Abstract
A generic approach is presented that allows high-resolution NMR spectroscopy of water/oil droplet emulsions in microfluidic devices. Microfluidic NMR spectroscopy has recently made significant advances due to the design of micro-detector systems and their successful integration with microfluidic devices. Obtaining NMR spectra of droplet suspensions, however, is complicated by the inevitable differences in magnetic susceptibility between the chip material, the continuous phase, and the droplet phases. This leads to broadening of the NMR resonance lines and results in loss of spectral resolution. We have mitigated the susceptibility difference between the continuous (oil) phase and the chip material by incorporating appropriately designed air-filled structures into the chip. The susceptibilities of the continuous and droplet (aqueous) phases have been matched by doping the droplet phase with a Eu3+ complex. Our results demonstrate that this leads to a proton line width in the droplet phase of about 3 Hz, enabling high-resolution NMR techniques.
Highlights
NMR spectroscopy is one of the most important analytical tools available to chemistry, biochemistry, and the life sciences
Droplet microfluidic systems are increasingly finding applications in chemistry 9 and the life sciences. 8,10 In this paper, we explore the possibility to obtain highresolution NMR spectra from small volumes of droplet emulsions on a chip
Managing susceptibility differences for an emulsion of droplets on a microfluidic chip adds additional complexity, since three different materials are involved: the chip, the continuous phase, and the droplet phase, all with different susceptibilities. We show that this can be mitigated in a two-step approach, which is based on the observation that most organic solvents in use as continuous phases for droplet microfluidics are less diamagnetic than water
Summary
NMR spectroscopy is one of the most important analytical tools available to chemistry, biochemistry, and the life sciences. Differences in magnetic susceptibility between the materials used for the microfluidic chip and the sample fluid, as well as the materials and geometry of the probe assembly, lead to a demagnetising field that varies continously over the sample volume.
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