How to Detect all Frequencies Simultaneously.

Imagine the following thought experiment. Construct not one but two oscillators placed with their magnetic field along the x axis in the diagram above, and two receivers detecting magnetisation along the y axis. For protons, each would scan a different frequency range of 300Hz at 1Hz/second. The time taken to accumulate a spectrum would be halved from that for a single oscillator to 300 seconds. One could keep on adding oscillators until each would be required to scan a range similar to the actual width of the resonance being measured, for protons about 0.5Hz. Actually, 1200 such oscillators could have a fixed frequency, 0.5 Hz apart. The entire spectrum could be recorded by 1200 fixed frequency receivers in 600/1200 = 0.5 second!

It is of course not practical to design a spectrometer with 1200 oscillators each generating a fixed frequency. Lets go back to using a single oscillator (transmitter) and use this to generate a pulse of electromagnetic radiation of frequency w but with the pulse truncated after only a few complete cycles (corresponding to a duration tp) so that the waveform has rectangular as well as sinusoidal characteristics. It can be proven that the frequencies contained within this pulse are within the range +/- 1/tp of the main frequency w. For the proton example above, a pulse of 1/600 = 0.001667 seconds duration would generate a range of frequencies covering +/- 600 Hz, ie representing a typical chemical shift range at 60 MHz. For technical reasons, a shorter pulse covering a wider frequency range is often used.

Return to Index page
Copyright (c) H. S. Rzepa and ICSTM Chemistry Department, 1994, 1995.