Utility of gas chromatography infrared spectroscopy (GC-IR) for the differentiation of isomers of related substances

Forensic laboratories routinely use gas chromatography mass spectrometry (GC-MS) in the identification of controlled substances using both retention time and electron impact ionization (EI) mass spectra. Certain drugs such as positional isomers of some fentanyl related substances (FRS) can produce indistinguishable EI mass spectra but may be differentiated using retention time. The core structure of fentanyl consists of an amide group, a piperidine ring, an aniline ring, and an N-alkyl chain, each providing opportunities for points of substitution that create FRS and corresponding positional isomers. For this study, the analysis by GC coupled to a vapor-phase infrared spectroscopy detector (GC-IR) was used as a complementary technique to GC-MS for the identification of positional isomers of FRS. The result is a novel fentanyl library consisting of 212 different FRS reference compounds. A collaboration among three different laboratories yielded correct identifications of twenty blind samples when searched against the GC-IR FRS library created at Florida International University (FIU). The expected limits of detection for fentanyl using GC-IR range between 0.10 and 0.19 mg/mL, depending on the sample introduction (injector) method and other instrumental parameters. The newly created GC-IR library and its GC-MS counterpart of 212 FRS are shared in the supplementary materials for future use by researchers and practitioners.

Automated summary

Forensic laboratories use both gas chromatography mass spectrometry (GC-MS) and gas chromatography Fourier-transform infrared spectroscopy (GC-IR) for the identification of controlled substances. A novel fentanyl library consisting of 212 different fentanyl related substances (FRS) reference compounds was created, which can be used for future research and identification purposes.