Formation of NMR Signal
It is known to us that all matter consists of atoms, which is the main particle in the nucleus and from which the NMR signal generates. For example, due to the positive electricity and rapid spinning itself, the hydrogen proton can be considered as a ring current. The electromagnetic theory states a truth that electricity generates magnetism. Thus, proton can be looked as a magnetic needle for the magnetic moment it produces. Difference of direction of the countless magnetic needles in each sample keeps the total magnetic moment to be zero by cancelling each other out.
Once a sample is placed in a static magnetic field, its original balance will be stricken. According to the principles of quantum mechanics, we know that the space orientations of nuclear magnetic moment in the external magnetic field are quantized. Namely the orientations are specified. For hydrogen, there are two orientations position which are of different energy, one high and the other low. Thereupon, the hydrogen nuclear splits into two orientations which are called Zeeman-split from its original balanced states. Vividly speaking, the process can be described like this: when a magnetic needle is put in a static magnetic field, two groups will be formed, one obedient and the other resistant. The obedient group has small energy, of low energy level and the same orientation as the static magnetic field; while the resistant group has large energy, of high energy level and the opposite direction to the static magnetic field. Moreover, that the amount of the obedient group is one in every 100, 000 nuclear larger than the resistant one forms, by accumulating that little one mentioned above to a mickle, a magnetization vector whose orientation is opposite to the main magnetic field and it is from which the NMR signal arises.
Once an appropriate radio-frequency field is imposed on the magnetic vector which rotates originally in the direction of the main magnetic field, a tilt will happen to the magnetic vector. Thus, NMR signal is formed. But which radio-frequency can be regarded as the appropriate one? As the previous analogy described, the magnetic vector can be compared to a sandbag which is perpendicular to the ground and the pulse, emitted by radio-frequency field, to a fist.
If only the strength of fist is big enough to match the weight of sandbag, the sandbag will be tilted and thereby a shadow will be formed on the ground. Then the shadow is just the NMR signal detected by instrument. The sandbag will gradually resume to the balance state, and the shadow will be smaller and smaller till it disappeared. Similarly, the NMR signal detected is also a decaying one.
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