From Halopedia, the Halo wiki
Trace Amounts is the ninth track included in the Halo: Original Soundtrack.
The soundtrack contains electronically generated sounds and a relatively slow tempo. The ominous tone reflects the Library's dark past.
The track plays in the Halo: Combat Evolved campaign level The Library, when John-117 exits a small tunnel in the second floor of the Library, up until the point where he reaches the locked door that leads into the third floor of the Library's index chamber.
The piece was arranged into Bad Dream in Halo: Combat Evolved Anniversary Original Soundtrack.
The name Trace Amounts is a reference to the effects that the Flood has on an atmosphere, according to 343 Guilty Spark. When scientists refer to the contents of an atmosphere, they say, for example, 21% oxygen, 78% nitrogen, and "trace amounts" of other gases.