In a mass spectrometer system, ions travel through an orifice in an inlet plate into a first vacuum chamber containing AC-only rods, and then through an orifice into a second vacuum chamber containing a standard quadrupole. The second vacuum chamber is held at low pressure, e.g. 0.02 millitorr or less, but the product of the pressure in the first chamber times the length of the AC-only rods is held above 2.25.times.10.sup.-2 torr cm, preferably between 6.times.10.sup.-2 and 15.times.10.sup.-2 torr cm, and the DC voltage between the inlet plate and the AC-only rods is kept low, e.g. between 1 and 30 volts, preferably between 1 and 10 volts. This produces a large enhancement in ion signal, with less focussing aberration and better sensitivity at high masses, and also allows the use of smaller, cheaper pumps so the system can be more easily transportable.