Hello everyone,
I am facing a consistent issue in my NMR spectra with an unwanted peak appearing at 1.25 ppm. This peak seems to vary with the amount of sample: it becomes more pronounced with smaller sample amounts and diminishes when the sample amount is larger.
Here are some details about my attempts to resolve this issue:
1. Ensured the purity of my solvent.
2. Thoroughly cleaned the NMR tubes.
3. Used fresh samples.
4. Heated the samples in an oven at 100°C.
Despite these efforts, the peak persists. Could anyone provide insights into the potential sources of this peak and suggest effective strategies to eliminate it?
Thank you in advance for your assistance!
Strategies to Eliminate the Unwanted Peak:
- Since the peak diminishes with larger sample amounts, increasing the sample concentration might help suppress the unwanted peak relative to the desired signals.
- Use solvent suppression methods in your NMR experiments to reduce the intensity of peaks arising from the solvent or other contaminants.
- Run a series of control experiments with blank solvents, clean tubes, and no sample to see if the peak appears.
- Compare the chemical shift and coupling pattern with known contaminants to identify the source.
- Sometimes switching to a different solvent can help isolate the problem. For instance, if you're using CDCl3, try using DMSO-d6 or acetone-d6 and see if the peak persists.
- Look for any reports or literature on common NMR contaminants that appear around 1.25 ppm. This can provide clues on the potential source.
- If you suspect water contamination, try adding a small amount of D2O to your sample to exchange with any residual protons and shift the water peak.
- Experiment with different sample preparation techniques, like filtering the sample or using different solvents for dissolution.
By systematically addressing these potential sources and employing these strategies, you should be able to identify and eliminate the unwanted NMR peak at 1.25 ppm.
John R Carney 1. I am using CDCl3 as a solvent. 2. i will try it.
3. No, there is no peak(s) in 13C NMR.
It’s possible the signal at 1.25 ppm is water dissolved in CDCL3 (http://ccc.chem.pitt.edu/wipf/Web/NMR_Impurities.pdf).
It turns out heating the NMR tube (or any glassware that comes in contact with the CDCl3)is insufficient for water removal. Water binds interstitially to the glass surface and con’t be removed with reasonably high temperatures.
We eliminated this problem by pre-soaking the NMR tube in D2O. The water remains as interstitial DOD. In extreme cases (very small natural product samples) glass syringes used to prepare the samples were also pre-treated with D2O.
The impurity could be t-butanol. We occasionally see this as an impurity in our analyses. In D2O it shows as a singlet at 1.25 ppm. We are not 100% sure about the source of the contamination, but it is likely from the coating on disposable centrifuge tubes and pipette tips.
- Badges
- Science topic
- Similar topics
- Filing