NMR Artifacts

 

MTR Resources

Even the best laid plans of NMR spectroscopists oft go awry. It may be as simple as a poorly positioned sample causing high-order shimming problems or as perplexing as persistent out-of-phase 3 Hz sidebands. Whatever their cause, these artifacts can ruin a spectrum or lead to an incorrect analysis. This page will attempt to list the most common artifacts in NMR spectroscopy that arise from incorrect sample preparation, improper instrument set-up, and processing problems. Below you will find spectra that contain a problem and the various causes for those problems as well as their resolution (if possible).

Please choose from the following list to find a resolution to your spectral artifact.

 

 

 

 

 

 
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well shimming

Spectrum 1: 0.1% Ethylbenzene in CDCl3 (expansion of the methyl triplet).

Problem: None.

Cause: Good work.

Result: Nice peak resolution and correct analysis.

Resolution #1: Keep up the good work!

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Z2 shim is off

 

Spectrum 2: 0.1% Ethylbenzene in CDCl3 (expansion of the methyl triplet).

Problem: Asymmetrically broadened peaks with non-Lorenzian shape.

Cause: Misadjustment of Z2 shim.

Result: Loss of resolution and potential missing peak splittings.

Resolution #1: Open the Lock interface (i.e. click acqi => LOCK) and adjust the LOCK PHASE to get optimal lock level. Click on SHIM and readjust Z2 in units of -4+ to get highest lock level. Now adjust Z1 in -4+ units to optimize. Continue to adjust Z1 and Z2 until no improvement is achieved.

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Z1 shim is wrong

Spectrum 3: 0.1% Ethylbenzene in CDCl3 (expansion of the methyl triplet).

Problem: Symmetrically broadened peaks with non-Lorenzian shape.

Cause: Misadjustment of Z1 shim.

Result: Loss of resolution and potential missing peak splittings.

Resolution #1: Open the Lock interface (i.e. click acqi => LOCK) and adjust the LOCK PHASE to get optimal lock level. Click on SHIM and readjust Z1 in units of -4+ to get highest lock level. Now adjust Z2 in -4+ units to optimize. Continue to adjust Z1 and Z2 until no improvement is achieved.

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Z1 and Z2 shims are incorrectly set

 

Spectrum 4: 0.1% Ethylbenzene in CDCl3 (expansion of the methyl triplet).

Problem: Asymmetrically broadened peaks with multiple maxima.

Cause: Misadjustment of Z1 and Z2 shim.

Result: Loss of resolution, potential missing peak splittings, and possible misassignment of peak splittings.

Resolution #1: Open the Lock interface (i.e. click acqi => LOCK) and adjust the LOCK PHASE to get optimal lock level. Click on SHIM and readjust Z1 in units of -4+ to get highest lock level. Now adjust Z2 in -4+ units to optimize. Continue to adjust Z1 and Z2 until no improvement is achieved.

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bad Z4 or Z3 shim

 

Spectrum 5: 0.1% Ethylbenzene in CDCl3 (expansion of the methyl triplet).

Problem: Asymmetrically broadened peaks with multiple maxima.

Cause: Misadjustment of Z3 and/or Z4 shim.

Result: Loss of resolution, potential missing peak splittings, and possible misassignment of peak splittings.

Resolution #1: Open the Lock interface (i.e. click acqi => LOCK) and adjust the LOCK PHASE to get optimal lock level. Click on SHIM and readjust Z3 in units of -4+ to get highest lock level. Now adjust Z4 in -4+ units to optimize. Continue to adjust Z1 and Z3 as well as Z2 and Z4 until no improvement is achieved.

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low signal-to-noise

Spectrum 6: 0.1% Ethylbenzene in CDCl3

Problem: Low Signal-to-Noise (S/N).

Result: Missing peaks, poor integration.

Resolution #1: Acquire more transients (scans). Increase in multiples of 4. Doubling S/N will require 4 X more transients.

Resolution #2: Add exponential multiplication. Type lb=0.3 wft to add 0.3 Hz linebroadening. Adding more than 0.3 may result in loss of important coupling information.

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glitch at spectrum center

Spectrum 7: 0.1% Ethylbenzene in CDCl3

Problem: Center glitch. Artifact at exact center of spectrum.

Cause: Slight imbalance in Quadrature detectors. Usually visible with low number of transients.

Result: Poor autophasing, potential structure misassignment.

Resolution #1: Determine if it is truly a center glitch by typing ds f centersw. If the red cursor falls on the artifact, it is likely a center glitch. Typically, acquiring more transients will eliminate the problem.

Resolution #2: If #1 does not resolve the issue, try typing wft('nodc'). This can eliminate the problem.

