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. It plays in the Halo: Combat Evolved campaign level The Library. The piece is remastered into Fragments of the 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.