Abstract: A tandem quadrupole mass spectrometer system having first, second and third quadrupole sections close coupled in series with one another. AC-only is applied to the center section and conventional AC and DC voltages are applied to the two end sections. The AC applied to all three sections is synchronized in frequency. The AC phase shift between each section is of magnitude between 0 and 0.1 cycles in absolute value, preferably between 0 and 0.03 cycles in absolute value, and in the preferred embodiment the AC phase shift between each section is essentially zero. The sections are spaced apart longitudinally by a very short distance not exceeding r.sub.o, the radius of the inscribed circle within the quadrupole rods.