Chromatography Forum

Hi all. I have a question. Hope you guys can help out ^^

I'm not too familiar with NMR data yet; can someone advise:

If NMR spectra showed that a sample is pure ie one or two peaks, can running the same sample in MS showed that the sample is not pure ie multiple peaks.


Please avise. Thanks a lot ^^

For NMR, as with other spectroscopic techniques, spectra for impurities are easliy lost in complex spectra when the impurities are present at low level. When you use GC-MS or LIC-MS, you add a separation so the contaminant can be separated from the compund for which you are checking purity.

NMR gives a measure of everything in the NMR tube that has a specific type of atom, such as hydrogen, C13, etc. The mass spectrometer gives a measure of molecules that can be ionized to form a detectable ion, and when attached to a chromatographic instrument, is limited to only those moleuces that can pass through the chromatographic column and form a peak.

Thanks Don, I understand what you mean ^^

Say for example, I have a sample shown to be 'pure' based on NMR spectra with only 1 or 2 peaks observed.

When I run the same samples in LCMSMS, the spectrum showed multiple peaks which unfortunately, seemed not to be able to piece together with the data from NMR spectra.

Does that mean:
i) There is contamination in the LCMS/MS instrument; thus giving rise to 'the noisy spectrum as compared to the NMR spectra'?

ii) Or, NMR spectra was seen to be 'pure' because of specific measurement of certain type of atoms ie 1H and 13C NMR spectra and also largely dependant on the instrument magnetic settings etc etc?

Due to specific parameter settings in NMR, does that mean not all 'compounds which are present in the samples will be 'rotated/vibrated' by the NMR?

iii) Compared to LCMSMS which has soft ionisation, most of the compounds ionisable will be detected by the instrument; hence more 'impurities seen'?

Please advise of any opinions.

Thanks so much ~

Don explained it:

using NMR you will detect any molecule containing hydrogen and carbon atoms (or others with different probes). Thus it gives you a better idea of the purity of your sample than MS.

However NMR is insensitive and low level impurities that are likely to be detected by MS may not be detected by NMR. If these impurities are much better ionized than your product of interest, the signal intensities in MS may be similar and you will erroneously conclude that your product is far from pure.

Last possibility: your LC-MS system is contaminated and the impurities you detect come from the solvent, column, injector etc.

Thanks Gaetan.

I have a better understanding now with the replies from you and Don.

I know this might be a no brainer question but I just need some confirmation especially for NMR:

I have actually isolated my compound fractions before running for NMR and MS.

Even if it's purified extracts, the possibilities are still there for NMR to show an intense peak for that compound while MS might showed you more things due to the better sensitivity, right?

If the impurities peak are strongly ionised by MS, we might be getting wrongly interpreted molecular weight or m/z as compared to the expected from NMR spectra of that particular compound, am I right?

Thanks ~

You must run a sample with everythign present excpet the analyte and a sample with the analylte through the LC/MS to ensure you are not looking at contamination from solvents and such ( as pointed out). Also, you need to be sure that you can see your compund in the LC/MS. You may need particular conditions for your compund to ionize - and if you are not at those conditions, you may be ionizing other stuff and missing your compund.

If you are doing NMR, all atoms of the type you are analyzing will be excited. (NMR is an atomic measurement, with the spectrum beign a measure of local electromagnetic field strength for each atom. And as long as you are looking at small molecues in liquid, molecuar motion does not have a lot to do with the NMR spectrum. This is the advantage of NMR - if you have a significant quantity of an impurity in solution, you should see a signal. LC techniques can be a bit tricky, because there is no universal detector for LC.

Ken this is well meaning advice: Sit in on a course or at least get a book on instrumental analysis if your chemistry background is strong enough.
This advice is based on your statements such as "will be 'rotated/vibrated' by the NMR? ". A NMR spectrometer doesn´t rotate or vibrate molecules.
Or:
"If the impurities peak are strongly ionised by MS, we might be getting wrongly interpreted molecular weight or m/z as compared to the expected from NMR spectra of that particular compound, am I right?"
The answer is NO. The ionization intensity is related to amount of ions you see.
Don and Gaetan have presented a valiant attempt to give some background, not much more can be done here.