The Importance of Mixing Your NMR Samples

It is very important that your NMR samples are mixed well before NMR data are acquired. If the sample has a concentration gradient (i.e. more concentrated at one end of the sample column compared to the other) it will be very difficult to shim the magnet over the entire volume of the sample as the magnetic susceptibility is not constant over the sample volume. As a result the NMR lines may be skewed and will be much broader than necessary. This will lead to a much lower signal-to-noise ratio based on signal heights. The figure below shows partial 300 MHz ¹H NMR spectra for 2-bromobutane in CDCL₃. The spectrum on the left was acquired on a sample where 1 drop of 2-bromobutane was added to CDCL₃ in an NMR tube. The tube was gently swirled but not shaken. The magnet was shimmed using a gradient shimming routine and the data collected. The spectrum on the right was acquired on the same sample except the tube was removed from the magnet, shaken and reinserted. The magnet was reshimmed with the same gradient shimming routine and the same number of scans were collected. The difference in the quality of the NMR data is obvious.

A Concentration Gradient Across Your Sample Leads to Poor NMR Data

While emptying the carousel of the AVANCE 400 daily, I notice that some samples have a concentration gradient (i.e. the sample is more concentrated at the bottom of the sample column compared to the top). The samples are not mixed. Such samples will always give NMR spectra of a lower than necessary quality. Why? - because the gradient shimming routine used on the spectrometer uses a magnetic resonance image of the deuterium concentration across the tube. If the concentration of the solvent is not constant across the sample volume, then the deuterium gradient shimming routine will not do well and you will get a poorly resolved spectrum. In fact you will also get a poor spectrum when manual shimming is carried out based on the lock signal.

Mix your samples well - get better data.