Enhancing Sensitivity in the Hyphenation of High-Performance Liquid Chromatography to Benchtop Nuclear Magnetic Resonance Spectroscopy at Isocratic and Onflow Conditions

The onflow hyphenation of high-performance liquid chromatography (HPLC) in adsorption mode with a benchtop ¹H nuclear magnetic resonance (NMR) spectrometer is described for the first time. Protonated solvents and isocratic conditions are used. The sensitivity was increased by choosing suitable NMR acquisition parameter as well as optimizing injection parameters and postacquisition data processing methods. With optimized conditions, the limit of detection (LOD) and the limit of quantification (LOQ) were LOD = 0.010 g L^-1 and LOQ = 0.031 g L^-1 for the methoxy ¹H of methyl paraben at 4.07 ppm, LOD = 0.038 g L^-1 and LOQ = 0.134 g L^-1 for the aromatic ¹H of pentyl paraben between 7.00 and 8.50 ppm. These are expressed in injection concentration and are comparable to existing HPLC hyphenation with high-field NMR spectrometers. The analysis of a 2 g L^-1 paraben mixture, far below the legal limits for usage in cosmetics, illustrates the applicability of the method. Taking advantage of the spectral resolution, chromatographically overlapping peaks are resolved using analyte-specific NMR elution traces. A methodology is detailed to facilitate the transfer of the optimized method to other (analyte) systems.