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The Anti-Mass Spectrometer is a room-sized piece of equipment located in Sector C's Test Lab C-33/a at the Black Mesa Research Facility that is used to analyze exotic materials such as Xen crystals. It was designed by Dr. Rosenberg.
Regular mass spectrometers are used to measure the mass and relative concentration of atoms and molecules, allowing the chemical composition and structure of a substance to be analyzed. This process does not work for exotic matter that has anti-mass (negative mass). The Anti-Mass Spectrometer works by scanning a sample with oscillating electromagnetic fields and beams of high-energy plasma, agitating the exotic matter of the Xen crystals. This results in currents of displacement energy within the crystal, which forms noticeable displacement fields that can be analyzed using advanced sensors developed by the Lambda labs. Samples are brought into the Test Chamber by a lift from the lower levels and is held by a cart to be pushed by a person.
Despite the high levels of energy used in the experiments (running the spectrometer requires the use of a HEV Suit, and in Half-Life: Blue Shift the process is shown to place visible strain on the entire facility's power sources), it was a typically safe procedure. The final experiment of the Black Mesa Anti-Mass Spectrometer, combining a highly unstable sample (GG-3883) and the disabling of certain safety features, triggered a Resonance Cascade and was the opening event of the Black Mesa incident. The Anti-Mass Spectrometer was heavily damaged during the Resonance Cascade, but not actually destroyed until Black Mesa itself was detonated.
 Half-Life: Decay
Richard Keller in the lower Anti-Mass Spectrometer Control Room.
The Materials Handler inside the delivery system.
 Half-Life 2
A faint depiction of a partially redone Anti-Mass Spectrometer test chamber during the opening sequence of Half-Life 2.