As noted earlier in the paper, the limitations of EI-MS are well-established. And given that EI-MS is usually preceded by GC, it is prudent to consider retention times (or retention indices) while trying to discriminate samples. We can simulate this experience by computing a combined RI and EI-MS dissimilarity prior to an AHC analysis (see Figure ). Because the measurements differ in range and interpretability, it is easiest to work with binary values based on whether the individual dissimilarities are above or below specified cutoff values. In particular, we can compute the combined dissimilarity of two fentanyl analogs using RI and EI-MS as ). Of the identified pairs of compounds that were indistinguishable with all standard library measurements, almost all were positional isomers.