Fourier-transform NMR

A major breakthrough occurred in 1966. Richard R. Ernst then discovered (together with Weston A. Anderson, USA) that the sensitivity of NMR spectra could be increased dramatically if the slow frequency sweep was replaced by short, intense radiofrequency pulses. The pulses cause a signal to be emitted by the nuclei. This signal is measured as a function of time after the pulse. It cannot be interpreted directly. Ernst discovered, however, that it was possible to extract the resonance frequencies from such a signal and to convert the signal into a NMR spectrum by a mathematical operation (Fourier* transformation, FT). This is performed rapidly in a computer. The whole process can be compared with stretching both arms over a piano and pushing all the keys at the same time. All the tones are there, but they are difficult to distinguish. A computer can discern the different tones (frequencies).
     Ernst's discovery is the basis of modern NMR spectroscopy, called FT NMR. It leads to a tenfold, and sometimes 100-fold increase in sensitivity since the pulse response contains information on all resonance frequencies at the same time. During the same time needed to record a single conventional spectrum, the FT experiment can be repeated many times and the results summed by a computer. FT NMR makes it possible to study small amounts of material as well as chemically interesting isotopes of low natural abundance.

* Fourier was a French mathematician, who lived 200 years ago.

The diagram a) shows a NMR signal from carbon-13 nuclei (which only occur in 1% of all the carbon atoms in nature) in ethyl benzene solution, obtained with the pulse technique by accumulating the response of the nuclear spins to two hundred pulses. The total experiment time was 20 minutes. After Fourier transformation, one obtains the carbon-13 NMR spectrum in the diagram b). If the experiment was performed with the old technique, in the same time one would only manage to perform a single sweep and the spectrum would look like the diagram c).