A shoebox-sized, 10-kg, handheld mass spectrometer, Mini 10, based on a rectilinear ion trap mass analyzer has been designed, built, and characterized. This instrument has evolved from a decade-long experimental and simulation program in mass spectrometer miniaturization. The rectilinear ion trap has a simplified geometry and high trapping capacity, and when used with a miniature and ruggedized pumping system, it allows chemical analysis while the instrument is being carried. Compact electronics, including an air core RF drive coil, were developed to control the instrument and to record mass spectra. The instrument runs on battery power, consuming less than 70 W, similar to a laptop computer. Wired and wireless networking capabilities are implemented. The instrument gives unit resolution and a mass range of over m/z 500. Tandem mass spectrometry capabilities are implemented using collision-induced dissociation, and they are used to provide confirmation of chemical structure during in situ analysis. Continuous monitoring of air and solution samples is demonstrated, and a limit of detection of 50 ppb was obtained for toluene vapor in air and for an aqueous naphthalene solution using membrane sample introduction.