Abstract
Abstract. Extractive electrospray ionization (EESI) has been a well-known technique for high-throughput online molecular characterization of chemical reaction products and intermediates, detection of native biomolecules, in vivo metabolomics, and environmental monitoring with negligible thermal and ionization-induced fragmentation for over two decades. However, the EESI extraction mechanism remains uncertain. Prior studies disagree on whether particles between 20 and 400 nm diameter are fully extracted or if the extraction is limited to the surface layer. Here, we examined the analyte extraction mechanism by assessing the influence of particle size and coating thickness on the detection of the molecules therein. We find that particles are extracted fully: organics-coated NH4NO3 particles with a fixed core volume (156 and 226 nm in diameter without coating) showed constant EESI signals for NH4NO3 independent of the shell coating thickness, while the signals of the secondary organic molecules comprising the shell varied proportionally to the shell volume. We also found that the EESI sensitivity exhibited a strong size dependence, with an increase in sensitivity by 1–3 orders of magnitude as particle size decreased from 300 to 30 nm. This dependence varied with the electrospray (ES) droplet size, the particle size and the residence time for coagulation in the EESI inlet, suggesting that the EESI sensitivity was influenced by the coagulation coefficient between particles and ES droplets. Overall, our results indicate that, in the EESI, particles are fully extracted by the ES droplets regardless of the chemical composition, when they are collected by the ES droplets. However, their coalescence is not complete and depends strongly on their size. This size dependence is especially relevant when EESI is used to probe size-varying particles as is the case in aerosol formation and growth studies with size ranges below 100 nm.
Highlights
Atmospheric aerosols are suspended particles in the air ranging from a few nanometers to several micrometers in diameter
We explored the dependence of the electrospray ionization (EESI) sensitivity on particle size using individual chemical standards and chemical mixtures with two different EESI source designs
We show that the EESI sensitivity decreases as the size of the particles increases
Summary
Atmospheric aerosols are suspended particles in the air ranging from a few nanometers (nm) to several micrometers (μm) in diameter. Progress has been made with the development of chemical ionization interfaces such as the Filter Inlet for Gases and AEROsols (FIGAERO) (Lopez-Hilfiker et al, 2014), Thermal Desorption Differential Mobility Analyzer (TD-DMA) (Holzinger et al, 2010; Wagner et al, 2018), and Chemical Analysis of Aerosol Online (CHARON) (Eichler et al, 2015) coupled to a mass spectrometer These techniques suffer from thermal decomposition of the analyte prior to ionization and/or ionization-induced fragmentation, impeding molecular speciation (Müller et al, 2017; Stark et al, 2017). We determined whether particles are fully extracted or if extraction is limited to the coated surface by coating monodisperse NH4NO3 particles of a fixed size with variable amounts of oxidation products in an oxidation flow tube reactor
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