Development of an LC‐Benchtop Nuclear Magnetic Resonance at 125 MHz for Use in Stop‐Flow Gradient‐Based Separations of Pharmaceutical Compounds

The development of a high-performance liquid chromatography system with a benchtop ¹H nuclear magnetic resonance (NMR) spectrometer for use in stop-flow mode with a gradient separation is described for the first time. Advancements in permanent magnet-based benchtop NMR technology have resulted in a field strength of 125 MHz for increased sensitivity with the use of an integrated flow cell. The size and accessibility of benchtop NMRs can allow easier access to NMR for many laboratories. Development was focused on analytical scale instrumentation with concentrations and injection volumes commonly seen in this scale. A combination of pharmaceuticals, specifically acetaminophen, caffeine, warfarin, and ibuprofen, which present a wide profile of different NMR spectra, was examined. Separations and subsequent NMR data collection were performed successfully in multiple common protonated solvents, as opposed to high volumes of costly deuterated solvents, to show the versatility of this new application. Each separated compound presented was successfully analyzed in online stop-flow mode within a single run and was repeated across solvent mixtures. All compounds were detected within a minimum signal-to-noise (SNR) ≥ 3 post-separation after NMR spectra were processed in MestReNova. These were then compared to individual samples tested in typical deuterated solvents for ¹H NMR to confirm the success of this approach.