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.
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Copyright (c) H. S. Rzepa and ICSTM Chemistry Department, 1994, 1995.