An NMR tube filled with a colorless sample, sealed with a green polyethylene cap and Parafilm

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.

Shigemi tubes

A Shigemi tube is a microscale NMR tube used with an ordinary-size NMR tube. Shigemi tubes may be appropriate for protein NMR experiments, where only a smaller sample is available. A corresponding smaller solvent volume is desired to maintain a higher sample concentration. The reduced sample depth is compensated for by solid glass on the NMR tube beneath the level of sample, which varies for the make of spectrometer. Once air bubbles have been expelled, the plunger is secured to the tube proper by parafilm. Ideally, the tubes are matched with the deuterated solvent used to have better spectrum resolution.^[1]

Plunger/insert centre, left 8 mm "bottom length" outer tube suitable for Bruker and Varian NMR spectrometers, right 15 mm outer tube suitable for Varian only]]

Fully assembled 8 mm Shigemi tube, insert secured to outer tube with parafilm

Close-up of the sample lying in between insert and "bottom length"

Sample preparation

Left to right: Flame, septum and polyethylene cap sealed NMR tubes

Usually, only a small sample is dissolved in an appropriate solvent. For ¹H NMR experiments, this will usually be a deuterated solvent such as CDCl₃. Sufficient solvent should be used in order to fill the tube by 4-5 cm (depending on the spectrometer).

The sample may be sonicated or agitated to aid dissolution, and solids are removed via filtering through a plug of celite in a Pasteur pipette, directly into the NMR tube.

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.

Cleaning

Aqua regia has been added to these NMR tubes to remove all traces of organic material.

NMR tube cleaning apparatus: 1: NMR tube, 2: Compound residue, 3: NMR tube cap, 4: NMR tube cap form a vacuum seal, 5: Cleaning Tube fits within NMR tube, 6: Solvent reservoir container, 7: Solvent/solvent level, 8: Vacuum applied, 9: Waste solvent .

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

Cleaning is performed usually by rinsing with the same (non-deuterated) solvent used to dissolve the initial sample. Dichloromethane or acetone are good choices because dichloromethane is similar in polarity to chloroform, a common NMR solvent, while acetone dissolves many organic compounds. Sonication and scrubbing with a pipe cleaner may helpful to remove traces of contaminants. If necessary, the tube may be filled with an oxidizing solution of aqua regia or piranha solution (H₂O₂/H₂SO₄). Chromic acid solutions are never used, due to traces of paramagnetic chromium left behind on the tubes causing interference with NMR experiments.^[2]

When 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 (left). 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,^[3] though it is costly and easy to destroy by shattering or breaking off the cleaning tube. Equivalent designs may be assembled from ordinary labware as well.^[4][5]

References

1. ^ "Shigemi NMR Microtube Assemblies". Sigma-Aldrich. Retrieved on 2008-06-27.
2. ^ "NMR-010: Proper Cleaning Procedures for NMR Sample Tubes". Wilmad. Retrieved on 2008-06-27.
3. ^ "Aldrich NMR Tube Cleaners". Sigma-Aldrich. Retrieved on 2008-06-27.
4. ^ "NMR Tube Cleaner and Drier". Torviq. Retrieved on 2008-06-27.
5. ^ Karsten Alstad Sørbye (2005-09-29). "NMR Tube Washer". spectroscopyNOW.com. Retrieved on 2008-06-27.

Gallery

A sample of polythiophenes in an NMR tube. Due to the high degree of conjugation, the sample fluoresces under ultraviolet light.	3 sealed NMR tubes	Teflon tap used to seal NMR tubes for air-sensitive samples - has a central bore starting at the top of the tap, which splits into a T-junction at the bottom. One of the openings of the inverted "T" can be seen in this picture.	A J. Young NMR tube attached to an adapter with a female 24/40 joint already greased. Note the hole resulting from the T-bore in the side of the PTFE plug.
A J. Young NMR tube from above looking down the hole that leads to the T-bore.