Coupling Low-Field Benchtop NMR On-line with High-Performance Liquid Chromatography and Off-line with Supercritical Fluid Chromatography for Pharmaceutical Analysis

Separation techniques such as high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC) are essential to the modern scientific environment, but these alone cannot provide the information required to fully determine the structure of an unknown analyte. Nuclear magnetic resonance spectroscopy (NMR) provides this capability. Here we will discuss the use of a benchtop low-field 125 MHz NMR, as a more accessible alternative to traditional high-field NMR. The successful coupling of benchtop NMR to an HPLC into a fully functional LC-NMR system is presented with gradient based separations. The LC-NMR system has been used to separate acetaminophen, caffeine, warfarin, and ibuprofen using both acetonitrile and methanol mobile phases. These are some of the most common organic phases, and each have their advantages in not only the separation but also the NMR data acquired due to differences in the solvent peaks. The methyl resonances of both organic solvents were used as the reference for analysis. These compounds present a representative sample of a variety of peak intensities in real world samples and present signals within the limits of detection (SNR ≥3) even in the most complex analyte, showing the viability of this new approach. For off-line SFC-NMR, a compound set similar to the LC-NMR, but also including indomethacin and naproxen were separated and fractions collected. Analysis in both protonated methanol direct from the SFC as well as samples reconstituted in deuterated solvents was performed to show low-field NMR suitability. The applications of low-field NMR to both of these approaches indicate the versatility of this class of instruments, especially when used in conjunction with both new and existing techniques.