In experiments involving precipitation/redissolution, 1.25 mL of the sample in the 10 mm NMR tube was extracted into a syringe containing 0.75 mL of 1 M sodium fumarate in D2O. The remaining 150 µL of reaction solution was lost as droplets in the transfer capillaries and containers. The 2 mL sample was then rapidly injected into 1 mL of 12 M HCl at room temperature on a glass sinter (grade 4) atop a vacuum flask. Fumaric acid crystals immediately precipitated out as a white solid. The residual solution was removed by vacuum filtration, and the fumaric acid crystals were redissolved in 1 mL of 3 M NaOD to produce the 'purified' sample.