RF microcoil design for practical NMR of mass-limited samples

This paper addresses practical issues involved in obtaining high resolution 1H NMR spectra from samples containing less than 10 nmol. Solenoidal microcoils have been constructed to: (a) assess the effects of magnetic susceptibility mismatches at 500 MHz, (b) increase the concentration sensitivity of microcoil probes, (c) incorporate a lock channel for 2D experiments and long 1D acquisitions, and (d) assess the total amount of the sample required (with respect to the coil length) to avoid line broadening due to edge effects. Compared to previously published microcoil results, sample volumes have been increased by a factor of 20 with a concomitant decrease in the required concentration (5-20 mM). Perfluorocarbon susceptibility matching remained effective at 500 MHz, allowing acquisition of high resolution NMR spectra. A lock channel has also been successfully incorporated in microcoil probes. The limits of detection for sucrose with a 10 min acquisition time were found to be 17.8 and 34.1 pmol for the single and double resonance coils, respectively. A sample length of approximately 10 times than that of the coil was required to avoid magnetic susceptibility artifacts.