A wireless-controlled miniature rectilinear ion trap mass spectrometer system, total weight with batteries 5.0 kg, consuming less than 35 W of power, and having dimensions of 22 cm in length by 12 cm in width by 18 cm in height, is characterized. The design and construction of the mass spectrometer including mass analyzer, vacuum system, electronics system, and data acquisition and processing systems, is detailed. The mass spectrometer is compatible with various types of ionization sources including a glow discharge electron impact ionization source used in the internal ionization mode, and various atmospheric pressure ionization sources, including electrospray ionization, atmospheric pressure chemical ionization, and desorption electrospray ionization, which are employed for external, atmospheric pressure ionization. These external sources are coupled to the miniature mass spectrometer via a capillary interface that is operated in a discontinuous fashion (discontinuous atmospheric pressure interface) to maximize ion transport. The performance of the mass spectrometer for large and small molecules is characterized. Limits of detection in the parts-per-billion range were obtained for selected compounds examined using both the internal ionization and external ionization modes. Tandem mass spectrometry and fast in situ analysis capabilities are also demonstrated using a variety of compounds and ionization sources. Protein molecules are analyzed as the multiply protonated molecules with mass/charge ratios up to 1500 Da/charge.Miniature mass spectrometers are useful because they combine the usual advantages of mass spectrometrysbroad applicability, high sensitivity, high specificityswith the convenience of fast in situ analysis. These features are important to many applications in the area of public safety, environmental protection, and industrial process monitoring, among others.1 After a decade of increased attention, progress in the development of hand-portable mass spectrometers is becoming increasingly rapid. Ion traps of a variety of types have been the mass analyzer of choice in most recent miniature mass spectrometers, a choice made mainly because of their inherent tandem MS capability, relaxed requirement for high vacuum, and flexible modes of operation.2-8 Batterypowered ion trap miniature mass spectrometers with weights below 10 kg have been developed and a few have began to appear as commercial products. [9][10][11][12]