TMS is shifted for 0,5 ppm towards metabolite peaks which match peaks in literature.
Spectral referencing is a complicated thing. Proton chemical shifts are normally quoted relative to TMS so your statement is inherently incorrect as TMS must be at 0 ppm. In reality, many people reference to either the residual solvent signal or to the spectrometer synthesised frequency. The location of all signals (and this includes TMS) will change based on temperature, concentration, matrix, pH, etc, so if you are referencing in a different way, you shouldn't expect the same answer.
To deal with your observation, are you sure that what you are seeing is TMS? People often use TSP, particularly in DMSO or polar solvents because TMS is only sparingly soluble. It is also possible to see silicon grease as though it were TMS.
Did the literature references have TMS peaks in them (or claim to be referenced to TMS)? If they are not correctly referenced to TMS, you will see differences.
Thank you so much for your answer!
I know that TMS is supposed to be the zero reference but then all the metabolites are removed for 0,5 ppm compared to the peaks from literature which were also measured according to TMS. We also scanned just TMS in a test tube and the peak was the same. It should be pure TMS, we scanned it directly from the bottle..
Also, we had an issue that the C6D6 peak was at 7,16 ppm when scanned alone but when we scanned it in a capillary tube that was exposed to dry ice and sealed under a flame the peak was at 6,37 ppm.
Are there any other possible reasons for such shifts?
I get the feeling that I am missing quite a lot of information from your experiment. What is the solvent in which you are running these spectra? Is this the same solvent as the literature that you quote? As for the C6D6 experiment, what was your solvent for this? Solvents can make a big difference in chemical shifts (e.g. CHCl3 in CDCl3 = 7.27, in DMSO = 8.32ppm)
There was no solvent, we scanned pure C6D6 both times. The difference was that we scanned one directly out of the bottle in a test tube and the other we put in a capillary tube and sealed the capillary using a technique with dry ice and a flame, so I thought that maybe that could affect C6D6 in some way and change it's peak?
As for TMS, we used the same technique to make a capillary and we scanned the capillary in a tube with a myoinositol solution and we also scanned it directly out of the bottle in a test tube. Both times the peak was at 0,4 ppm. The myoinositol peak was similar to the literature. We don't know what could possibly cause that
Sorry if it was confusing, I am new to spectroscopy so I don't quite know how to phrase everything :)
Dear Ana,
to me it is still not clear how you lock the spectrometer?
Let us know....
I am getting very confused. Firstly, I'm surprised that you are using benzene (we stopped using it in our labs a long time ago because it is a carcinogen). Secondly, I'm not sure what solvent you are running your metabolites in. Is this the same solvent as the literature? Was your myoinositol in solution in water or some other solvent? Chemical shifts vary hugely with solvent. Benzene in particular is extremely anisotropic and will give rise to very different chemical shifts. This paper is useful to see the differences: J. Org. Chem. 1997, 62, 7512-7515.
If you could answer the following questions, we may be able to help:
1) What solvent did you run your metabolites in?
2) What solvent was used in the literature reference?
3) How was the literature NMR data referenced?
4) How did you reference your data?
Dear Alfred, thank you for your answer! We are using Bruker Biospec 70/20 USR and it doesn't have the lock system but we use drift compensation in our sequence. Also, we use deuterated benzene just because we had access to it in our lab.
The answers to your questions John:
1) We ran myoinositol and other metabolites in water, but we ran C6D6 and TMS in capillaries that were placed in the cryotube containing the myoinositol solution. So C6D6 and TMS never mixed with water and the metabolites. Could they still affect the measurments?
2) I went to check the literature reference and the solvent for myoinositol was D20. I suppose that could influence the measurements. Thank you for bringing that to my attention!
3) The literature NMR data was referenced to DSS, also a possible problem.
4) We wanted to reference our data to TMS or C6D6 but then we ran into these issues. We used water as a point of reference because it's peak was consistent, but since we plan to scan tissue in the future water might not be so reliable since it's concentration may vary.
Thank you again for your help!
Ah. The light begins to show. If you reference in a different way, you cannot expect to see the same chemical shifts. TMS in C6D6 will have a very different chemical shift to DSS in D2O. Any reason why you can't use DSS? I think that this would get around a lot of your problems.
Hello Ana!
Could you please elaborate on how the TMS reference peak was being observed at 0.5 ppm?
- Badges
- Science topic
- Similar topics
- Physics
- Condensed Matter Physics
- Solid State Physics
- PEAKS