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

Previous studies demonstrated the feasibility of using 100-μm inner diameter planar spiral inductors (microcoils) as detectors in ¹H 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-μm inner diameter, 147-μm outer diameter). The capillary was positioned 50 μm 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 μm, an outer diameter of 200 μm, trace width of 10 μm, trace spacing of 10 μm, and trace height of 3 μm. At 5.9 T (250 MHz) in ¹H-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.