LC-NMR/MS and LC-SPE-NMR/MS: technical realization, possibilities and limitations

The hyphenation between chromatographic and spectroscopic techniques is well established and approved in various fields, e.g. pharmaceutical or natural products analysis. It combines commercially available analytical HPLC equipment with an NMR spectrometer and a mass spectrometer using specially developed interfaces according to the type of application.

LC-NMR/MS directly transfers the separated peak into an NMR flow probe and via split into the ion source of a mass spectrometer. Chromatographic separation is done using mixtures of protonated organic and deuterated aqueous mobile phase, resulting in strong background resonances from the organic solvent which needs to be suppressed in order to detect small signals originating from the analytes. Chromatographic peaks can be send either directly to the NMR or stored in loops for subsequent transfer into the NMR probe. This direct coupling is especially used for polar and/or labile compounds present in high to moderate concentrations.

Post column solid phase extraction was first introduced 1998 by Griffiths and Horton: peaks eluting from a reversed phase column are diluted by aqueous phase and trapped on small SPE cartridges packed with reversed phase material. Elution into the NMR flow probe or tube can be done with deuterated organic solvent preferably after drying of the cartridge with nitrogen gas in order to remove residual protonated solvent. This indirect coupling can be used for medium polar to unpolar compounds down to very low concentrations due to the possibility of multiple peak trapping.

In combination with cryogenically cooled NMR probes highest NMR sensitivity can be achieved.