NMR tube

An NMR tube is a thin glass walled tube used to contain samples in Nuclear magnetic resonance spectroscopy. Typically NMR tubes come in 5 mm diameters but 10 mm and 3mm samples are known. It is important that the tubes are uniformly thick and well-balanced to ensure that NMR tube spins at a regular rate (i.e. that they do not wobble), usually about 20 Hz in the NMR spectrometer.

Sample preparation

1. 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.
2. 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.
3. The sample may be sonicated or agitated by drawing and expelling from a Pasteur pipette to aid dissolution.
4. 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. Filtering of deuterated solutions should be avoided due to the cost of these solvents.
5. The NMR tube is loaded with the sample to the required height.
6. Normally the deuterated solvent used will contain an NMR reference standard (such as TMS). However, if an NMR reference standard is required, this can be added carefully to the sample. Modern NMR machines can reference to solvent peaks, and so an internal reference is not always required.
7. 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. The tube should be labeled.

Cleaning

NMR tubes are hard to clean because of their small bore. Tubes are cleaned best before the sample has dried.

1. 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.
2. If traces of contaminants still exist, the tube is sonicated with an appropriate solvent, and scrubbed with a pipe cleaner.
3. If necessary, the tube may be filled with an oxidizing solution of aqua regia or piranha solution (H₂O₂/H₂SO₄, not chromic acid!)
4. Once the NMR tube is determined to be clean, it is triple-rinsed with distilled water, acetone, and left to air-dry or dry in an oven at low-temperature.

NMR tube cleaner

A better alternative to the use of potentially hazardous oxidizers is an NMR tube cleaner (right). It is an apparatus which uses a vacuum to flush solvent and/or a detergent solution through the entire length of the NMR tube.

In this apparatus, the NMR tube 1 (with the cap 3 fixed to the base of the NMR tube), is placed 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.

This sort of apparatus is commercially available, though it is costly and easy to destroy by shattering or breaking off the cleaning tube.

See also

• Laboratory glassware

Gallery