Controlling LC-SPE-NMR systems

There are several stages of the LC-SPE-NMR process that should be monitored closely to ensure an efficient isolation and concentration of the target analyte, for instance analyte break-through and compound transfer from the LC-SFE to the NMR probe. In this study, analyte break-through monitoring wits performed with a UV detector and a mass spectrometer placed after the SPE unit. Easy break-through was a problem when attempting multiple trapping of various compounds using C-18 SPE cartridges with the original commercial system. However, oil lowering the flow rate over the SPE system and using SPE cartridges packed with porous carbon, the number of trappings possible increased five-fold. To increase control over the online SPE-NMR transfer, a gradient pump-UV system was used to elute compounds trapped on an SPE to an NMR probe, The analyte band was placed ill the active volume of the probe by a stop-now mechanism. The modified LC-SPE system was also coupled with off-line NMR analysis for determination of it degradation product of the insecticide monuron, present in the low ppm range.