The NMR spectrometer Magnet Probe Coils Transmitters Amplifiers and pre-amplifiers Receiver ADC

The NMR spectrometer

The Magnet History First magnets were built using ferromagnetic material= permanent magnet Then Electromagnets: i.e. field was generated by wiring of conducting material Now: cyomagnets: i.e. electromagnets made of superconducting wire. A “cutted” magnet

Cryomagnets Superconducting wire has a resistance approximately equal to zero when it is cooled to a temperature close to absolute zero (-273.15o C or 0 K) by emersing it in liquid helium. Once current is caused to flow in the coil it will continue to flow for as long as the coil is kept at liquid helium temperatures. The length of superconducting wire in the magnet is typically several miles.

Magnet

The Probe The sample probe is the name given to that part of the spectrometer which accepts the sample, sends RF energy into the sample, and detects the signal emanating from the sample. It contains the RF coil, sample spinner, temperature controlling circuitry, and gradient coils. Picture an axial cross section of a cylindrical tube containing sample. In a very homogeneous Bo magnetic field this sample will yield a narrow spectrum

B0 homogeneity In a more inhomogeneous field the sample will yield a broader spectrum due to the presence of lines from the parts of the sample experiencing different Bo magnetic fields.

The coils You may find one or more RF coils in a probe. RF coils create the B1 field which rotates the net magnetization in a pulse sequence. They also detect the transverse magnetization as it precesses in the XY plane. Most RF coils on NMR spectrometers are of the saddle coil design and act as the transmitter of the B1 field and receiver of RF energy from the sample. The resonant frequency,n , of an RF coil is determined by the inductance (L) and capacitance (C) of the inductor capacitor circuit. n=1/2p(LC)1/2 RF coils used in NMR spectrometers need to be tuned for the specific sample being studied.