FAQs

• Consider purifying solvents used for chromatography or crystallisation such as petrol; nearly all commercial solvents contain involatile impurities which are insignificant when working with multi-gram quantities of compound, but become very objectionable when working on the small scale quantities high-field NMR has made possible.

• Precision 5 mm tubes must be used (available from CRL Stores). These should be periodically checked for cracks and for scratches around the bottom of the tubes, and discarded if necessary.
• NMR tubes that are shorter than 7 inches are already broken and should be discarded.
• Samples must be made up to a solvent depth of 4.0–4.5 cm. If you allow the liquid column to fall below 4 cm, the instrument will find it more difficult to lock and shim. Making samples too deep, although less detrimental, will dilute your sample and waste solvent.
• It is good practice to filter your samples before placing them in the NMR tube as floating debris or a cloudy solution often yield poor spectra.
• Ensure that sample tubes are clean on the outside before placing them in the instrument, to avoid contamination of the probe (which is a very serious matter!). If you break a tube whilst loading a turbine, clear up the mess with some methanol. Do not use chlorinated solvents as these will destroy the depth gauge plastic!

Open-access

You will typically require the following sample quantities for acceptable results:

• 200 MHz - ca. 5 mg
• 400 MHz - ca. 2 mg for ¹H, or 20 mg for ¹³C

When handling oils, the quantity drawn up into a pipette by capillary action should be sufficient for a ¹H spectrum.

Service

Service samples are most often run on the NEO600, which is equipped with a cryogenic probe and is thus substantially more sensitive than the open-access machines. This allows us to handle samples which are very dilute, going down to sub-milligram quantities.

Nevertheless, it is still in your best interests to submit as much sample as can be spared — this means that you are more likely to get your data back faster, and will also yield spectra of higher quality which are easier to interpret.

• Make sure you choose an appropriate deuterated solvent — in particular, check their boiling and melting points to ensure that your solvent is still a liquid at the temperature you intend to study it at. Do not go within 10 °C of the boiling or melting points. Common solvents used for high-temperature work are toluene-d₈ or DMSO-d₆; and for low-temperature work, CD₂Cl₂, MeOD or DMSO-d₆.
• Use only NMR tubes made from Class A glass ("Pyrex"), such as the Norell S-400 ("Select Series") tubes. These can be obtained from Stores. Cheap / disposable tubes can warp or break at higher temperatures and will be rejected if submitted to the service.