One field where these are found is nuclear magnetic resonance (NMR) spectroscopy, where it identifies radio frequency (RF) coils that are smaller than 1 mm.
The detection limits of micro-MRI or MRM can be pushed further by taking advantage of microsystem fabrication techniques. In general, the RF receiver coil should closely conform to the sample to ensure good detection sensitivity. A properly designed NMR probe will maximize both the observe factor, which is the ratio of the sample volume being observed by the RF coil to the total sample volume required for analysis, and the filling factor, the ratio of the sample volume being observed by the RF coil to the coil volume.
The miniaturization of NMR probes thus involves two advantages:
# Increased sensitivity without which the analysis of such low concentration compounds would be impossible, and
# Increase of filling factor by matching the probe to the sample volume. Still, the extraction of the NMR spectra of samples having smaller and smaller volumes is a real challenge. Either these reductions of volume are dictated by the difficulties of production of sufficiently large samples or by the necessities of miniaturization of the analysing system, in both cases a careful design of the radiofrequency coils, ensuring an optimum reception of the NMR signal, are required.