Du, Yong, Ronghui Zhou, Maria-Jose Ferrer, Minda Chen, John Graham, Bill Malphurs, Greg Labbe, Wenyu Huang, and Clifford R. Bowers. “An Inexpensive Apparatus for up to 97% Continuous-Flow Parahydrogen Enrichment Using Liquid Helium.” Journal of Magnetic Resonance 321 (December 2020): 106869.
https://doi.org/10.1016/j.jmr.2020.106869.
Nuclear spin hyperpolarization derived from parahydrogen can enable nuclear magnetic resonance spectroscopy and imaging with sensitivity enhancements exceeding four orders of magnitude. The NMR signal enhancement is proportional to 4xp À 1, where xp is the parahydrogen mole fraction. For convenience, many labs elect to carry out the ortho–para conversion at 77 K where 50% enrichment is obtained. In theory, enrichment to 100% yields an automatic three-fold increase in the NMR signal enhancement. Herein, construction and testing of a simple and inexpensive continuous-flow converter for high paraenrichment is described. During operation, the converter is immersed in liquid helium contained in a transport dewar of the type commonly found in NMR labs for filling superconducting magnets. A maximum enrichment of 97.3Æ1.9% at 30 K was observed at 4.5 bar and 300 mL/min flow rate. The theoretically predicted 2.9-fold increase in the signal enhancement factor was confirmed in the heterogeneous hydrogenation of propene to propane over a PdIn/SBA-15 catalyst. The relatively low-cost to construct and operate this system could make high parahydrogen enrichment, and the associated increase in the parahydrogen-derived NMR signals, more widely accessible.