In comparison with handling of samples for other spectroscopic techniques samples for NMR analysis require special care. Failure to adhere to recommended practices can result in gross errors or even accidents. In the present article some guidelines are suggested to help avoid such situations.
Sample size of 1-10mg is sufficient for proton NMR of an organic compound in a short time span of around 10-15 minutes. Larger quantities (5-50mg) are needed for low sensitivity 13C NMR measurements and field exposure time requirement is also longer (5-30minutes). Larger molecules such as polymeric materials can require sample sizes even above 50mg and still longer observation times.
Suspended Solid Particles
Suspended solids in a solution disturb the magnetic field homogeneity resulting in band broadening. All solutions should be filtered using appropriate filters to remove any suspended solids before introduction into NMR tubes.
The NMR tube should be filled to an appropriate depth of 4.5-5.0cm. A scale can be used for the purpose but if a mark is made on the tube for consistent filling it should be cleaned with an appropriate solvent and dried before insertion into the sample tube cavity.
The shim coils correct for distortions in the magnetic field. Low sample depths can require increased shimming times whereas excessive filling can add to the deuterated solvent costs which are expensive.
The NMR tube should be clean from both inside and outside. Solid particles sticking to outside walls can disturb magnetic field homogeneity. Routinely the tube should be washed with water, followed with acetone and dried. It should be cleaned with glass wool. A brush should not be used for cleaning as it can erode the tube leading to impairment of spectral resolution. Tube cap contamination of sample cannot be ignored. It is advisable to cover the top with a Teflon tape and then cap it. Never invert the tube for mixing but instead use a vortex mixer.
Never write the sample details along the tube length. It is advisable to wrap a sticker or tape label around the top end of the tube or even better if the tape is affixed to the cap. A flap or dangling loose paper label should not be affixed to the tube as the tube spins at a high rate inside the magnet cavity.
Oxygen in the air has paramagnetic properties and any air dissolved in the sample should be removed by degassing for analysis requiring high sensitivity. The sample should be prepared in an inert environment and run at the earliest after preparation.
It is advisable to test the sample solubility in an ordinary protonic solvent first and then adopt the corresponding deuterated solvent to save on the higher cost of deuterated solvents. Most of the solvents are toxic in nature so you need to familiarize with their safety aspects before handling them for sample preparation.
It goes without saying that full care and precautionary measures need to be adopted in sample preparation to ensure consistent sensitivity and safety of operations.