In situ Zero- to Ultralow-field Control of Spin Orders in Hyperpolarized Organic Solvents

Zero- to ultralow-field (ZULF) NMR promises chemical sensing with portable devices, but weak thermal polarization and low natural isotopic abundance of heteronuclei limit its sensitivity. Here we combine Signal Amplification by Reversible Exchange (SABRE) and SABRE-relay with in situ ZULF NMR detection, and introduce optimized field-cycling pulse sequences and sample geometry to deliver label-free J-spectra from neat solvents. We achieve single-shot J-spectra of [15N]-acetonitrile (MeCN) at natural abundance (∼0.35 %) with SNR of ∼500 in neat acetonitrile and additionally resolve the rare isotopologues [1-13C,15N]-MeCN and [2-13C,15N]-MeCN (∼0.004 %). In acetonitrile-d3, we observe 15N–2H J-spectra, demonstrating sensitivity to quadrupolar nuclei, and also obtain single-shot spectra of natural-abundance [15N]-pyridine in pyridine. The elimination of sample shuttling minimizes relaxation losses and enables precise spin-order manipulation, while scalable, magnet-free hardware supports non-invasive measurements through metal enclosures. These results establish hyperpolarized ZULF NMR as an information-rich practical platform for solvent fingerprinting at natural isotopic abundance, paving the way for inline process analysis.