NMR instrumentation

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

Magnet

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. The homogeneity of the magnetic field created by the primary magnet is improved by using a set of shims.

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.

Probe

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. In addition, 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).

Magnetic field gradients are controlled by an independent subsystem. Magnetic field gradients are generated by passing current through coils of appropriate geometry.

MR transceiver

The MR transceiver contains two major subsystems: The transmitter and the receiver subsystems

Transmitter subsystem: It consists of the RF synthesizers and amplifiers. This subsystem is responsible for generating pulse sequences contains pulses of specified frequency, amplitude, phase, shape and duration at specified times. Multiple RF synthesizers are required because many MR experiments require simultaneous application of RF pulses of different frequencies. Earlier MR systems used waveform generators for RF synthesis with subsequent phase modulation. However, more recent systems rely on DDS (direct digital synthesis).

RF switch is responsible for coupling either the Transmitter or the Receiver subsystem to the probe. This ensures that the sensitive receiver subsystem is not overloaded with the high powered RF signal generated by the transmitter system. Also, the receiver is 'blanked' during the transmission and for a short duration afterward.

Receiver subsystem: This consists of the components: Preamplifier, Amplifier, Mixer and the Analog-to-Digital converter (ADC). The Mixer is used to subtract a reference frequency of specified phase from the observed signal, resulting a signal of lower frequency that can be easily digitized.