ONLINE USE OF NMR DETECTION IN SEPARATION CHEMISTRY

The direct on-line coupling between important separation and extraction techniques such as high-performance liquid chromatography (HPLC), supercritical fluid chromatography (SFC), supercritical fluid extraction (SFE) and capillary electrophoresis (CE) and proton high-field nuclear magnetic resonance (NMR) spectroscopy is described. The resolution of the H-1 NMR spectra obtained in HPLC-NMR, SFC-NMR and SFE-NMR coupling under continuous-flow conditions is similar to conventionally recorded NMR spectra. In CE-NMR coupling signal line widths are degraded but the resolution of CE-NMR spectra is improving continuously. The detection limit of a HPLC-NMR separation in acetonitrile-D2O for aliphatic signals of a low-molecular-mass compound (300 Da) is 500 ng of injected compound in the continuous-flow mode at a 600 MHz NMR spectrometer with a 120 mu l flow cell. A similar detection limit in the nanogram range with a 5 nl flow cell is reached in a CE-NMR separation at a 300 MHz NMR spectrometer. In SFC-NMR coupling the whole proton spectral range can be observed without any solvent windows. SFE-NMR offers the advantage of directly monitoring the extraction process.