NMR Panopticum

 

Introduction

Every now and then one sees a spectrum that does not look as expected. In most cases, this is because we actually didn't know what to expect, or our endeavours produced a compound different from the desired one, or a mixture, or ...

This is not what I want to display on this page. The horror show that I would like to present here will be a collection of spectra that show some sort of artefacts, "features" or in general an appearance that will be different from a spectrum recorded under proper conditions.

The purpose will be to help you to identify, and potentially remedy, the cause of these artefacts. Some of the examples are known from textbooks on NMR, some others have been observed in our routine use of NMR, solids or liquids.

A click on the picture will bring you to a page giving more details and providing a PDF file showing the spectrum at higher quality.

More Peaks Than Expected

DC Offset of the FID
	A constant voltage offset of the FID in the real or imaginary part will cause a "peak" exactly in the middle of the spectrum.

 

Quadrature Detection Image
	A quadrature detection image will appear if the two channels of the receiver are electronically imbalanced. The so called quad image can be identified by looking at the strongest peak in the spectrum, measuring its separation from the transmitter frequency (the exact center of the spectrum), and going the same distance from the center to the other side of the spectrum.

 

Lost Lock Signal
	The response of the solvent deuterium signal is monitored to keep its frequency "locked" at a constant value. Loss of the lock signal during acquisition causes the spectrometer to go into a sweep mode, where the magnetic field is swept through a specified range in the hope to locate the deuterium resonance and to reestablish the field-frequency lock.

 

Interference with Neighbor
	A neighboring NMR spectrometer operating at the same field strength and running proton-decoupled carbon-13 NMR causes decoupler sidebands in the proton spectrum.