Two miniature mass spectrometer systems based on a microengineered quadrupole mass filter have been developed. One of the instruments has a footprint of 27 cm x 20 cm and is intended for laboratory use when space is at a premium. The other is portable and intended for use in the field. It is battery powered, weighs 14.9 kg, and is housed in a rugged case. This is the first example of a portable mass spectrometer incorporating an analyzer fabricated using microelectromechanical systems (MEMS) techniques. The starting material for construction of the filters is a bonded silicon on insulator substrate, which is selectively etched using batch processing techniques to form coupling optics and springs that accurately hold 0.5 mm diameter stainless steel rods in the required geometry. Assembled filters measure 35 mm x 6 mm x 1.5 mm and are mounted, together with an ion source and channeltron detector, in small, interchangeable cartridges, which plug into a 220 cm(3) vacuum chamber. Recovery from accidental contamination or when servicing is required can be achieved within 5-10 min, as the cartridge is easily exchanged with a spare. A potential application to environmental monitoring has been investigated. The headspace above water spiked with dibutyl mercaptan was sampled with a solid phase microextraction (SPME) fiber, which was then injected directly into the vacuum chamber of the mass spectrometer. Using this method, the limit of detection was found to be approximately 5 ppm for a 15 s sampling period.