The Environmental Control and Life Support System (ECLSS) on Space Station utilizes the Stand Alone Major Constituent Analyzer (SAMCA) for monitoring major atmospheric constituents from several sampling locations within the station. This paper describes the design of the flight SAMCA instrument and contains: system overview, SAMCA interface description, mechanical description, operational mode summary, and performance capabilities.The SAMCA monitors six major constituents: oxygen, nitrogen, carbon dioxide, hydrogen, water vapor, and methane. In order to monitor these gases the SAMCA uses a permanent-magnet, single-focusing mass spectrometer. This configuration was originally developed for the Skylab metabolic analyzer and most recently used for the Spacelab's Gas Analyzer Mass Spectrometer, GAMS. For the SAMCA application, the mass spectrometer and its support electronics were repackaged to meet Space Station requirements.The SAMCA is installed in the ECLSS Atmospheric Revitalization (AR) rack. The SAMCA interfaces with the AR rack via two manual mate connectors which supply electrical power and provide a 1553B communication bus for data and control. Other manual mate connectors provide the pneumatic interface for sample acquisition. The SAMCA mounts to the AR rack with two drawer slides and four screws on the front panel.In order to comply with maintainability requirements the SAMCA flight design has been divided into seven Orbital Replaceable Units (ORUs): ORU 01, Data and Control Assembly; ORU 02, Major Constituent Analyzer Assembly; ORU 04, Power Supplies; ORU 05, Sample Pump Assembly; ORU 06, Sample Distribution Assembly; ORU 07, EMI Filter; and ORU 08, Verification Gas Assembly. All ORUs are mounted on the chassis assembly or Additional Maintenance Item (AMI).