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Exploring Chemistry in Microcompartments Using Guided Droplet Collisions in a Branched Quadrupole Trap Coupled to a Single Droplet, Paper Spray Mass Spectrometer
journal contribution
posted on 2017-10-19, 00:00 authored by Michael I. Jacobs, James F. Davies, Lance Lee, Ryan D. Davis, Frances Houle, Kevin R. WilsonRecent studies suggest
that reactions in aqueous microcompartments
can occur at significantly different rates than those in the bulk.
Most studies have used electrospray to generate a polydisperse source
of highly charged microdroplets, leading to multiple confounding factors
potentially influencing reaction rates (e.g., evaporation, charge,
and size). Thus, the underlying mechanism for the observed enhancement
remains unclear. We present a new type of electrodynamic balancethe
branched quadrupole trap (BQT)which can be used to study reactions
in microdroplets in a controlled environment. The BQT allows for condensed
phase chemical reactions to be initiated by colliding droplets with
different reactants and levitating the merged droplet indefinitely.
The performance of the BQT is characterized in several ways. Sub-millisecond
mixing times as fast as ∼400 μs are measured for low
velocity (∼0.1 m/s) collisions of droplets with <40 μm
diameters. The reaction of o-phthalaldehyde (OPA)
with alanine in the presence of dithiolthreitol is measured using
both fluorescence spectroscopy and single droplet paper spray mass
spectrometry. The bimolecular rate constant for reaction of alanine
with OPA is found to be 84 ± 10 and 67 ± 6 M–1 s–1 in a 30 μm radius droplet and bulk solution,
respectively, which demonstrates that bimolecular reaction rate coefficients
can be quantified using merged microdroplets and that merged droplets
can be used to study rate enhancements due to compartmentalization.
Products of the reaction of OPA with alanine are detected in single
droplets using paper spray mass spectrometry. We demonstrate that
single droplets with <100 pg of analyte can easily be studied using
single droplet mass spectrometry.
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reaction ratesreaction rate coefficientsmicrodropletBQTbulk solutionalaninecolliding dropletsGuided Droplet CollisionsBranched Quadrupole Trapquadrupole trappolydisperse sourcephase chemical reactionsSingle DropletOPAPaper Spray Mass Spectrometerstudy reactionsExploring Chemistryfluorescence spectroscopydroplet paper spray mass spectrometrypaper spray mass spectrometrydroplet mass spectrometrystudy rate enhancements30 μ m radius droplet