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Many mass spectrometry methods using various ionization sources provide bulk composition of airborne particles, but little is known about the surface species that play a major role in determining their physicochemical properties that impact air quality, climate, and health. The present work shows that the composition of surface layers of atmospherically relevant submicron organic particles can be probed without the use of an external ionization source. Solid dicarboxylic acid particles are used as models, with glutaric acid being the most efficient at generating ions. Coating with small diacids or products from α-pinene ozonolysis demonstrates that ions are ejected from the surface, providing surface molecular characterization of organic particles on the fly. This unique approach provides a path forward for elucidating the role of the surface in determining chemical and physical properties of particles, including heterogeneous reactions, particle growth, water uptake, and interactions with biological systems.
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New Mechanism of Extractive Electrospray Ionization Mass Spectrometry for Heterogeneous Solid Particles
Real-time in situ mass spectrometry analysis of airborne particles is important in a number of applications, including exposure studies in ambient air, industrial settings, and assessing impacts on visibility and climate. However, obtaining molecular and 3D structural information is more challenging, especially for heterogeneous solid or semi-solid particles. We report a study of extractive electrospray ionization mass spectrometry (EESI-MS) for the analysis of solid particles with an organic coating. The goal is to elucidate how much of the overall particle content is sampled, and the sensitivity of this technique to the surface layers. It is shown that for NaNO3 particles coated with glutaric acid (GA), very little of the solid NaNO3 core is sampled compared to the GA coating, while for GA particles coated with malonic acid (MA), significant signals from both the MA coating and the GA core are observed. However, conventional ESI-MS of the same samples collected on a Teflon filter and extracted detects much more core material compared to EESI-MS in both cases. These results show that for the experimental conditions used here, EESI-MS does not sample the entire particle, but instead is more sensitive to surface layers. Separate experiments on single component particles of NaNO3, glutaric acid or citric acid show that there must be a kinetics limitation to dissolution that is important in determining EESI-MS sensitivity. We propose a new mechanism of EESI solvent droplet interaction with solid particles that is consistent with the experimental observations. In conjunction with previous EESI-MS studies of organic particles, these results suggest EESI does not necessarily sample the entire particle when solid, and that not only solubility but also surface energies and the kinetics of dissolution play an important role.
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- Award ID(s):
- 1647386
- PAR ID:
- 10074944
- Date Published:
- Journal Name:
- Analytical chemistry
- Volume:
- 90
- Issue:
- 3
- ISSN:
- 0003-2700
- Page Range / eLocation ID:
- 2055-2062
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract. Extractive electrospray ionization (EESI) has been a well-knowntechnique for high-throughput online molecular characterization of chemicalreaction products and intermediates, detection of native biomolecules, invivo metabolomics, and environmental monitoring with negligible thermal andionization-induced fragmentation for over two decades. However, the EESIextraction mechanism remains uncertain. Prior studies disagree on whetherparticles between 20 and 400 nm diameter are fully extracted or if theextraction is limited to the surface layer. Here, we examined the analyteextraction mechanism by assessing the influence of particle size and coatingthickness on the detection of the molecules therein. We find that particlesare extracted fully: organics-coated NH4NO3 particles with afixed core volume (156 and 226 nm in diameter without coating) showedconstant EESI signals for NH4NO3 independent of the shell coatingthickness, while the signals of the secondary organic molecules comprisingthe shell varied proportionally to the shell volume. We also found that theEESI sensitivity exhibited a strong size dependence, with an increase insensitivity by 1–3 orders of magnitude as particle size decreasedfrom 300 to 30 nm. This dependence varied with the electrospray (ES)droplet size, the particle size and the residence time for coagulation in theEESI inlet, suggesting that the EESI sensitivity was influenced by thecoagulation coefficient between particles and ES droplets. Overall, ourresults indicate that, in the EESI, particles are fully extracted by the ESdroplets regardless of the chemical composition, when they are collected bythe ES droplets. However, their coalescence is not complete and dependsstrongly on their size. This size dependence is especially relevant whenEESI is used to probe size-varying particles as is the case in aerosolformation and growth studies with size ranges below 100 nm.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.analchem.7b01464
