Why the ion trap is called quadrupole

[Ricardinio]

I would like to understand what in the design or the functioning of the ion trap referes to "QUADRUPOLE"?

[MG]

The mathematics governing the two analyzers (quadrupole and quadrupolar ion trap) are similar. The difference is that with the quad, the ions are always unbounded with respect to the z-axis.

[Ricardinio]

In fact, I thought that quadrupole in the quadr instrument referes to the four rods with different polarities

[MG]

Ricardinio, I think you are correct as to the source of the name. However, many of the commerically available ion traps are based upon the Paul trap, which is also known as a quadrupolar ion trap. To my knowledge, it is called "quadrupolar" because the mathematics governing the motion of ions in the Paul trap is similar to that in the quadrupole (four rods) analyzer.

[JI2002]

Maybe it's better to say physics in Quadrupole Ion Trap is similar to Quadrupole MS.