The Available RF Field for MAS NMR Probes

In order to rotate an equilibrium magnetization vector from the z axis into the transverse plane, one must provide a pulse with an oscillating magnetic field transverse the static field, B_o, at the Larmor frequency of the nucleus being observed. This is usually provided with a vertically oriented Helmholtz coil for liquids and a horizontal solenoid coil for solids. In both cases the coils provide radio frequency fields transverse to the static magnetic field. MAS coils are solenoids oriented at 54.7 degrees from the static magnetic field. They produce an oscillating radio frequency field at the magic angle. It is only the horizontal component of this field which is capable of rotating magnetization vectors. The available radio frequency field is therefore only 82% of that compared to an identical horizontal solenoid coil.