Where is the library used for chemical identification stored?

• A. External database
• B. A temperature map
• C. A mass spectrometer reading
• D. Onboard library

Answer: (D)

The key idea is that chemical identification relies on comparing the measured data to known reference patterns, and having those references immediately available makes the process fast and reliable. That’s why the library is stored in the instrument itself—in its onboard memory—so you can perform matching in real time without needing an external connection. External databases can be used for updates or larger reference sets, but the location being asked about is the onboard storage. A temperature map isn’t related to identifying chemicals from spectra, and a mass spectrometer reading is the experimental data, not where the reference library lives.