1.3. Pulse and Signal Generation
In nuclear magnetic resonance (NMR) spectroscopy a sample is placed in a machine called a spectrometer. The spectrometer then applies a powerful external magnetic field and aligns the nuclei (see Section 1.2). Afterwards, the machine rapidly pulses very specific amounts of energy at very specific intervals into the sample. This is done concurrently to all nuclei of the same element and isotope in the system. The energies and intervals are nuclei- and experiment-specific and require significantly more background to discuss. However, the practical consequences of these “pulse programs” are relatively straightforward.
The sample is pulsed with energy such that the two alignments become temporarily equally populated (Figure 1.6). Then the machine collects information about the energy that is released by the sample as the nuclei return to their previous distribution of states. Again, this is done concurrently to all nuclei of the same element and isotope in the system.
Figure 1.6 – Concept Behind NMR Pulse and Signal Recording.
The energy that is released depends on several factors including the initial energy difference (ΔE). Because this value is itself dependent on several conditions, different nuclei, even of the same element and isotope, release different amounts of energy at different rates.
The period during which the nuclei return to their original states and information is collected is called Free Induction Decay (FID) and the resulting graph is an FID interferogram (Figure 1.7). These are typically very complex and cannot be interpreted directly.
Figure 1.7 – Example of an FID Interferogram.
Using a Fourier Transform (a mathematical process that transforms one function into another that describes it) the computer processes the information and changes the “time” domain into a “frequency” domain. The result is a new graph called an NMR Spectrum that is typically much more easily interpretable (Figure 1.8).
Figure 1.8 – Example of an NMR Spectrum with Frequency Domain.
