"It's - It's not shutting down; it's... Argh!" - Scientist (Half-Life)
The device used by scientists at the Black Mesa Research Facility to analyze samples of various materials from Xen and elsewhere. Judging by the related science of mass spectrometry, it was most likely used for measuring the mass-to-charge ratio of charged anti-particles.
Its location in the middle of the Test Chamber gives the Anti-Mass Spectrometer an even more futuristic appearance as it and almost fills out the room completely with rotors, emitters, and likewise high-tech components. See Test Chamber.