Sensitive and robust electrophoretic NMR: instrumentation and experiments.
Although simple as a concept, electrophoretic NMR (eNMR) has so far failed to find wider application. Problems encountered are mainly due to disturbing and partly irreproducible convection-like bulk flow effects from both electro-osmosis and thermal convection. Additionally, bubble formation at the electrodes and rf noise pickup has constrained the typical sample geometry to U-tube-like arrangements with a small filling factor and a low resulting NMR sensitivity. Furthermore, the sign of the electrophoretic mobility cancels out in U-tube geometries. We present here a new electrophoretic sample cell based on a vertically placed conventional NMR sample tube with bubble-suppressing palladium metal as electrode material. A suitable radiofrequency filter design prevents noise pickup by the NMR sample coil from the high-voltage leads which extend into the sensitive sample volume. Hence, the obtained signal-to-noise ratio of this cell is one order of magnitude higher than that of our previous U-tube cells. Permitted by the retention of the sign of the displacement-related signal phase in the new cell design, an experimental approach is described where bulk flow effects by electro-osmosis and/or thermal convection are compensated through parallel monitoring of a reference signal from a non-charged species in the sample. This approach, together with a CPMG-like pulse train scheme provides a superior first-order cancellation of non-electrophoretic bulk flow effects.
Hallberg
Fredrik
F
Division of Physical Chemistry and Industrial NMR Centre, Department of Chemistry, Royal Institute of Technology (KTH), Teknikringen 36, Plan 6, SE-10044 Stockholm, Sweden.
Furó
István
I
Yushmanov
Pavel V
PV
Stilbs
Peter
P
D001965:Bromides / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy / Q000295:instrumentation*; D019075:Electrophoresis, Capillary; D004867:Equipment Design; D000644:Quaternary Ammonium Compounds / Q000737:chemistry
D001965:Bromides; D000644:Quaternary Ammonium Compounds; C051470:ammonium bromide