UCSD SSPPS NMR Facility: Care and cleaning of NMR tubes

Every NMR user ends up with a collection of used NMR tubes. These tubes are often cleaned and used over and over, however, some care needs to be taken when cleaning tubes to prevent them from being damaged. Damaged tubes give poorer quality spectra, and may even break the spectrometer probe. This post provides some guidelines on best practices for cleaning NMR tubes.

The purpose of an NMR tube is to hold the sample in a perfect cylinder within the magnetic field. Any deviation will result in broadening of the peaks and a loss of resolution. For this reason any cleaning procedure that will affect the concentricity or camber of the NMR tube needs to avoided.

Several NMR tube manufacturers provide guides to cleaning¹, and most users will have their own cleaning protocols that incorporate many of these steps. To some extent the steps taken depend on what was in the tubes and what the cleaned tubes will be used for. In general, however, cleaning requires the following four steps:

1. Remove old solution

The old solution should be removed from the tube before the solvent has evaporated. This prevents the solute from sticking to the surface of the tube, making cleaning more difficult. Extra long Pasteur pipettes that will reach to the bottom of an NMR tube can be purchased to help with this². It is also possible to use a flame to melt a standard glass Pasteur pipette and draw out an extended pipette.

2. Soak

If the old solution was not removed before the solvent evaporated it may be necessary to soak off any deposits. If the original solvent does not remove the material, aqua regia or piranha solution may be used. Do not use chromic acid to clean NMR tubes. Chromium is paramagnetic and residual ions in your NMR tube will lead to peak broadening. Do not use base to clean tubes as it will etch the glass. Brushes are best avoided as they may scratch the internal surface of the tube.

3. Rinse

After removing the old sample, the tube needs to be rinsed. Using a series of solvents of differing polarity will ensure that everything is removed. The last solvent used should be one that evaporates easily. A final rinse with the deuterated solvent that will be used for the next sample will leave a smaller residual solvent signal.

Devices for rinsing 3 mm and larger tubes are commercially available³. These use a vacuum to move solvent through the NMR tube. Several papers describing how to do this with standard lab equipment have appeared recently⁴. For 1.7 mm NMR tubes, one of the facility's users, Brenda Andrade, used a length of teflon tubing projecting from a vacuum flask to remove solvent from the capillary tubes, like the commercial device from Wilmad^3b.

4. Dry

Following rinsing, the tubes need to be dried. Drying under vacuum is the best option. Using an oven may cause the tubes to bend. While the increased camber may not be visually detectable, a distortion of 0.3 mm is large enough to exceed the tolerances of modern NMR probes and may damage the spectrometer.

These guidelines are written with 5 mm tubes in mind. For 1.7 mm tubes I generally recommend they be treated as disposable. The quality of 1.7 mm tubes is comparable to economy 5 mm tubes, and at less than $5.00 each the cost of 1.7 mm tubes is probably less than the cost of the solvents and time spent cleaning them.

References

1. (a) Wilmad - Proper Cleaning Procedures for NMR Tubes

(b) New Era - Cleaning NMR sample tubes

2. (a) Norell - Pasteur Pipettes for NMR Tubes

(b) ChemGlass- Glass Pasteur Pipettes, NMR Tubes

3. (a) Wilmad - Economy Tube Washers/Cleaners for NMR and EPR Sample Tubes

(b) Wilmad - Universal Solvent Jet Tube Cleaner

4. (a) Vacuum Desiccator as a Simple, Robust, and Inexpensive NMR Tube Cleaner
Thanh Binh Nguyen
Org. Process Res. Dev. 2016, 20, 2, 319

(b) Easy-to-Assemble NMR Tube Cleaner Made from Common Laboratory Equipment
Sean C. Butler
J. Chem. Educ. 2021, 98, 10, 3405–3408