Studies on liquid chromatography-nuclear magnetic resonance (LC-NMR) with special emphasis on preconcentration on solid phase extraction (SPE) in a LC-SPE-NMR system.

ABSTRACT LC-NMR combines the powerful separation technique liquid chromatography (LC) with the structural information obtained of nuclear magnetic resonance (NMR) spectroscopy. The technique has become an important analytical tool to isolate and determine the structure of compounds of interest. However, a large amount of the compound, at least 20 µg, is needed to obtain good quality NMR spectrum. This large amount of the analyte can be obtained by preconcentration on a solid phase extraction (SPE) column. One µLC-NMR and two LC-SPE-NMR systems with on-line and off-line NMR analysis were studied. In the on-line µLC-NMR system, no NMR information was obtained due to the low amount of the analyte. A commercial LC-SPE-NMR system was studied using commercially available 2 mm I.D. x 10 mm Hysphere C18 SPE cartridges. Unfortunately, there were some difficulties with trapping large amounts of the target compound on the SPE cartridges. Therefore, an in-house LC-SPE system was designed and a 1 mm I.D. x 5 mm Hypercarb SPE column was used to isolate the target compound. The analytical column was a 2.1 mm I.D. x 150 mm, 3.5 µm Zorbax-SB-C18 column. The Hypercarb SPE column provided higher retention, and allowed injection of larger volumes of the analyte solution without breakthrough of the target compound as compared to the Hysphere C18 cartridge. The target compound was desorbed from the SPE column with 100 % ACN-d3 and manually transferred to a NMR tube for subsequent NMR analysis. Upgrading of the quality of some instrumental parts was necessary to improve the performance of the system. Different compounds have been used to gain knowledge of the LC-NMR techniques, and identification of a degradation product of monuron was explored using the in-house designed LC-SPE system with off-line NMR analysis