Here are some tips and tricks to make your life easier and solve some of the common problems encountered when submitting samples to the NMR Service or using one of the spectrometers.
Preparing your sample - When making up your sample try to avoid floating impurities or other solid material in your NMR tube. These will affect the quality of your spectrum. If necessary, filter your solution before putting it in the NMR tube. Aim to get about 0.7ml of solution in your NMR tube. The individual vials of solvents that the NMR Service sell are 0.75ml, sufficient for one sample at the correct depth in the tube. Too little solvent means that the air-liquid boundary may fall in the area of the probe coils leading to field inhomogeneity. Too much solvent means your sample concentration is lowered leading to a requirement for a longer acquisition time.
NMR tubes - Your NMR tubes should be straight, free from scratches, and a minimum of 17.5cm in length. This will mean that you can use your tube in any of the systems with a sample changer. Short tubes may fail to be picked up by the sample changer, which could stop the automatic run. You should always use Wilmad 528PP tubes in the departmental spectrometers - other makes of tube may slide too easily in the spinners, break more readily and not be manufactured to such fine tolerances leading to inhomogeneity. The NMR Service reserve the right to remove any non-standard tubes from the open-access spectrometers. You should take care when placing a cap on your tube, especially if the top of the tube is chipped. We sometimes have second-hand Wilmad 528PP tubes for sale at half the normal price - please ask in B28.
Inserting your tube into a spinner - You should take care when you insert a tube into a spinner (turbine). Grip the tube towards the bottom and gently insert into the spinner. Do not exert too much force - you may find that rotating the tube as you push it helps. Keep your hand near to the spinner as you push the tube further into it. Gripping the tube at the top and attempting to insert it into a spinner can create unnecessary forces which may cause the tube to snap with the potential for injury. When using a ceramic spinner for variable temperature work you should take extra care as they tend to be tighter than the normal spinners.
Breakages - We are all human, and sometimes things go wrong. If you break a tube in an NMR room, please try and clean up as much as possible, collecting the broken glass and disposing of it in a safe manner. If you break a tube when it is in a spinner, please bring the spinner to the NMR Service in B28. We have a tool for removing the glass from the spinner, and can clean it and return it to service. Please do not throw the spinners away - they are very expensive to replace. If a tube breaks inside the magnet, you should stop work immediately and inform the NMR Service. If this occurs outside normal working hours, then you should leave a large note on the instrument to warn others not to use it. Please email or leave a note for us - we will endeavour to clean inside the magnet and probe and restore the instrument to service as quickly as possible.
At the spectrometer - When you have inserted your sample into the magnet it is good practice to load the default shimset. The previous user of the instrument may have been working with an unusual sample and the shims may be a long way from their default position. This could make the sample difficult to lock and shim. Type rsh and select the shimset named current or similar. This will provide a good starting point for shimming your sample. You should not assume that the probe is tuned correctly either. Check the tuning using atma where fitted or by typing wobb and manually adjusting the knobs on the underside of the probe. If you have any problems with locking and shimming your sample, or tuning the probe then please see us in B28 as we will be able to help you get the best possible result.
Variable temperature NMR - When running experiments at very high or low temperatures you should use a ceramic spinner. If you are using the BB ATM 500 spectrometer (Glengrant), you should use the bore gas system which helps maintain the bore and o-rings in the magnet nearer to room temperature. Please see the NMR staff in B28 if you do not know how this works. You should use the default configuration files when changing between temperatures, as these have heater power and gas flow calibrated. If you need a different temperature, select the nearest temperature file, and just change to the exact temperature required manually. The chiller units will lower the temperature to -5C, below this you will need to use the liquid Nitrogen tank and apparatus. See the NMR Staff if you have not used this before. When using the TCI 500 spectrometer (Glenlivet), please be aware of the upper temperature limit of the probe which is 45C. Increasing the temperature beyond this limit will trigger the Cryoprobe to warm itself up, thus ruining your experiment.
Filling in a submission form - The sample submission form (available on the NMR Home page) helps us decide on the length of experiments we will run. Please take a little time to fill it in accurately and correctly. The sample weight and molecular weight of your sample are the most important pieces of information. The difference between 2mg and 0.5mg could be the difference between hundreds and many thousands of scans for a carbon experiment. If you don't know the exact molecular weight then make a rough calculation to the nearest hundred. Don't forget to fill in your solvent details - we don't like having to sniff your tube to find out. Try to write clearly, especially your name, telephone number and email address - we sometimes need to contact you before running your sample. Your personal code should relate to an entry in your lab book, and this is what we will name the experimental files prefixed by your group initials. For example jrn-abc1-25-prep. Please include the structure if you know it, it does help us to decide if the NMR experiment is successful or otherwise.