1) Magnet
The first requirement for NMR is a strong magnet, usually a superconducting
magnet, which generates a strong focussed magnetic field called B0
running through the center of the instrument. Important characteristics
of any magnet are strength of the field, stability of the field and homogeneity
of the field. Stronger magnets are preferred since strength determines
how far the nuclei are split apart in energy. The further they are apart,
the better the resolution of the NMR. Most magnets are in the range of
6-18 Teslas, which corresponds to a 1H operating frequency of
250-750 MHz. Superconducting magnets (ie they have zero electrical resistance)
are used on modern instruments, but they require liquid helium at 4 K to
maintain their superconductivity and they are very expensive. The purpose
of the magnet is to align all the NMR active nuclei and split them into
their respective spin states.
2) Transmitter Coil & Receiver Coil
The transmitter coil is placed perpendicular
to B0 and generates a magnetic field B1 which
oscillates back and forth. This field causes the nuclei to change their
spin. The reciever coil picks up the signal as the nuclei relax back to
their original position. A brief pulse of RF radiation from the
transmitter coil causes resonance in the nuclei (ie all nuclei have coherent
spins) and a signal is induced in the receiver coil. This signal is then
processed and turned into the NMR spectrum.
3) Signal Processing & Storage
The signal from the coil is digital and is stored
in a large microcomputer which is dedicated to the instrument. It collects
and processes the Free Induction Decay (or FID) signal. Performing a Fourier
Transformation on this signal gives us the normal NMR spectrum which we
are used to seeing.