Abstract: A cycloidal mass spectrometer having a housing which defines an ion trajectory volume, an electric field generator for establishing an electric field within the ion trajectory volume, and an ionizer for receiving gaseous specimens to be analyzed and converting the same into ions which travel through orthogonal electric and magnetic fields and impinge upon a collector. The spectrometer is designed to have a plurality of ions of different mass to charge ratios impinging on the collector generally simultaneously. A processor determines the mass distribution of the ions impinging upon the collector. A plurality of electric field plates are electrically insulated from each other and may be sealed so as to define the ion trajectory volume. In another embodiment, an assembly of electric field plates are disposed within a vacuum enclosure. A miniature ionizer preferably has a miniature filament. The cycloidal mass spectrometer and ionizer may be miniaturized so as to provide for a small, readily portable instrument.