Nanoliter volume, high-resolution NMR microspectroscopy using a 60-mu m planar microcoil

Previous studies demonstrated the feasibility of using 100-mu m inner diameter planar spiral inductors (microcoils) as detectors in H-1 nuclear magnetic resonance (NMR) microspectroscopy. However, high-resolution NMR applications were not possible due to poor spectral resolution and low signal-to-noise ratio (SNR). These limitations in performance have now been largely overcome by using a nonconductive liquid fluorocarbon (FC-43) to minimize the effects of susceptibility mismatch between materials, and by carefully optimizing the microcoil geometry for maximum SNR. In this study, liquid samples were loaded into a fused silica capillary (75-mu m inner diameter, 147-mu m outer diameter). The capillary was positioned 50 mu m above a 3.5-turn microcoil so that approximately 1 nL of the sample was present in the sensitive region of the microcoil. The microcoil was fabricated on a gallium arsenide substrate with an inner diameter of 60 mu m, an outer diameter of 200 mu m, trace width of 10 mu m, trace spacing of 10 mu m, and trace height of 3 mu m. At 5.9 T (250 MHz) in H-1-NMR microspectroscopy experiments using a spectral width of 1 kHz, 4096 sampled data points, and a recovery delay of 1 s, a SNR of 25 (per acquisition) and a spectral linewidth of less than 2 Hz were obtained from a sample of water. These results demonstrate that planar microcoils can be used for high-resolution NMR microspectroscopy. Such coils may also be suitable for localized NMR studies at the cellular level and as detectors in capillary electrophoresis or microbore liquid chromatography.