Definition of quadrupole

1. a “double dipole”;  a molecule with a quadrupole will have no dipole moment because the two dipoles present in the molecule cancel each other out.  A good example is carbon dioxide. Definition for  quadrupole mass filter : used in mass spec, this tool has four voltage-carrying rods which serve to give ions which travel between them oscillations.  Only ions with the right mass/charge ratio (m/z) can undergo these oscillations without hitting one of these rods.