Nuclear Magnetic Resonance:
a short Introduction

NMR is an initialism for Nuclear Magnetic Resonance, a mature method well-suited to quantification and structural elucidation of a wide variety of samples. But what exactly does this mean? Consider each word in turn:

  • Matter - at least that which we are familiar with as the 'stuff' around us - consists of atoms. Atoms themselves consist of a positively-charged nucleus surrounded by a cloud of negatively-charged electrons. The 'nuclear' refers only to this nucleus, rather than to anything to do with radioactivity. If the charges of the nucleus and the electrons do not balance out, then we call the matter ionic.

  • Going even further, nuclei themselves consist of positive particles called protons and neutral particles called neutrons. It is the number of protons in the nucleus that determines which element the nucleus belongs to. All atoms and ions of a particular element are referred to as isotopes. For example, all isotopes of lithium (Li) have three protons in their nucleus.
    Magnetic

  • Nuclei have a quantum-mechanical property called 'spin'. This can be related to the magnetic properties of the nucleus: if the spin is non-zero, then the nucleus acts as if it were a tiny magnet. Each of these nuclear magnets are vanishingly weak, but when a macroscopic sample containing many nuclei is placed in a strong external magnetic field, they tend to align to give a correspondingly macroscopic - but still tiny - net magnetisation.
    Resonance

  • If a nuclear magnetic moment (or more generally any micro- or macroscopic moment) is placed in a magnetic field, then its orientation will rotate, or 'precess', around the field. An analogy is the 'wobbling' of an old-school wooden top when it is tilted away from the vertical. This precession occurs with an extremely well-defined resonant frequency, typically in the radio range, for a particular isotope and background field.

This phenomena allows us to manipulate and measure samples with isotopic specificity by placing them in a strong magnetic field, pulsing them with radio waves, and recording the signals generated - a very attractive method given that is is non-contact, repeatable, and requires no consumables.

An in-depth introduction is available here: Nuclear Magnetic Resonance an introduction
 
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