Journal
ANALYTICAL CHEMISTRY
Volume 89, Issue 22, Pages 12511-12519Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.analchem.7b03704
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Funding
- Condensed Phase and Interfacial Molecular Science Program, in the Chemical Sciences Geosciences and Biosciences Division of the Office of Basic Energy Sciences of the U.S. Department of Energy [DE-AC02-05CH11231]
- Laboratory Directed Research and Development (LDRD) program at Lawrence Berkeley National Laboratory
- NSF [DGE-1106400]
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Recent 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 balance-the 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 similar to 400 mu s are measured for low velocity (similar to 0.1 m/s) collisions of droplets with <40 mu m 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 spectrometry. The bimolecular rate constant for reaction of alinine with OPA is found to be 84 +/- 10 and 67 +/- 6 M-1 s(-1) in a 30 mu m 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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