Electrodynamic balance–mass spectrometry of single particles as a new platform for atmospheric chemistry research

Abstract

Abstract. New analytical techniques are needed to improve our understanding of the intertwined physical and chemical processes that affect the composition of aerosol particles in the Earth's atmosphere, such as gas-particle partitioning and homogenous or heterogeneous chemistry, and their ultimate relation to air quality and climate. We describe a new laboratory setup that couples an electrodynamic balance (EDB) to a mass spectrometer (MS). The EDB stores a single laboratory-generated particle in an electric field under atmospheric conditions for an arbitrarily long length of time. The particle is then transferred via gas flow to an ionization region that vaporizes and ionizes the analyte molecules before MS measurement. We demonstrate the feasibility of the technique by tracking evaporation of polyethylene glycol molecules and finding agreement with a kinetic model. Fitting data to the kinetic model also allows determination of vapor pressures to within a factor of 2. This EDB-MS system can be used to study fundamental chemical and physical processes involving particles that are difficult to isolate and study with other techniques. The results of such measurements can be used to improve our understanding of atmospheric particles.

Single-particle experiments measuring humidity and inorganic salt effects on gas-particle partitioning of butenedial

“…analyte signal compared to previously (Birdsall et al, 2018). This could potentially be explained by our previous technique undersampling the peak shape.…”

“…Previous work in our laboratory has developed a technique termed electrodynamic balance-mass spectrometry (EDB-MS) to levitate individual charged droplets with diameter on the order of 10 μm in an electrodynamic balance (EDB) and then measure the droplet's composition with mass spectrometry (MS) (Birdsall et al, 2018). Other groups have developed methods to measure vapor pressures of organic compounds using highly precise particle diameter measurements using optical sizing techniques (Krieger et al, 2012).…”

“…Microdroplets were generated, levitated, and measured via mass spectrometry using the same electrodynamic balance-mass spectrometry (EDB-MS) apparatus as previously described (Birdsall et al, 2018). In brief, a droplet-ondemand particle generator injects a ~140 pL droplet from an aqueous solution into the EDB levitation chamber, through a charged coil that gives the droplet charge.…”

“…As previously described (Birdsall et al, 2018), droplet sizes were characterized after initial trapping and before the ejection procedure using a spring point technique, which relies on a quantitative relationship of the visible onset of droplet instability (the "spring point") in the EDB with the voltage conditions and the droplet diameter (Davis, 1985). The droplet 5 diameters determined for this study used the same calibration data using 18 μm diameter polymethylmethacrylate (PMML) spheres as previously.…”

“…PEG-6 was used as an internal standard to account for droplet-to-droplet variability in total amount of signal measured with the mass spectrometer because it shows a clear peak at 283 m/z in the mass spectrum (MH + ) and its vapor pressure is low enough for its evaporation to be negligible over the timescale of the experiments. We 25 also used PEG-6 as an internal standard in a previous set of experiments (Birdsall et al, 2018). Based on those previous results, the viscosity of a PEG-6 matrix is sufficiently low to expect no condensed-phase diffusion effects limiting the evaporation rate of butenedial.…”