James Eills, William Hale, Manvendra Sharma, Matheus Rossetto, Malcolm H. Levitt and Marcel Utz
We show that high-resolution NMR can reach picomole sensitivity for micromolar concentrations of analyte by combining parahydrogen-induced hyperpolarization (PHIP) with a high-sensitivity transmission line microdetector. The para-enriched hydrogen gas is introduced into solution by diffusion through a membrane integrated into a microfluidic chip. NMR microdetectors, operating with sample volumes of a few μL or less, benefit from a favorable scaling of mass sensitivity. However, the small volumes make it very difficult to detect species present at less than millimolar concentrations in microfluidic NMR systems. In view of overcoming this limitation, we implement PHIP on a microfluidic device with a 2.5 μL detection volume. Integrating the hydrogenation reaction into the chip minimizes polarization losses to spin−lattice relaxation, allowing the detection of picomoles of substance. This corresponds to a concentration limit of detection of better than 1 μM s , unprecedented at this sample volume. The stability and sensitivity of the system allow quantitative characterization of the signal dependence on flow rates and other reaction parameters and permit homo- (1H−1H) and heteronuclear (1H−13C) 2D NMR experiments at natural 13C abundance.