A Magnet is an essential component of MMR spectrometer. Its function is essentially to expose the sample to a strong homogeneous magnetic field. The sensitivity and resolution that can be achieved are in direct proportion to the applied field strength.
Continuous wave 60 or 90 MHz instruments were popular in earlier days for routine NMR studies. Early day instruments used bulky permanent magnets but at the start of this century technological advances permitted developments of smaller tabletop instruments which could meet the requirements of educational institutes and research laboratories. Such instruments require stable power supplies and water cooling for removal of heat generated during operation to prevent field drifts.
Discovery of superconductivity has brought about the much needed revolution in NMR spectroscopy. It introduced magnets capable of generation of stable magnetic fields. Such magnets employed electromagnets made from coils of superconducting materials. When cooled to cryogenic temperatures such coils lose all electrical resistance and conduct very large currents without generation of heat. This has made it possible to achieve stable magnetic fields of strengths above 20 Tesla (1 Tesla=\(10^4 \)gauss) at 800- 900 Hz levels.
Liquid helium is used as a coolant in such systems. The magnet assembly is housed inside an insulated cryostat. The outer jacket of the cryostat contains liquid nitrogen at 77° K to prevent evaporative loss of liquid helium.
Superconducting magnets offer several advantages to the NMR spectroscopist:
Low Electricity Consumption
High kilowatt or megawatt power supplies are not necessary for field generation. Once the field is achieved the power supply can be switched off as under superconducting conditions the field becomes self generating. This brings about significant reduction in electrical operational costs.
Highly Stable fields
High field intensities ranging from 20 – 30 Tesla are possible which can be sustained under extremely low drifts. Such homogeneous high intensity fields are of immense value in NMR spectroscopy of large complex molecules (molecular weights up to several thousand daltons)
Low Space Requirement
In comparison to permanent magnets and electromagnets superconducting magnets have lower footprints.
Large Scope of Applications
Stable fields of high intensity and homogeneity permit investigations of some complex molecular species such as biological molecules, drug discovery molecules and investigations involving magnetic resonance imaging.
The apparent limitations in applications involving superconducting magnets are additional cost of cryogenic liquids and lack of emergency shutdown procedures.However, such limitations are far outweighed by the increase in scope of applications.