Radio-frequency inhomogeneous

In an inhomogeneous radio frequency (RF) field, ions can be guided from one part of a mass spectrometer to another. 

Figure 10.3 Diagram of an NMR experiment. The sample lies between the poles of a powerful magnet, and is spun rapidly around its long (x) axis in order to compensate for any unevenness (inhomogeneity) of the magnetic field. Radio frequency receiver coils form a helix around the x-axis, and transmitter coils spiral around the y-axis.

By Earnshaw s theorem, it is impossible to make a trap for ions static electric fields, but Penning in 1936 developed a stable ion trap that was a combination of a static uniform magnetic field and inhomogeneous electrostatic fields. In 1958 Paul and his associates made a successful electric quadrupole ion trap without a magnetic field by having the electric field oscillate sinusoidally at radio frequencies. 

GerUch D. (1992) Inhomogeneous electrical radio frequency fields A versatile tool for the study of processes with slow ions. Adv. Chem. Phys. LXXXII 1 176. 

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