Tubes and Sample Volumes

Tubes and sample volumes

The price of NMR tubes are surprisingly high. The prices reflect the need to produce tubes that conform to strict requirements of straightness (camber) and concentricity, as deviation from these can produce undesirable artifacts in spectra, usually in the form of ‘spinning-sidebands’.
Generally, the higher quality (and more expensive) tubes are required on higher field instruments, and this is particularly so for proton work.
The diameter of the tube is generally determined by the dimensions of the probe to be used, or more precisely the probe’s rf coils.


Tube dimensions (outerdiameter)

Solute volume µ/mL

Solute column height/mm

10

2500

40

8

1500

40

5

500

40

3

150

40

1.7

30

20

1

5

10

Sample tube diameters and their associated working sample volumes and heights NMR tubes should be kept clean, dry, free from dust and must also be free from scratches on the glass as this distorts the cylinder in which the sample is confined. New tubes may not always clean when delivered, although this is often assumed to be the case.
For routine washing of tubes, rinsing with a suitable organic solvent, such as acetone, or with distilled water a few times is usually sufficient.
Tubes should not be dried by subjecting them to high temperatures for extended periods of time as this tends to distort them. If oven drying is used, the tubes must be laid on a flat tray and heated for only 30 min, not placed in a beaker or tube rack as this allows them to bend.
If the removal of all traces of water from the tube is important, then it is necessary to rinse the tube with D2O prior to drying to ensure the exchange of water adsorbed onto the surface of the glass. Unused tubes should be stored with their caps on to prevent dust from entering. Finally, it is also important to avoid contamination of the outside of the NMR tube (which includes fingerprint oils) as this leads to the transfer of the contaminants into the probe head. The accumulation of these contributes to degradation of the instrument’s performance over time.