“…As of 2017, the DEA has formed a signature program for fentanyl and fentanyl analogs to study inter-sample variations akin to that of the Heroin Signature Program, established in 1977 [26][27][28] [6, 8, 9, 11-15, 18-20, 30-35]. While attempts to profile heroin using an inorganic analysis of elemental impurities have been investigated [7,[36][37][38][39], the method has yet to become adopted for provenance determination, but rather shows promise for conducting street-level sample comparisons [40,41]. Budic and Klemenc reported the first instance of microwave-assisted acid digestion for the analysis of major crustal elements in heroin by inductively coupled plasma-atomic emission spectrometry (ICP-AES) [37].…”