In this paper, we define the concept of measurement diversity using agglomerative hierarchical clustering, and characterize the measurement diversity of fentanyl analogs across four common analytical measurement strategies: (a) GC retention indices, (b) full scan mass spectra collected using electron ionization mass spectrometry (EI-MS), (c) precursor m/z and product ion mass spectra (MS 2 ) collected at multiple collision energies using electrospray ionization tandem mass spectrometry (ESI-MS/MS), and (d) in-source collision induced dissociation (is-CID) full scan mass spectra collected at three orifice 1 energies using direct analysis in real time mass spectrometry (DART-MS). Examples of each measurement type using fentanyl are presented in Figure . We also discuss how these four measurements can be combined to increase measurement diversity and subsequently improve our ability to unambiguously identify compounds.