Nuclear Magnetic Resonance Spectroscopy (NMR) is closely related to Magnetic Resonance Imaging (MRI). Both techniques use the magnetic properties of atomic nuclei to get information about a sample. MRI measures the amount of a given nucleus at a given place (spatial distribution). NMR measures the amount of the nucleus in each chemical environment (chemical or molecular distribution). The distribution is called a spectrum, and is analogous to the optical spectra of substances which are responsible for their visible colors. You might say we are measuring the "magnetic color" of the substance.

Both techniques use almost the same equipment, so they are often combined for a more comprehensive study of a sample.

What are those spectra ?

A spectrum (plural spectra) is a graph showing the amount of energy (sometimes called the "signal") at a specific wavelength or frequency. The signal is plotted on the y axis (vertical) and the frequency on the x axis (horizontal).

In NMR spectra, the vertical amplitude corresponds quite closely to the number of atoms with a particular frequency. The frequency corresponds to the chemical environment. For example, the hydrogen (H) atoms in water (H2O) appear at about 4.8 ppm. Thus we can say what compounds are present (from the X axis position) and how much of each is present (from the Y axis amplitude). In other words, NMR spectroscopy provides qualitative and quantitative chemical analyses.

NMR Trivia: NMR spectra are usually plotted with an unusual X axis. It increases from right to left, rather than left to right as usual! The units are "parts per million" or "ppm".