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.
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