The development of a center-grounded load coil in inductively coupled plasma-mass spectrometry (ICP-MS) has been reported to reduce greatly the so-called "secondary discharge" between the plasma and mass-spectrometer interface. Presumably, this secondary discharge results from a high floating ac potential in the plasma relative to the sampler or skimmer and can complicate attempts to optimize or characterize the ICP-MS instrument. With the center-grounded load-coil configuration, each half of the load coil attempts to couple into the plasma a potential of opposite polarity; however, because of the relatively high conductivity of the discharge, the net voltage remains near zero. Although several workers have explored alternative load-coil geometries in an effort to achieve the same ends, the center-grounded load-coil configuration is simple and effective.
Pengyuan Yang, Barbara Ross, and Gary M. Hieftje, "Modification of an ICP Impedance-Matching Network for Center-Grounded Load-Coil Operation," Appl. Spectrosc. 43, 1093-1095 (1989)