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Quadrature image

Spectrum 8: 4 Hz doped H20 sample.

Problem: Quadrature image. The red cursor is at the spectrum center. The quad image appears equidistant from the center and usually has dispersive phase when compared to its true peak.

Cause: Slight imbalance in Quadrature detectors. Usually visible with low number of transients.

Result: Poor autophasing, interference with 'real' signals, potential structure misassignment.

Resolution #1: Acquiring more transients will eliminate the problem. Take a minimum of 4 transients. More is better.

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Receiver overflow

Spectrum 9: 4 Hz doped H20 sample.

Problem: Receiver overflow causing significant distortions in spectrum.

Cause: Gain is set too high. Only should occur when autogain is turned off.

Result: Useless spectrum.

Resolution #1: Turn autogain on by typing gain='n' and reacquire the spectrum. If you get a message similar to 'gain driven to 0...', your sample is too concentrated and should be diluted to get a good spectrum. It could still be run by reducing the pulse width to 1 or less (e.g. type pw=1).

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ADC overflow

Spectrum 10: 4 Hz doped H20 sample.

Problem: ADC (Analog-to-Digital Converter) overflow resulting in spectral distortion.

Cause: Gain is set too high, which causes too much signal to pass to the digitizer. A signal that triggers a ‘receiver overflow’ may dissipate enough in time as not to trigger an ‘ADC overflow’. Only should occur when autogain is turned off.

Result: Useless spectrum.

Resolution #1: Turn autogain on by typing gain='n' and reacquire the spectrum. If you get a message similar to 'gain driven to 0...', your sample is too concentrated and should be diluted to get a good spectrum. It could still be run by reducing the pulse width to 1 or less (e.g. type pw=1).

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baseline is rolled

Spectrum 11: 0.1% Ethylbenzene in CDCl3.

Problem: Rolling baseline.

Cause: Improperly set zero- and first-order phase parameters. Very broad background peaks from solid material will also cause this problem (this is typical for 19F NMR using standard probes).

Result: Difficult to interpret and impossible to phase.

Resolution #1: Reset the phasing parameters to zero by typing rp=0 lp=0, then type aph0 to perform automatic zero-order phasing. Finish up by clicking Phase, then click and hold on the rightmost peak and drag the mouse up or down to get proper phasing for the right peak (if you run out of room, click Phase again and continue). Now click and hold on the leftmost peak and drag mouse up or down to correct left phase. Redo right end if necessay. When completed, type ds and hit return.

Resolution #2: Reset the phasing parameters to zero by typing rp=0 lp=0, then click Phase, then click and hold on the rightmost peak and drag the mouse up or down to get proper phasing for the right peak (if you run out of room, click Phase again and continue). Now click and hold on the leftmost peak and drag mouse up or down to correct left phase. Redo right end if necessay. When completed, type ds and hit return.

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aromatic peaks are folded

Spectrum 12: 0.1% Ethylbenzene in CDCl3.

Problem: Aliased or folded Peaks (compare to Spec. 6 and note the peaks around 0 ppm). This is more typical for carboxylic acids and certain hydrides.

Cause: Improperly set spectral window.

Result: Peaks that are outside the spectral window will fold over to the opposite end of the spectrum by the amount they are outside the window. In the above example, the aromatic peaks that should be at 7.2 ppm are folded to ~0.5 ppm. This can lead to incorrect spectral assignment.

Resolution #1: Increase spectral window to accomodate all expected peaks and rerun experiment. This is done by typing setsw(upper ppm, lower ppm), where upper ppm is the upper limit for your spectrum and lower ppm is the lower limit. If you want a spectrum from 20 ppm to -10 ppm, type setsw(20,-10) then type ga.

Resolution #2: An alternative is to double the size of the existing spectral window by typing sw=sw*2. Rerun the experiment with ga to get the new spectrum.

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not locked

Spectrum 13: 4 Hz doped H20 sample.

Problem: All Peaks have multiple tops.

Cause: Running an experiment with the Lock turned ON, but not locked on the deuterated solvent.

Result: All peaks with have many maxima and spectrum cannot be interpreted.

Resolution #1: Stop experiment if it is running. Open the LOCK interface by clicking acqi. Click on LOCK, turn LOCK OFF, move LOCK slider to find "zero beat", click LOCK ON, reduce lock power to appropriate level (20-32 for CDCl3, 5-20 for other solvents), click CLOSE (see Locking for more help). Rerun spectrum.

Resolution #2: If you do not have a lock solvent, open the LOCK interface by clicking acqi. Click on LOCK, turn LOCK OFF. Rerun experiment with lock off.

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tilted baseline

Spectrum 14: 0.1% Ethylbenzene in CDCl3.

