The analytical challenges of cyclopropylfentanyl and crotonylfentanyl: An approach for toxicological analysis

New psychoactive substances (NPS) are increasingly being seen in forensic casework globally and encompass a number of types of drugs including "designer opioids", especially fentanyl analogues, which are of particular concern due to their high potency and significant risk of toxicity. They are often sold as heroin or mixed with other illicit drugs and therefore users may be unaware they are taking such hazardous compounds. Two fentanyl analogues that have recently been detected are cyclopropylfentanyl and crotonylfentanyl. In order to accurately determine the prevalence of such compounds in clinical and forensic casework, including potential toxicity, they need to be correctly identified using definitive and defensible techniques. Cyclopropylfentanyl and crotonylfentanyl are structural isomers, and it has previously been highlighted that these 2 compounds are analytically difficult to specifically identify owing to their similarity in structure and chromatographic behaviour. To further investigate in an attempt to overcome this problem, analysis of certified reference material using high performance liquid chromatography with diode array UV detection (HPLC-DAD), liquid chromatography-tandem mass spectrometry (LC-MS/MS), and liquid chromatography-quadrupole time-of-flight-mass spectrometry (LC-QToF-MS) has been performed. Whilst the compounds were shown to have an identical mass-spectral fragmentation pattern, they had different UV spectra. This was coupled with a discernible difference in retention time with the HPLC conditions applied, allowing differentiation of the 2 compounds. Using this approach, cyclopropylfentanyl was positively identified and subsequently quantified in 4 fatalities with the exclusion of crotonylfentanyl.