radio-frequency pulses (1 – 1000 MHz) to excite molecules placed within a
strong magnetic field (0.5 – 20 Tesla). Molecules containing NMR-active nuclei
produce detectable signals as they relax back to the ground spin state. The
detected peaks appear shifted from a reference standard based upon the specific
chemical environment surrounding the nuclei. Each unique nuclear environment
gives rise to a peak at a specific chemical shift. Peak signal intensities can
be integrated to determine the relative number of nuclei occupying each unique
environment. Peaks can also be split based upon local chemical neighbors; which
provides spatial proximity between coupled chemical environments. The most
common nucleus analyzed is 1H because of its high natural abundance, high sensitivity,
and occurrence in organic molecules. Other common nuclei amenable to NMR
spectroscopy include 13C, 31P, 19F, and 15N.
NMR can analyze
neat liquids or dissolved organic solids (preferably in deuterated solvents)
with concentrations down to 100 mM. Chemical identification can be made based
upon processing raw data and fitting the resultant spectrum against
theoretically simulated data. Spectral libraries are not required.
Identification is most confident when the sample contains a single chemical.
Chemicals best analyzed with NMR include drugs, explosives, chemical weapons,
solvents, and other organic molecules (with molecular weights below 500 amu).
vary greatly in form factor, ranging from very large high power instruments to
much smaller benchtop models. A number
of commercial NMR instruments are available as the technique is very
mature. NMR spectrometer performance and
price is dependent on the instrument setup, especially as to the RF and magnet
used for data acquisition. Current miniaturized NMR technology uses similar,
although not identical instrumental configurations.