For each of the 197 fentanyl analogs investigated, we performed four AHC analyses using the four measurement types discussed previously. Figure shows parts of the dendrogram created using retention indices as the discriminating measurement, with an overlayed magnification of 24 analogs and a table discussing three analogs (norsufentanil, N-methyl cyclopropyl norfentanyl, and cyclopropyl norfentanyl). From this, we can observe several groups consisting of two to four compounds with retention indices within 10 arbitrary units (a.u.) (e.g., cyclopropyl norfentanyl and N-methyl cyclopropyl norfentanyl) and a few compounds that would be uniquely identifiable We studied how the measurement diversity changed for each measurement as a function of dissimilarity cutoff level (Figure ). The diversity of retention indices was never a perfect 1, even with a dissimilarity level of 0 (Figure )-there are a pair of fentanyl analogs (𝛽-Methylfentanyl and p-Fluoro acryl fentanyl) that happen to have identical retention indices in the NIST GC-Methods library. The diversity of mass spectral measurements (Figures ) is 1 with a dissimilarity cutoff value of 0 As expected, the diversity of all measurements decreased with increasing cutoff values. In practical application, repeat RI and mass spectral measurements are only roughly reproducible and thus, will always have dissimilarity values greater than 0 a.u. In particular, we might expect retention indices of the same molecule to differ by upwards of 10 a.u. and for the replicate mass spectra to have similarity values of approximately 0.9 a.u. (or dissimilarity values of 0.1 a.u.) even when measured under identical conditions. With measurements collected under nonidentical conditions (e.g., when building a reference library), one might observe even larger discrepancies between measurements of the same compound. Using a dissimilarity threshold of 10 a.u. for retention index and 0.1 a.u. for mass spectral measurements (red points in Figures ),