Multiple Electrically Decoupled Coils

The first liquid-state application, reported by Fisher et al. [18], used two coils, both tuned to the carbon frequency of 75.44 MHz at 7.4 T. The coils were 

The concept of using decoupled coils has been developed further by Li et al. [19]. In this case, an RF switch was used for time-domain multiplexing of the signals into a single receiver channel. A four-coil system was constructed for operation at 6 T in a wide-bore (89 mm) magnet. Each coil was fabricated by using 17 turns of 50 pm diameter copper wire wrapped around a 180 pm i.d., 355 pm o.d., polyimide-coated fused silica capillary, giving an observe volume 

Another CE separation method that has been adapted to on-line NMR detection for trace level separations is capillary isotachophoresis [22]. In this case, after the separation, the analyte bands are slowly moved through the capillary until they lie directly within the coil. Precise positioning of the analyte bands in the NMR detection coil can be difficult. A recent enhancement is the use of several NMR detection coils on a single separation capillary [23], In this way, the first coil acts as a scout coil and is optimized for sensitivity (not necessarily linewidth) to locate the analyte band as it moves through the coil. After an analyte band is detected, the flow is stopped after the appropriate time-interval so that the analyte bands are now located in the second coil, which is used to acquire high-resolution NMR spectra. 

The use of capillary separations, an NMR probe that contains multiple coils, and the associated capillary fluidics to deliver the samples to and from the coils is the next step in probe development. A future exciting development will be the interfacing of such intelligent NMR probe and fluidic systems with other integrated detection modalities such as fluorescence, absorbance and mass spectrometry to provide an integrated system capable of delivering unprecedented structural information from complex samples. 

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