NMR Tube Thickness and Signal-to-Noise-Ratio

The amount of NMR signal is expected to be proportional to the amount of sample inside the coil of the NMR probe.  As a result, the signal-to-noise ratio for samples run in NMR tubes with thick walls is expected to be lower than that for comparable samples run in NMR tubes with thinner walls due to a reduced filling factor of the NMR probe coil.  I was curious to see how much of a difference in signal-to-noise ratio there would be.  0.68 mL of  CDCl₃ (99.8 % D) was put in 5 mm NMR tubes with wall thicknesses of 0.38 mm and 0.80 mm.  The NMR tubes were New Era Entepprises NE-MP 5 (4.20 mm ID) and Norell S-300 (3.43 mm ID), respectively.  The samples are shown here:

The height of the sample column for the thick-walled tube is obviously higher due to the smaller inner diameter of the tube.  In this case, much of the sample will be "invisible" to the NMR measurement as it is outside of the active NMR probe coil volume and therefore "wasted".  Single scan proton NMR spectra were run for these samples on a 300 MHz instrument.  A third sample was prepared by removing some sample from the thick-walled NMR tube such that the column height was equal to the sample in the thin-walled tube.  The volume for this sample was 0.45 mL and it was run under identical conditions to the other two.  Care was taken to shim the magnet and tune and match the NMR probe reproducibly.  The data, processed with 0.5 Hz of line broadening, are plotted side by side in the figure below:

The 0.68 mL sample in the thin-walled tube (blue) gave a signal-to-noise-ratio of 566.  The 0.68 mL sample in the thick-walled tube (red) gave a signal-to-noise-ratio of 339 and the 0.45 mL sample in the thick-walled tube (green) gave a signal-to-noise-ratio of 369.  The difference in the signal-to-noise-ratios for the two samples in the thick-walled NMR tube may very well be the same within experimental error as the signal-to-noise-ratio is very sensitive to magnet shimming.  One would expect them to be similar based on the fact that both samples have volumes exceeding the active volume of the probe coil.  From the data, one sees a 35-40% loss in signal on going from a thin-walled to a thick-walled NMR tube.  It is instructive to look at the volume corrected signal-to-noise-ratio of the 0.45 mL sample in the thick-walled NMR tube compared to the 0.68 mL sample in the thin-walled NMR tube.  If the signal-to-noise ratio for the 0.45 mL sample is multiplied by (0.68 mL/0.45 mL), the corrected value is 557 which is very likely the same as the 566 value measured for the 0.68 mL sample in the thin-walled NMR tube within experimental error.  From these observations, one can conclude that the signal-to-noise-ratio loss is entirely due to the reduction in sample volume within the coil.         

11B Background Signals

Unfortunately NMR probes and NMR tubes cannot be "transparent" for all of the isotopes one may want to observe. Depending on the NMR probe, it is very common to have background signals for ¹⁹F, ²³Na, ²⁷Al, ²⁹Si, ⁶⁵Cu, ¹⁰B and ¹¹B. These background signals must be taken into account when interpreting NMR spectra. The background signal for ¹¹B on a Bruker AVANCE 300 with a 5 mm broadband probe is shown in the figure below with several different types of NMR tubes commonly used for routine work in our laboratory. The probe was tuned before running each spectrum and the spectra were collected with proton decoupling. The left hand panel shows the background signal for the NMR probe and the other spectra show the combined background of the probe and the indicated NMR tubes. It is obvious that the magnitude and shape of the ¹¹B background depends on the type of NMR tube used. It should be noted that all of the major NMR tube manufactures offer quartz NMR tubes which have little (if any) ¹¹B background signal.

23Na Background in NMR Tubes.

When we measure NMR spectra, we want to observe the NMR spectrum of our sample only. However, NMR probes, tubes (or MAS rotors) must be made of real materials, so we should expect some NMR background signals for the materials present inside (or near) the coil. The user should be aware of the presence of these signals. Instrument companies expend a great deal of effort to minimize the number and intensity of background signals in their NMR probes (this is one of the reasons they are so expensive). The figure below shows three ²³Na NMR spectra taken on a high resolution NMR spectrometer. The top trace is the spectrum of an aqueous solution of NaCl, the middle trace is that of an empty NMR tube and the lower trace is that of an empty NMR probe. One can see that although the NMR probe is free of a ²³Na background signal, the NMR tube is not.