UniversitÓ di  Firenze
 
NMR-Problem Solving
Department of  Chemistry

(A PDF version of this page is available from the Handouts site)

Resources
This page contains pertinent information to help you when a problem arises. Please look through the list below to identify and find resolutions to the most common problems at our facility. If your problem is not listed, please contact the staff at 3537. In addition, please fill out our NMR incident report so that we may be better able to serve your needs. The incident report should be used even if you are able to fix the problem. Click here to get the NMR Incident report. For spectral problems, see NMR Artifacts.
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What do I do if...
 Handouts
 NMR Artifacts
 
What do I do if....
I broke a tube in the NMR
The computer screen is blank
 
The console is unresponsive to commands
The temperature is too high/low
I can't lock on my sample
 
I can't access a workspace (e.g. exp1, exp2)
I can't eject my sample
 
my shimming is poor
I have poor signal/noise
my sample is not spinning
the baseline rolls or is not flat
   
 
   

 

Broken Tube in the Magnet:

THIS MUST BE REPORTED AS SOON AS POSSIBLE TO MINIMIZE DAMAGE TO THE PROBE. DO NOT ATTEMPT TO REMOVE SAMPLE! PLEASE CONTACT NMR STAFF IMMEDIATELY (CALL 3537, 3478, OR 3477). IF STAFF IS UNAVAILABLE, LEAVE MESSAGE AT 3567. ALWAYS LOG-OFF OF THE INSTRUMENT AND HANG A SIGN ON MONITOR STATING; 'BROKEN SAMPLE: DO NOT USE INSTRUMENT'.

Console is unresponsive to Commands:

1. With the right mouse button, click once on the Solaris desktop background to reveal a menu.  Click on Programs or Tools, then over to select Terminal… A Terminal window will pop-up.


2.  In the Terminal window, type ‘su acqproc’ and hit Return.  You will get a message indicating that acqproc is being ‘killed’.  Also, in the ACQUISITION STATUS window in VNMR, STATUS should now read “inactive”.


3. Proceed to the NMR console, which is the big grayish box.  See below for location of reset button for various instruments.

4. Push the reset button.

LOCATION OF RESET BUTTON FOR EACH INSTRUMENT
Instrument
Reset Button Location
I400: Inova-400 Open the left door of the consolle. Look for the switch with the written word,'reset'.
Mercury 400  


5. Wait for at least 40 seconds.


6. In the terminal window type su acqproc and hit Return.  This will take a few moments.   Look at the ACQUISITION STATUS window in VNMR.  Wait for STATUS to read “idle”.


7. In the VNMR command line (i.e. where you usually type e), type su and hit Return.  The NMR should now be ready for use. 

 

Can't Lock on Sample: (PDF version)

This can be caused by several problems. Common reasons and their solutions are listed below:

Common Reasons for Inability to Lock
Cause
Result
Solution
No deuterated solvent No deuterium signal to lock on. Use a deuterated solvent like CDCl3.
Shimming is very poor The signal is so broad that it is not well observed. Type fixshims to get a good start.
Suspended particles This causes line broadening, which makes the signal intensity weaker. Filter your sample.
Too concentrated There is little deuterated solvent to get a lock. Especially problematic when using CDCl3. Dilute your sample or use a solvent with more deuteriums (e.g. C6D6).
Improperly gauged When injected, the sample is not in the coil. Eject sample and ensure that the sample resides between the white lines on the gauge.
Lockpower is too low The sample is not receiving enough rf signal to flip the deuterium spins. Increase lockpower to 80% of max before trying to lock.
Lockpower is too high (for multi-resonance samples) With solvents having more than one deuterium resonance (e.g. CD3OD, THF-d8), high lock power will cause all resonances to be excited. This makes locking on a single resonance difficult. Reduce the lockpower by 20 and find the lock signal. If you see the signal with 'bumps', reduce lockpower further.
Lockgain is too low The signal from your sample is not amplified enough for you to see the signal. Increase Lockgain to maximum.

I Can't Access a Workspace (e.g. exp1, exp2):

I Can't Eject my Sample:

Poor Shimming: (Click HERE to get a more detailed procedure for shimming. PDF format)

Common Reasons for Poor Shimming
Cause
Result
Solution
No deuterated solvent No deuterium signal to lock on and, thus nothing to shim on. Use a deuterated solvent like CDCl3.
Initial shimming is poor Your peaks are very broad. Load a set of shims to get a good start.
Suspended particles This causes line broadening, which makes the signal intensity weaker. Filter your sample.
Too concentrated Concentrated samples can be viscous, which will cause line broadening. Dilute your sample.
Too little solvent When injected, the sample is not properly positioned in the coil and the liquid/air interface is 'seen' by the coil. This will make shimming very difficult. Add more solvent.
Improperly gauged When injected, the sample is not properly positioned in the coil and the liquid/air interface is 'seen' by the coil. This will make shimming very difficult. Eject sample and ensure that the sample resides between the white lines on the gauge.
Cheap, old, or stored in hot oven NMR tube Poor quality NMR tubes are either very slightly out-of-round and/or have uneven wall thickness. This results in a flucuation in the field and thus poor line shape. Throw away the NMR tube and get a tube that is, at the very least, rated for the field you are using. We recommend the Wilmad 528 series as a minimum.

 

Poor Signal/Noise:

The Computer Screen is Blank:

The Temperature is too High/Low:

My Sample is not Spinning:

My baseline rolls or is not flat (for 2D Spectra, see the Processing, phasing, and plotting 2D phase sensitive data Handout):