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Default NMR Tube Thickness and Signal-to-Noise-Ratio

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. The use of thick-walled NMR tubes is advantageous in the case where a sample can be concentrated into a smaller volume (eg: 0.45 mL vs 0.68 mL). In such a case, running the sample in a thick-walled NMR tube puts more of the sample of interest is in the coil and one expects a higher mass-normalized signal-to-noise ratio. The use of thicker walled NMR tubes seems to be a viable (and simpler) alternative to using susceptibility matched Shegemi type tubes however there are likely to be problems in the limit of very thick walls due to susceptibility differences between the glass and the sample.




Source: University of Ottawa NMR Facility Blog
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