A tandem quadrupole mass spectrometer system in which an AC-only section is close coupled with a standard AC-DC section with the rods of the two sections closely longitudinally adjacent each other. The rods of the AC-only section are of open structure, formed by thin wires, to allow gas to be introduced into the system and to escape through the open structure rods. The system is particularly suited for use with a tandem quadrupole system consisting of three sections, namely an AC-DC section, an AC-only section, and an AC-DC section all close coupled, with a target gas introduced into the AC-only section to induce CID of ions traveling through the system. In one arrangement the rods of the AC-only section have solid center portions between which the target gas is introduced, and open structure end portions through which the target gas may flow away so that little of it enters the end AC-DC sections. In another embodiment, gas is beamed directly through a short open structure section by placing an appropriate gas dynamic beam forming device such as a collimated hole structure or a gas dynamic free jet on one side and an appropriate vacuum pumping arrangement on the other.