University of Ottawa NMR Facility Web Site

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Monday, October 15, 2007

The Consequences of Setting the Receiver Gain Too High

The receiver gain of an NMR spectrometer is much like the volume control on a radio. When it is set too high there will be distortion in the NMR signal. The FID will be clipped near the beginning of the signal. The Fourier transform of this distorted signal is a distorted NMR spectrum. The figure below shows what this distortion looks like.
Many spectrometers will calculate the receiver gain automatically however you should be aware that this automatic calculation is not always perfect and that the receiver gain may have to be set manually. On a Varian spectrometer the receiver gain is the "gain" parameter. On a Bruker spectrometer the parameter is "rg". In both cases higher numbers mean a higher receiver gain.

5 comments:

Unknown said...

I'm using a Bruker Avance 300 spectrometer. When using "RGA" rg value jumps up to a very high value more than 3000 and not depending on sample concentration. What could be causing this error? How can I vary manually the rg value in a Avance 300 spectrometer?

Glenn Facey said...

Unknown,
If RGA finds a very high value, the signal is very weak. You can adjust the receiver gain manually by typing "rg" on the command line and entering an appropriate value.
Glenn

Anonymous said...

What can cause a very weak signal? Does it mean that there is a problem with the spectrometer?
Thank you so much in advance

Glenn Facey said...

Anonymous,

What nucleus are you observing?

It could be a spectrometer problem. Put in the ethylbenzene standard sample and run a signal-to-noise test. Compare this to the spectra provided by the service engineer after installation.
Glenn

Anonymous said...

I'm observing hydrogen nucleus.