To test precipitation/redissolution as a method to purify hyperpolarized fumarate solutions, the same reaction and field cycle procedure was used as before, with a 60 s bubbling duration chosen as it led to formation of a high concentration of fumarate (150 mM) at a relatively high polarization level. After performing the reaction and magnetic field cycle, the C-polarized fumarate solution was split into a control sample and a sample that was purified. The purified solution was transported to the benchtop NMR spectrometer for signal acquisition following a 𝜋/2 pulse. Immediately following this, the control sample was measured in the same manner. For both samples, after the hyperpolarized NMR signals had fully relaxed, a thermal equilibrium 1 H NMR spectrum was acquired to quantify the fumarate concentration. The time between sample ejection from the reactor and detecting C spectra of the purified and control samples was approximately 30 s and 34 s, respectively. Further experimental details are provided in the Materials and Methods section.