Here are your options on collecting NMR data when you have only a limited amount of sample.
1. Use a standard setup and get more sample or collect more scans. Yeah yeah..... I know ..... if you could do this you wouldn't have a problem.
2. Reduce the volume of the solution keeping the same mass and lift the NMR tube such that all of the sample is in the receiver coil. This is not a very desirable option as the magnetic susceptibilities of the solution inside the coil and the air outside the coil are very different and will result in a severe distortion of the magnetic field. It is very difficult to shim the magnet to compensate for this effect and the resolution and signal-to-noise ratio in your spectrum will suffer greatly.
3. Reduce the volume of the solution keeping the same mass and use a "Shigemi" tube. These tubes have plastic parts which go above and below the sample which is centered in the receiver coil. The plastic parts are chosen to have the same magnetic susceptibility as the solvent used to prepare your solution.
4. Reduce the volume of the solution keeping the same mass and use an NMR tube of a reduced diameter with a probe designed for such a tube. This is an option we have at Ottawa U. on the Varian INOVA 500 for which we have a 3 mm probe.
5. Use a Cryogenically cooled probe. These probes have cryogenically cooled coils and electronics which dramatically reduce the amount of noise thereby increasing the signal-to-noise in the spectrum. They are very expensive to both purchase and maintain and not currently an option at Ottawa U. Maybe we can ask Santa.....