Solvent quality, effect, flow-enhanced

The effectiveness of a polymeric flow enhancer is influenced decisively by the state of solution and the solvation characteristics. In the case of polyelectrolytes, in particular, the chemical nature plays a significant role, e.g., it was found for poly(acrylamide)-coacrylate that a significant increase in effectiveness arises with the increasing number of ionic groups. It is therefore necessary to consider, for example, such factors as the question of critical concentration, polymer-polymer and polymer-solvent interactions, the thermodynamic quality of the solvent, the proportion of ionic molecular groups and their behavior in the presence of lower-molecular-weight charge carriers. 

A common drawback to LC-NMR analysis derives from the interference of the resonances of the mobile phase, usually much higher than those of the analyte itself. To overcome this problem, the mobile phase coming from the liquid chromatograph is efficiently suppressed with techniques such as water-suppression enhanced-through Ti effects (WET). This allows the separation of analytes in typical RP-HPLC conditions using common solvents such as MeOH or acetonitrile, yet usually replacing water by D2O to achieve better quality spectra however, in the case of analyte signals coincident with the suppressed solvent resonances, those of the analytes will also be suppressed. On the other hand, in spite of the usefulness of LC-NMR for stmcmral elucidation of flavonoids, this is not a sensitive LC detection technique. Therefore, to improve sensitivity, transients can be accumulated by operation in two different modes the on-flow and stop-flow modes. In both cases, the analytes from an LC column pass into an NMR microflow probe that typically has an active volume of 60-120 p.L, comparable with the conventional NMR sample size of 500-600 p,L. 

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