A compact mass spectrometer apparatus is presented to enable accurate qualitative and quantitative analyses of target ions. The apparatus can operate in a relatively poor vacuum in the range of 10.sup.-2 to 10.sup.-3 torr compared with the conventional requirement of 10.sup.-6 to 10.sup.-8 torr while providing precision results with lesser number of electrodes than the conventional mass spectrometer. The separation of the ionic species is achieved through two effects: flight time differentials produced by varying mass/charge ratios of the sample ions; as well as high frequency resonance separation by synchronizing the injection of ions with high frequency electric field applied to an electrode system having equi-potential space and high frequency space. The resulting dispersion in the wide energy spectra of the sample ions serves to accurately identify the sample ions both qualitatively and quantitatively. The spectra dispersion is achieved by modulating the ion beam with application of suitable magnitude and phase angle of the high frequency voltage, and allowing only those ions having the maximum kinetic energy to pass through to a collector electrode. The analyses are based on measurements of the maximum ion current flowing in the collector electrode.