Guide: Preparing a Sample for NMR analysis – Part I

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful technique used in various scientific fields and it is an indispensable tool for scientists to elucidate and probe the structural and chemical properties of molecules. Whether you are a beginner or an experienced user, it is paramount to prepare a high-quality NMR sample to obtain the most accurate and reliable results possible. In solution-state NMR, selection of the correct NMR solvent and ensuring an appropriate sample volume in the NMR tube are crucial factors to obtain good quality spectra.

Recommendations

Before preparing the NMR sample, ensure that all the glassware and tools are clean and dry to avoid external contaminants.

It is critical that the solvent does not interfere significantly with the signals of interest in the spectrum. Furthermore, one must consider its purity, accessibility, reactivity, and capacity to dissolve compounds to form a homogeneous solution. In solution-state NMR, it is standard to use a deuterated solvent to dissolve the samples. In proton (¹H) NMR, using deuterated solvents avoids overwhelming the signals of interest with those arising from the solvent. Deuterium (²H) has a different resonance frequency than proton in an NMR experiment, making it “invisible” in the ¹H NMR spectrum.

In general, always keep in mind that ‘’like dissolves like’’, meaning that polar solvents will dissolve polar compounds, and non-polar solvents will dissolve non-polar compounds.¹ There are several deuterated solvents available, and we list the most common ones and some of their properties below.

Another benefit of using a deuterated solvent is that the instrument can lock onto its deuterium signal, meaning the instrument can correct for minor deviations in the magnetic field whilst conducting an experiment. Deuteration is never absolute, hence, there will always be the presence of the mono-proteo version of the solvent.³ However, the residual solvent signal can negate the need for a reference signal such as tetramethylsilane, TMS (δ = 0 ppm). After the addition of your solvent to your sample, you will need to ensure that the solution is homogeneous and does not contain any particles in suspension. There are multiple techniques to help achieve this, and we will discuss these in our next blog entry.

The final step is to ensure that the volume transferred to the NMR tube is sufficient for the experiment.

Sample preparation is often overlooked by most users, but it is a very important step to obtain good quality spectra. Please feel free to contact us regarding the preparation of NMR samples or to ask us questions about how this can benefit your chemical applications and don’t forget to check out our next blog entry for tips on how to remove particles from your sample.

References

[1] S. A. Richards; J. C. Hollerton, Essential Practical NMR for Organic Chemistry, Hoboken, NJ: Wiley, 2011, p. 11 – 21.
[2] Blog: To D₂O or not D₂O? (Accessed February 7, 2024)
[3] Blog: Deuterated Solvents (Accessed February 15, 2024)