NMR Spectroscopy

Preparing an NMR Sample

What you will need

To prepare an NMR sample for submission to the CTL 300 MHz spectrometer you will need the following:

all of which are available for collection from the prep room hatch as well as:

A photo sohwing a tray on the preproom hatch containing clean NMR tubes, NMR lids, submission lables and deuterated chloroform
A photo showing all the items required for preparing an NMR sample

How to make up the sample

1. Ensure that your compound is soluble in the solvent selected. If you are unsure then test the solubility of your compound using protic solvent first.

A photo showing two sample vials containing solvent. The left vial shows the compound fully disolved in the solvent. The right vial shows that the compound is insoluble
Both vials contain the same compound. The left is made up in Deuterated Water (soluble) the right in Deuterated Chloroform (insoluble)

2. Prepare a pipette filter with cotton wool and clamp this above an NMR tube so that the filtrate will run straight into the NMR tube.

3. Add an amount of compound to a sample vial. The amount required varies depending on the information you wish to obtain. Proton NMR requires very little sample (as 1H accounts for 99.9% of hydrogen atoms) whereas 13C NMR requires much greater amounts (13C is around 1.1% abundant). The molecular mass of your sample also makes a difference as it is the concentration of the solution that is important, but for most low molecular mass compounds this makes little difference. The same sample can be used to record multiple NMR experiments. Typically 20 mg of compound is sufficient to record proton and carbon NMR spectra.

4. Add approximately 1 mL of detuterated solvent using a Pasteur pipette and use the pipette to dissolve the sample.

Full solvation of the product can be aided buy continuously sucking up and dispensing the solvent in the sample vial.

5. Transfer the solution into the pipette filter and allow the solution to run through into the NMR tube.

If the liquid is struggling to pass through the cotton wool plug, a small amount of back pressure can be applied by adding a pipette teat to the top of the filter and squeezing.

6. Ensure there is 4 cm depth of solvent in the NMR tube. If there is insufficient solvent, top the tube up using a clean Pasteur pipette. Avoid overfilling NMR tubes as the solvents are expensive. Once happy with the depth of the sample fit a NMR tube cap

7. Complete the relevant NMR sample submission label to your lab

8. Attach the label to the tube by feeding the tube through the holes then complete the NMR submission online form with the corresponding details to that on the label.

9. Leave the made up sample at the submission station opposite the prep room hatch, also make note of your sumbission details for locating your sample data at a later date

Preparing an NMR Sample

Points to note

Deuterated solvents are very expensive. Only use brand new clean Pasteur pipettes to take NMR solvents from the bottle. Even tiny amounts of contamination affect NMR results.

It is essential all samples are filtered to ensure samples are homogeneous. Any solid particles in a sample will result in poor NMR spectra.

Samples with insufficient depth result in poor NMR spectra being recorded. Excess depth is a waste of expensive deuterated solvent.