Frequently Asked Questions

Avance 500 with 5mm X/H/P triple resonance broadband probe
Avance 300 with QNP probe
XX = Best choice
 

XX = Best choice

Please have some idea of how much sample is in your tube and use the following as a rough guide for pure compounds (of molecular weight 600 g/mol or less) on the AVANCE 400.
 

The most common reason for such behavior is that you have under-filled your sample tube. When you push the "LOCK" button on the command panel a default set of shims are called in to the magnet after which you are prompted to input the solvent you are using so the lock can be automatically established.  The default shims are good for a properly filled NMR tube ( ~ a 5 cm column of solution).  When an under-filled tube is used the shims are far off and hence the lock signal is very low.  To compensate for the low lock signal, the auto-locking routine increases the lock power to the point where the lock is saturated (causing the unstable behavior).  Reduce the lock power until the lock signal is stable and adjust the lock gain so the lock level is conveniently on scale then start shimming (or better still, fill your tube properly!).  You will see similar behavior if you do not set the depth of the NMR tube properly with the depth gauge.

Some possible reasons are:

• The magnet is not properly shimmed. - This will often be a problem if you try to autoshim the magnet when your sample is under-filled or if you did not set the depth of the sample correctly with the depth gauge.
• Check both the depth and quantity of sample in your NMR tube.  Your tube should contain a column of sample that is 50-70 mm high.
• There is a significant concentration gradient across your sample - Remove the NMR tube from the magnet, shake the sample to insure adequate mixing.  Put the sample back in the magnet and re-shim.
• There are floaters in your sample or it is a suspension - Filter your sample or find a better solvent.
• Your sample contains paramagnetic compounds - This will broaden out and shift your NMR lines - That's life.
• You tried to autoshim with a poor lock signal or no lock signal - In order for autoshimming to work the lock must be established with a reasonably strong signal.   If you cannot find the lock signal for your sample or think the lock signal for your sample is unusual or noisier than it should be then seek help from the NMR lab staff.

You must sign out a lab key from the NMR lab staff in room 416.

If you used methanol-d4, THF-d8 or toluene-d8 as a solvent, it could be that the lock was established on the wrong deuterium signal.  Try re-establishing the lock.  If the same problem persists, seek help from the NMR Facility staff.

The signal to noise ratio in an NMR spectrum is proportional to the amount of sample in the NMR tube and the square root of the number of scans.  For example, a factor of 10 improvement in the signal to noise ratio will require a factor of 10 increase in the amount of sample or a factor of 100 increase in the number of scans.

There isn't really an answer to this question.  The height  of the lock signal depends on all of the following: solvent, amount of solvent in the NMR tube, lock power, lock gain, lock phase and magnet homogeneity.

This is usually the result of having the receiver gain set too high.  Sometimes the automatic receiver gain optimization does not work very well for 2D experiments as it uses only the first increment to test the receiver gain.  Often the signals for subsequent increments are larger and can saturate the receiver.  Run your experiment again using the same receiver gain that was used in a conventional proton spectrum of the same sample.  You can find this by typing "rg" on a Bruker system or "gain?" on a Varian system while looking at your proton spectrum.  Note the value of the receiver gain and use the same value in your COSY or NOESY spectra.