NMR Sample Preparation
If I Can Prepare an NMR Sample So Can You
(Slightly Modified from the GPL Wikipedia article)
- Typically only a small amount of sample is required (e.g. 10-20 mg), however depending on the NMR experiment performed, a smaller or larger quantity can be used.
- The appropriate solvent is added. For 1H NMR this will usually be a deuterated solvent such as CDCl3. The amount of solvent required will typically be the amount required to fill the tube by 4-5 cm. It is possible to do nmr with non-deuterated solvents but this is not straightforward and requires extra knowledge about the instruments.
- The sample may be sonicated or agitated by drawing and expelling from a Pasteur pipette to aid dissolution.
- Ideally the sample should not contain any suspended matter and if required the solution should be filtered. This can be done by forcing the solution through a plug of celite in a fresh Pasteur pipette directly into the NMR tube.
- The NMR tube is loaded with the sample to the required height.
- Modern NMR machines can reference to solvent peaks, and so an internal reference is not usually required. When an internal reference is required use TMS or, for H2O, DDS.
- The NMR tube is then usually sealed with a polyethylene cap, but can be flame sealed or sealed with a Teflon 'Schlenk' tap or even a very small rubber septum. Parafilm may be wrapped around the cap to reduce solvent evaporation or special tight-seal caps may be used. The tube should be wiped clean and labeled.
NMR tubes are hard to clean because of their small bore. Tubes are cleaned best before the sample has dried.
Alternatively, an NMR tube cleaning apparatus can be used. The NMR tube 1 (with the cap 3 fixed to the base of the NMR tube), is place upside down on the apparatus. The NMR tube fits over an inner tube 5 linked to the solvent reservoir 6. The NMR cap rests on the outer tube of the apparatus 4. A vacuum is applied (usually via a water aspirator via the vacuum inlet). The NMR tube cap forms a vacuum seal. Solvent 7 is drawn from the solvent reservoir 6 and is forced to the base of the NMR tube and flushes the NMR tube out 9 with solvent cleaning it. Note to complete the vacuum a flask is attached to the NMR tube cleaning apparatus.
- The tube is rinsed with the (non-deuterated) solvent used to dissolve the initial sample. If unknown, dichloromethane or acetone are good choices because dichloromethane is similar in polarity to chloroform, a common NMR solvent, while acetone dissolves many organic compounds.
- If traces of contaminants still exist, the tube is sonicated with an appropriate solvent, and scrubbed with a pipe cleaner.
- If necessary, the tube may be filled with an oxidizing solution of aqua regia or piranha solution (H2O2/H2SO4, not chromic acid!)
- Once the NMR tube is determined to be clean, it is triple-rinsed with distilled water, acetone, and left to air-dry or dry on a flat surface in an oven at low temperature (35oC).