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Precise and accurate charge measurements on microdroplets are essential for understanding the role of charge in modulating microdroplet chemistry, including reaction kinetics, ion distribution, and interfacial dynamics. Despite the availability of various charge measurement techniques, existing contactless techniques either lack the sensitivity to accurately detect charges with ∼1 fC precision or lack the ability to measure charge on micron-sized particles, leaving a significant gap in the field. Here, a new technique is presented to directly measure the net charge of microdroplets exiting a quadrupole electrodynamic trap (QET) using induced charge detection. With this method, the charge droplets induce on a cylindrical electrode (Qinduced) is detected using a homebuilt charge sensitive pre-amplifier (CSP). The long time constant of the CSP (1.02 ± 0.01 s−1) facilitates accurate measurement of Qinduced on slow-moving microdroplets that interact with the detection electrode for up to 100s of ms. The new charge detection method is validated by comparing Qinduced with the charge of droplets measured using a Faraday cup (QFaraday cup) for roughly 2900 droplets with different net charges, sizes, and velocities. Regardless of droplet properties, Qinduced closely correlates with QFaraday cup with absolute differences averaging <5 fC (i.e., 1% accuracy). While the charge detection system is coupled to a QET, it could easily be adapted for other droplet-based measurements (e.g., droplet train experiments). Ultimately, the induced charge detection system presented here will support future studies exploring how charge influences the physical and chemical processing of microdroplets, such as understanding how charge can drive accelerated chemistry in microdroplets. 

Efficient 13C hyperpolarization of ketoisocaproate is demonstrated in natural isotopic abundance and [1-13C]enriched forms via SABRE-SHEATH (Signal Amplification By Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei). Parahydrogen, as the source of nuclear spin order, and ketoisocaproate undergo simultaneous chemical exchange with an Ir-IMes-based hexacoordinate complex in CD3OD. SABRE-SHEATH enables spontaneous polarization transfer from parahydrogen-derived hydrides to the 13C nucleus of transiently bound ketoisocaproate. 13C polarization values of up to 18% are achieved at the 1-13C site in 1 min in the liquid state at 30 mM substrate concentration. The efficient polarization build-up becomes possible due to favorable relaxation dynamics. Specifically, the exponential build-up time constant (14.3 ± 0.6 s) is substantially lower than the corresponding polarization decay time constant (22.8 ± 1.2 s) at the optimum polarization transfer field (0.4 microtesla) and temperature (10 °C). The experiments with natural abundance ketoisocaproate revealed polarization level on the 13C-2 site of less than 1%—i.e., one order of magnitude lower than that of the 1-13C site—which is only partially due to more-efficient relaxation dynamics in sub-microtesla fields. We rationalize the overall much lower 13C-2 polarization efficiency in part by less favorable catalyst-binding dynamics of the C-2 site. Pilot SABRE experiments at pH 4.0 (acidified sample) versus pH 6.1 (unaltered sodium [1-13C]ketoisocaproate) reveal substantial modulation of SABRE-SHEATH processes by pH, warranting future systematic pH titration studies of ketoisocaproate, as well as other structurally similar ketocarboxylate motifs including pyruvate and alpha-ketoglutarate, with the overarching goal of maximizing 13C polarization levels in these potent molecular probes. Finally, we also report on the pilot post-mortem use of HP [1-13C]ketoisocaproate in a euthanized mouse, demonstrating that SABRE-hyperpolarized 13C contrast agents hold promise for future metabolic studies.