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Abstract Synthetic opioids such as fentanyl account for over 71,000 of the approximately 107,000 overdose deaths reported in the United States in 2021. Fentanyl remains the fourth most identified drug by state and local forensic laboratories, and the second most identified drug by federal laboratories. The unambiguous identification of fentanyl‐related substances (FRS) is challenging due to the absence or low abundance of a molecular ion in a typical gas chromatography‐mass spectrometry (GC‐MS) analysis and due to a low number of fragment ions that are similar among the many potential isomers of FRS. This study describes the utility of a previously reported gas chromatography‐infrared (GC‐IR) library for the identification of FRS within a blind, interlaboratory study (ILS) involving seven forensic laboratories. Twenty FRS reference materials, including those with isomer pairs in the library, were selected based on either their presence in the NIST library and/or some similarity of the mass spectra information produced. The ILS participants were requested to use the Florida International University (FIU) GC‐MS and GC‐IR libraries supplied by FIU to search for matches to their unknown spectra generated from in‐house GC‐MS and GC‐IR analysis. The laboratories reported improvement in the positive identification of unknown FRS from ~75% using GC‐MS alone to 100% correct identification using GC‐IR analysis. One laboratory participant used solid phase IR analysis, which produced spectra incompatible with the vapor phase GC‐IR library to generate a good comparison spectrum. However, this improved when searched against a solid phase IR library.
