the coil. However, the filling factor of this coil was less than In this paper we show how the filling factor, and therefore 5%, meaning that high concentrations were necessary to the signal-to-noise ratio, of very small solenoidal microcoils achieve sufficient signal-to-noise (S/N). In this Communican be increased by almost an order of magnitude compared cation, we show that the filling factor of solenoidal microto previously published results, with minimal degradation of coils can be increased by up to a factor of seven, correspondspectral resolution. We also empirically determine at what ing to a reduction in data acquisition time of 50-fold, with dimensions the magnetic susceptibility of the coil materials minimal degradation in the spectral resolution. We anticipate produces substantial line broadening in high-resolution that these new coils will be particularly important in studying spectra.
both biological molecules that can only be used in very low Nuclear magnetic resonance spectroscopy is one of the concentrations and molecules with low solubility. We also most powerful methods available for determining molecular show that the filling factor can be increased further, to greater structure. Relative to many other analytical techniques, howthan 50%, but the linewidths achieved increased to more ever, it is inherently insensitive. For example, Fourier-transthan 3 Hz. form infrared spectroscopy commonly has limits of detection Solenoidal microcoils were constructed by wrapping wire (LODs) as low as 10 012 to 10 015 mol (1) and mass specaround fused silica capillaries. The wire used was 50 mm trometry has achieved LODs in the 10 018 mol range (2, 3), diameter, 99.99% copper (California Fine Wire, Grover but LODs for NMR are typically quoted as 5 1 10 09 mol Beach, California), and was coated with 6 mm thick polyure-(4). Many authors have shown that the mass sensitivity can thane. The fused silica capillaries (Polymicro, Phoenix, Aribe increased by using NMR ''microcoils,'' loosely defined zona) were available with various outer diameters and wall as coils having a diameter of 2 mm or less (5-10). There thicknesses, and were used without removal of the polyimide remain, however, fundamental questions concerning the coating. The capillaries were first wiped to remove contamiminimum number of spins that can be detected inductively, nants, rinsed under a high-pressure water jet, washed with and still result in high-resolution NMR spectra.
deionized water, sprayed with compressed air to remove Rugar et al. (11) have shown that the mechanical detecdust, and allowed to dry. The capillary was then centered in tion schemes proposed by Sidles ( 12) can significantly rea pin vise and the pin-vise holder was tilted upward at an duce the LODs for both electron paramagnetic resonance angle of approximately 20Π. A 5 cm length of wire, weighted and NMR. However, due to the inhomogeneous nature of at both ends, was draped over the capillary for two turns. the magnetic field, high-resolution spectroscopy is not yet
The wire was glued to the capillary using nonpermanent possible with this technique. Wu et al. (13) used a number cyanoacrylate adhesive and allowed to dry. One weight (3.0 of solenoidal microcoils with diameters in the range 355 to g) was then removed, and the pin vise was rotated to start 695 mm diameter for both static and dynamic NMR measurewrapping the coil. Details of the actual wrapping process ments, but only achieved linewidths on the order of 7-10 have been published elsewhere (14,15). For an n-turn coil, Hz. The most successful results so far have been obtained
12 / n turns were wrapped, and the final turn was glued by Olson et al. (14), who showed that a solenoidal microcoil with permanent adhesive. The nth from final turn was glued (355 mm diameter) increases the mass sensitivity by more with the nonpermanent adhesive and the entire assembly was than two orders of magnitude when compared with a stanallowed to dry. In the final step, the remaining weight was dard 5 mm probe. High spectral resolution was achieved via a perfluorinated ''susceptibility matching'' fluid surrounding removed, the extra turns were unwound up to the nth from 83