What is NMR Spectroscopy?
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
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".
C.D.Gregory: April 24, 1996