Problem: Baseline is crooked.

Cause:Typically caused by a first point distortion of the FID.

Result: Slanted spectrum and inaccurate integrals.

Resolution #1: Type f cdc dc to display full spectrum, clear previous drift correction, and apply new drift correction. If you have peaks at the edge of the spectrum, this may cause a more tilted baseline. In this case, either acquire spectrum with larger spectral window (type sw=sw*2 ga) and apply f cdc dc or expand region with no peaks at the edge and type f cdc dc.

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spinning sidebands

Spectrum 15: 0.1% Ethylbenzene in CDCl3 with TMS as internal standard (expansion of TMS peak). Spinning speed is 20 Hz.

Problem: Spinning sidebands at multiple of spinning speed.

Cause: Improperly set non-spinning shims (X, Y, ZX, and ZY shims). May also come for a bad NMR tube.

Result: Additional peaks that make interpretation difficult.

Resolution #1: First make sure that your NMR tube is good. It should be rated for the field you are using and free from any defects. It should not of been stored in the oven. Once you have a good tube, open LOCK interface (click acqi =>LOCK) and turn off spinning. Increase LOCK GAIN and/or LOCK POWER to maintain lock (be sure not to saturate). Click SHIM and then right mouse click the arrow next to SHIM in the middle of the window. Choose first order non-spinning shims. Adjust X and Y. Now adjust X and ZX then Y and ZY. Finish with X and Y. Click on LOCK and start spinning, reduce LOCK POWER and GAIN to appropriate level. Click SHIM and right mouse click next to the middle SHIM and choose Z shims. Adjust Z1 and Z2.

Resolution #2: If resolution #1 does not work, you can either run the sample non-spinning or increase the spinning rate to 30-35. Do not increase it too high because this can cause vortexing and lead to bad spectra.

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too low digital resolution

Spectrum 16: 0.1% Ethylbenzene in CDCl3 with TMS as internal standard (expansion of CH2 peak).

Problem: Poor digital resolution.

Cause: Improperly set acquisition time or Fourier number is too small.

Result: Inaccurate peak shape, poor integration, missing fine structure.

Resolution #1: For standard small molecule proton NMR, the acquisition time should be at least 2 seconds and preferably 4 seconds. Check acquisition time by typing at?. If it is too small, type at=4 to set it to 4 seconds and rerun experiment. If acquisition time is good, you can increase the Fourier number by typing fn=4*np wft. This will set fn up to 4 times the number of acquired data points. All additional points will be zeros and add no additional noise. This technique is called 'zero filling'.

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poor integration due to short recycle delay

Spectrum 17: 0.1% Ethylbenzene in CDCl3 with TMS as internal standard.

Problem: Poor integration. The aromatic signals (around 7.2 ppm) should integrate to 5.

Cause: Typical cause for large errors between aliphatic and aromatic integrals is too short recycle delay.

Result: Inaccurate integration and possible wrong structural asssignment.

Resolution #1: The most common cause of this problem (assuming baseline is flat, phasing is good) is having a recycle delay that is too short. Check the current recycle delay by typing d1? (that's d and the number one). Note the value and then increase it by a factor of 3. Thus, if d1 is 1, increase it to 3 or even 5. Rerun the experiment. If integration is still wrong, increase d1 further and repeat experiment. If you want to be accurate, obtain your molecule's T1's (see T1 measurement opens PDF) and set d1 + at to be 5 times the longest T1 of your molecule.

Resolution #2: An alternative to #1 is too reduce the pulse angle according to the Ernst equation to get the best signal with the given d1 and known T1. This approach is not recommended because it is intended for obtaining the best S/N for a given recycle delay and T1 and will not allow for full relaxation between scans.

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trucation artifacts

Spectrum 18: 0.1% Ethylbenzene in CDCl3 with TMS as internal standard (Methyl expansion).

Problem: Truncation artifacts or so-called 'sinc wiggles'.

Cause: Acquisition time is too short and the FID is clipped at the end causing artificial modulation.

Result: Potential obscuring of peaks.

Resolution #1: This usually occurs with large peaks and a short acquisition time. Check the acquisition time by typing at?. If it is 2 or less, type at=4 to set to 4 seconds and rerun experiment. Otherwise, increase acquisition time by typing at=at*2. Rerun experiment.

Resolution #2: If it is small (much less than in example), add exponential multiplication by typing lb=0.3 wft (this is for proton data). You can use bigger lb values, but this will reduce resolution and in these cases it is best to use resolution #1.

 


Last Updated:  February 17, 2009   - WebMaster
URL: http://www2.chemistry.msu.edu/facilities/nmr/NMR Artifacts.html

 

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