NMR instrumentation

A typical NMR spectrometer contains three components: 1) Magnet 2) Probe 3) RF electronics.

For any system at equilibrium, in the absence of a magnetic field, all the nuclear spin states are equally populated and hence there is no net polarization due to the nuclear spins. Therefore, it is necessary to introduce an external magnetic field which leads to preferential population of the lower energy nuclear spin states. The energy differences between the different nuclear spin states are proportional to the strength of the magnetic field. Therefore, higher magnetic field lead to greater separation between the energy levels and greater polarization at equilibrium. The required magnetic field is usually provided by an external magnet. High magnetic fields (1 Tesla to 17 Tesla) are generally preferred for high resolution, high sensitivity NMR spectroscopic experiments. In general, higher magnetic fields provide higher signal to noise ratio as well as higher resolution. Most high resolution NMR spectrometers used by chemists and biologists use superconducting magnets. However, NMR spectrometers with lower resolution may use permanent magnets or electromagnets. It is also feasible to carry out certain types of NMR experiments in much weaker fields - in fact many NMR spectroscopic experiments have been conducted using the earth's Magnetic field.

The probe in an NMR spectrometer is responsible for coupling the radio frequency electromagnetic field generated by the RF electronics to the sample. It is also responsible for detecting the NMR signal (through induction) and passing it to the receiver. The receiver is a component of the RF electronics that is responsible for amplification of the NMR signal and for its convertion to a lower frequency so that the signal can be digitized. The RF electronics is responsible for generating a radio frequency signal in the form of a sequence of pulses with specified frequency, amplitude, phase and duration for each pulse.

In addition the the above, most modern NMR spectrometers are equipped with the following subsystems: Coils for magnetic field gradients (pulsed and constant), field frequency lock, sample spinning, sample temperature regulation (VT).