U.S. flag

An official website of the United States government, Department of Justice.

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

NCJ Number
Forensic Chemistry Volume: 29 Dated: July 2022
Date Published
July 2022

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.


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. In the current project, 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. (Publisher abstract provided)

Date Published: July 1, 2022