Capillary isotachophoresis/NMR: extension to trace impurity analysis and improved instrumental coupling

Building upon its promising initial performance, the online coupling of capillary isotachophoresis (cITP) to nuclear magnetic resonance (NMR) is extended to trace impurity analysis. By simultaneously concentrating and separating dilute charged species on the basis of their electrophoretic mobility, cITP greatly facilitates NMR structural elucidation. cITP/NMR appears particularly attractive for identifying trace charged synthetic and natural organic compounds obscured by large excesses of other components. A 9.4 microL injection of 200 microM (1.9 nmol) atenolol in a 1000-fold excess of sucrose (200 mM) is analyzed by cITP/NMR. A microcoil, the most mass sensitive NMR probe, serves as the detector as it provides optimal NMR observation of the capillary-scale separation. cITP successfully isolates the atenolol from the sucrose while concentrating it 200-fold to 40 mM before presentation to the 30 nL observe volume microcoil, thereby enabling rapid 1H NMR spectral acquisition of atenolol (experimental time of 10 s) without obstruction from sucrose. For this particular probe and sample, the stacking efficiency is near the theoretical limit as 67% of the sample occupies the 1 mm long microcoil during peak maximum. A multiple-coil probe with two serial 1 mm long microcoils arranged 1 cm apart has been developed to facilitate peak trapping and sample band positioning during cITP/NMR.