demonstrate that, while isoprene does not oligomerize in emulsions, it does undergo
protonation and oligomerization in electrosprays, both with and without pH-adjusted
water, confirming that C-C bonds form along myriad high-energy pathways during
electrospray ionization. We also compared our experimental results with some quantum
mechanics simulations of isoprene molecules interacting with hydronium at different
hydration levels (gas versus liquid phase). In agreement with our experiments, the kinetic
barriers to protonation and oligomerization of isoprene were inaccessible under ambient
conditions. Rather, the gas-phase chemistries during electrospray ionization drove the
oligomerization of isoprene. Therefore, we consider that ESIMS could induce artifacts in
interfacial reactions. These findings warrant a reassessment of previous reports on
tracking chemistries under ambient conditions at liquid-vapor interfaces via ESIMS.
Further, we took some high-speed images of electrosprays where it was possible to
observe the main characteristics of the phenomena, i.e. Taylor cone, charge separation,
and Coulomb fission. Finally, we took the freedom to speculate on possible mechanisms
that take place during electrospray ionization that affected our system and possibly may
influence other common analytical techniques on ESIMS.
| Identifer | oai:union.ndltd.org:kaust.edu.sa/oai:repository.kaust.edu.sa:10754/626292 |
| Date | 12 1900 |
| Creators | Gallo Junior, Adair |
| Contributors | Mishra, Himanshu, Biological and Environmental Sciences and Engineering (BESE) Division, Nunes, Suzana Pereira, Cavallo, Luigi |
| Source Sets | King Abdullah University of Science and Technology |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | 2018-12-04, At the time of archiving, the student author of this thesis opted to temporarily restrict access to it. The full text of this thesis became available to the public after the expiration of the embargo on 2018-12-04. |
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