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I realize anything NMR-related is essentially physics, but I figured this was a better place to ask this. I have this issue fairly regularly (with one instrument in particular) where my baseline ends up looking like the one in the image below. The (inverted) sine wave-shape is what I'm referring to, if it's not obvious.

wavy nmr

Is there something specific that causes this baseline shape? Is there a way to fix it? Preferably I'd want to use something that fixed the signal/input, but if I have to resort to a processing method, I'd settle for that.

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This baseline roll seen in this spectrum is an example of acoustic ringing, and is quite normal for many low frequency NMR experiments that are recorded over wide sweep widths. It occurs as a consequence of slight breakthrough of the excitation pulse, as it rings down to zero. Ideally, you would like to start collecting signal at your receiver immediately after your excitation pulse, however it does take a small amount of time (microseconds) for the pulse to decay to zero. If your pre-scan delay (typically configured for your spectrometer during the configuration process) is too short, you can get this ringing artifact. It is more pronounced for weaker samples, but usually nothing to worry about.

To correct this as an experimental acquisition parameter, you would make your pre-scan delay longer; for Topspin, this is the parameter DE in your acquisition parameter list. It is normally defined during spectrometer configuration, and needs to be as short as practically possible to maximise your signal detection - typically less than 10 microseconds. For some nuclei, much of the signal has already decayed during this pre-scan period, and require some backward linear prediction to help. I'd speak to your local friendly NMR spectroscopist before mucking about with changing parameters like DE.

It is perfectly normal, and you should be able to get rid of it by applying a simple baseline correction algorithm - for Topspin you should use the commands abs or absd. This should work for your spectrum. As a word of caution when applying baseline corrections to your 1H spectrum, this will also apply an automatic integration for you, overwriting any integration you may have done previously. You can avoid this by using the optional n switch (abs n). For severe cases of baseline roll, apply a partial basline correction using absf between the limits of absf1 and absf2. Your Topspin manual can provide a fuller description, including other baseline correction modules.

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  • $\begingroup$ Awesome, this is really helpful. I'll try the baseline correction and see how that goes. If I have to do anything beyond that, I'll ask one of our NMR guys to help me out before moving on to other measures. $\endgroup$ – SendersReagent Apr 23 '16 at 12:15
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This seems to happen alot when you try to auto phase spectra with poor signal to noise. Longer acquisition times more concentrated samples and more delay time to relax between pulses will quiet down the baseline and allow for better phasing.

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  • $\begingroup$ I tried phasing this one myself. Any time it flattened out, I had signals at ~90°. This wasn't finished, but when I let these things all night even, I come back to less noise but the same kind of shape. Also, that thing was pretty concentrated. RG was like 25 for the proton. $\endgroup$ – SendersReagent Apr 22 '16 at 23:00
  • $\begingroup$ Is this in vnmrj? When was the last time the carbon channel on the instrument was tuned? $\endgroup$ – StevieD Apr 22 '16 at 23:04
  • $\begingroup$ This is Bruker Topspin. I hadn't tuned it at this point. I did check it, though. It looked pretty good. I might have gotten it slightly better but not much. This NMR isn't as good for X nuclei as it is for H, it's a BBI. I can always just turn around and use one of the BBO's, but that fixes the results and not the problem. $\endgroup$ – SendersReagent Apr 22 '16 at 23:08
  • $\begingroup$ Did you filter the solution after you made up the nmr sample? Particulates in the sample will also cause this, as will any biphasic or immiscible liquids. This used to happen to me all the time with air sensitive organometallics that would decompose and precipitate out during longer acquisitions. $\endgroup$ – StevieD Apr 22 '16 at 23:16
  • $\begingroup$ I did the filter it, but it had just come off a column. Could still be an issue, these compounds seem to have solubility issues. Great in DCM, terrible in everything else, including chloroform. I could have tried DMSO, but then I couldn't recover it very easily. $\endgroup$ – SendersReagent Apr 22 '16 at 23:24

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