Guide: Preparing a Sample for NMR Analysis – Part II

Nuclear magnetic resonance (NMR) spectroscopy is a valuable tool for analyzing molecules, and thus making good NMR samples is essential to obtain good spectra. In Part I of this guide, we touched upon several solvents commonly used to dissolve samples for NMR experiments. In addition, we highlighted the optimal volume for obtaining a good spectrum. In this part, we will delve into techniques that ensure sample homogeneity, and we will discuss the impact of the quality of the NMR tube on data quality.

Recommendations
As a precaution, ensure that all the glassware and tools used are clean and dry before preparing the NMR sample to prevent contamination of your sample.

In NMR, sample homogeneity is important to maintain the uniformity of the magnetic field, which allows for reliably obtaining high quality spectra.¹ Samples containing solid particles can disrupt this uniformity leading to spectra with broad lines. Reducing the broadness of these lines, even with processing, is not always possible.² Thus, this can make interpretation of signals in NMR spectra more difficult, as seen in Figure 1.

Figure 1. ¹H (59.5 MHz) NMR spectra of acetaminophen extracted from Tylenol^® without using centrifugation (left) and with using centrifugation (right).⁵

After dissolving your sample, it is very common to use techniques such as filtration and centrifugation to aid in removing suspended particles. These techniques are explained in more detail below:

Another important point to consider when preparing your samples for NMR analysis is the tube quality. Compared to traditional high-field NMR instruments (¹H operating frequencies > 300 MHz), low-field spectrometers can use lower quality NMR tubes and still obtain good spectra. However, using too poor of a tube will always result in poor spectra, no matter the field strength. To achieve good spectra, ensure that the tube is straight (camber) and that the inner (ID) or outer (OD) diameters are relatively parallel to one another (concentricity).⁵ Note that camber is the measure of the lack of straightness and concentricity is the largest variation of the wall thickness of an NMR tube.⁵ You can get these numbers from your tube provider and most of them will also recommend to you specific tubes that cater to the magnet strength that you intend on using. Furthermore, if the NMR tubes selected are sufficient for spectrometers with a ¹H operating frequencies of 100 MHz or above, then it is very likely the quality of the tube is sufficient for use with our instruments.

During the preparation of your NMR sample, focus should be placed on the elimination of solid particles and the quality of the NMR tube to optimize the quality of your NMR spectrum. This can reduce spectral artifacts, line broadening, and improve reliability. By using the techniques mentioned above, you will be rewarded with spectra that are easier to interpret, thus providing you with more useful information. 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.

References
[1] S. A. Richards; J. C. Hollerton, Essential Practical NMR for Organic Chemistry, Hoboken, NJ: Wiley, 2011, p. 11 – 21.
[2] Blog: To Apodize or not to Apodize the age old question
[3] Sample Experiment: Quantification of Active Pharmaceutical Ingredients Present in Over-the-Counter Medication via ¹H NMR Spectroscopy
[4] Y. Wang; J. Wang, LCGC North America, 2022, 40, 386-392.
[5] Sample Tubes in NMR Spectroscopy, Dalhousie University https://cdn.dal.ca/content/dam/dalhousie/pdf/Diff/nmr3/general/NMRTubes.pdf (Accessed February 20, 2024)