The data used in this study was extracted from various NIST databases . The EI mass spectra, ESI MS 2 mass spectra, and RI values were selected from pre-release versions of the NIST 23 EI-MS, ESI-MS/MS, and GC Methods libraries, respectively. DART-MS mass spectra were selected from the most recent release of the NIST DART-MS Forensic Database (version 7, Grasshopper) . Articles describing how these measurements are recorded and how libraries/databases are constructed/evaluated can be found in the literature . We only considered fentanyl analogs for which we had all four measurements in this study; details about the complete set of 197 fentanyl analogs are in Table .