Sample Preparation

• NMR tube:

All our solution NMR instruments require 5 mm tubes, where 5 mm refers to the outer diameter of the tube. The length of the NMR tube is usually 7 or 8 inches (17.8 or 20.3 cm). NMR tubes are available from Stores in Chemistry Department or can be ordered through UCL MyFinance. To avoid instrument downtime, you must use NMR tubes free from defects, such as scratched surface, cracked, chipped or fractured top, etc.  NMR tubes must not be inserted into oil baths at any stage. Avoid drying your NMR tubes in a hot oven. Washing NMR tubes with acetone at the final stage of cleaning, then leaving them upside down in a fume hood overnight is usually sufficient. Please make sure that the surface of NMR tubes is clean before taking them into the NMR lab. This is to avoid dirt building up inside the probe in the magnet. Once the sample is ready for running NMR experiments, you must clean the surface of the tube using acetone. In the NMR lab, please wipe the bottom ~10 cm of your tube with a medical wipe before placing it in a sample changer.

• Amount of sample:

The sample amount depends on the experiment you are performing. For proton NMR spectra of organic compounds with a molecular mass less than 600, the quantity of material required is usually 1-10 mg. A higher concentration could lead to broad or asymmetric lines if the sample is not mixed well and the sample concentration varies along the solution height in the NMR tube. For samples with colour, this is often visible. Shake your samples well to avoid the concentation variation along the solution height in the NMR tube. For ¹³C NMR, the higher the concentration the better, since it is ~6000 times less sensitive than ¹H. For a satisfactory signal-to-noise ratio, ~3 mg per inequivalent carbon is needed on 500, 400 and 300 MHz instruments with room temperature probes. As an example, there are 4 inequivalent carbons in phenol. On halving the sample quantity, the number of scans (NS) can be increased 4 times to achieve the same level of the signal-to-noise ratio. On the 600 and 700MHz instruments with cryoprobes, there is usually no need for changing NS if you have >5 mg of sample dissolved in ~0.5 mL of solvent.

• Solvent:

If you are not sure which solvent to use, start with non-deuterated solvents until you find the appropriate solvent. The most used NMR solvent by organic chemists is deuterated chloroform (CDCl₃), but it could decompose acid sensitive compounds. As there is only one deuterium atom per molecule in CDCl₃ and the molecular weight of chloroform is relatively high, the ²H signal search at the start of the NMR experiment may fail for very concentrated samples or those with paramagnetic impurities. Other popular solvents include D₂O, H₂O+D₂O (9:1 in volume), acetonitrile-d₃, acetone-d₆, benzene-d₆, toluene-d₈, DMSO-d₆, THF-d₈ and CD₂Cl₂. Some of these solvents are available from Stores in Chemistry Department (contact Tony Field, uccaaac@ucl.ac.uk). In some cases, you could also use usual "protonated" solvents with ¹H instead of ²H. These will require special solvent suppression experiments depending on the number of solvent peaks in the ¹H NMR spectrum. Please see (8) in http://www.ucl.ac.uk/nmr/problems describing an example when H₂O is used as a solvent. Following these instructions, you could run your samples in other "protonated" solvents. However, you need to consider such issues as distortions arising from the solvent suppression and chemical shift calibration.

• Amount of solvent:

Dissolve your sample in a solvent selected by you. Mix it well and transfer it into an NMR tube. Filter the solution of your sample into the NMR tube if there are undissolved parts left. An appropriate volume is 0.5 mL or 40 mm solution height in a 5 mm NMR tube. 

• Water in solvents:

Solvents are likely to contain a small amount of water when purchased and once the solvent container is opened, the water content will increase due to absorption from the atmosphere. You could store your solvents over molecular sieves, but be aware of the particles from the sieves. The chemical shift of water and other trace impurities in various solvents can be found by searching for "NMR chemical shifts of solvents".

• Cap and label:

Ensure that the polyethylene cap is pushed fully onto the tube (avoid cuts and injuries!). Do NOT use/attach any paper label, glue, sellotape, parafilm, Teflon tape, any other tape, improper sealing or anything else with/to the NMR tube. Seek advice if you do need to use parafilm. A thin strip of parafilm (not more than two layers) may be wrapped around the joint between the tube and the cap, not lower than 3 mm from the bottom of the cap. Using an ultra-fine tip permanent marker, a short code can be written at the top of the NMR tube, not lower than 3 cm from the top of the NMR tube. You could also label the cap both on the top and on the side using permanent markers. 

• Sample collections and cap colours: 

The NMR lab must not be used for sample storage and NMR tubes must be collected from the lab without delay, as some solvents, such as CDCl₃, evaporate, even with parafilm wrapped around the cap. Samples for which experiments have finished can be found in sample changers, beakers on the workstation desks and in the old fridge opposite to the 300 magnet. For ease of identification, NMR tube caps of different colour must be used by the research groups, e.g. to distinguish 5 different groups from KLB using aqua blue caps, please write letters A (the EA group), W (JDW), K (KS), S (TDS) and C (VMC) using a permanent marker on the top of the cap. The cap colours assigned to different groups are: