Limited-sample NMR using solenoidal microcoils, perfluorocarbon plugs, and capillary spinning

This study demonstrates three improvements to mass-limited NMR using solenoidal microcoils as detectors: (1) sample confinement using liquid perfluorocarbon plugs to increase the observe factor, (2) design and incorporation of a capillary spinner to improve spectral line widths, and (3) facile sample changing via the use of a capillary insert. The probe is constructed to spin a fused silica capillary of 530 microns i.d., 700 microns o.d. inside a solenoidal coil wound around a 0.8 mm i.d., 1 mm o.d. glass capillary. The smaller capillary contains the sample, and capillaries with different samples can be exchanged easily. In high-resolution limited sample microcoil NMR studies published thus far, the length of the sample plug has been 7-10 times the length of the solenoid to avoid line broadening from volume magnetic susceptibility (chi v) mismatches at both ends of the sample. This arrangement is not efficient since it places most of the sample volume outside of the coil observe volume. It is shown here that the observe factor cannot exceed 23% if the sample plug is bracketed by air, without substantial line broadening occurring. However, if the sample is bracketed by two liquid perfluorocarbon plugs, the observe factor can be increased to 70% while maintaining high spectral resolution. This is equivalent to improving the limit of detection by a factor of 3, or reducing the total data acquisition time for a given signal-to-noise by a factor of 9. It is also shown that, for the 440-nL sample plug used in this study (bracketed by the perflurocarbon plugs), sample spinning can improve the spectral resolution from 1.5 (nonspinning) to 0.6 Hz (spinning). This corresponds to a further improvement in the limit of detection of 2.5, or just over a factor of 6 decrease in data acquisition time.