Fentanyl

Fentanyl, AcrylFentanyl, OcFentanyl and FuranylFentanyl were purchased from LGC Standards (LGC Standards S.r.L., Sesto San Giovanni, Milan, Italy) and www.chemicalservices.net (accessed on 5 October 2022). The test compounds were dissolved in absolute ethanol up to 20 mM stock solution and stored at −20 °C. Absolute ethanol concentration was maintained in the range 0–1% in all experimental conditions, to avoid potential solvent toxicity.

All experiments were performed using the following LC-MS-grade solvents and additives: acetonitrile (CHROMASOLV^TM, Honeywell International Inc., Charlotte, NC, USA); formic acid (Optima^TM, Fisher Chemical, Thermo Fisher Scientific Inc., Waltham, MA, USA); methanol (CHROMASOLV^TM, Honeywell International Inc., Charlotte, NC, USA); and water (LiChrosolv^®, Merck KgaA, Darmstadt, Germany).
The TFME coatings were prepared using concentrated hydrochloric acid (Fluka^TM, Honeywell International Inc., Charlotte, NC, USA), N,N-dimethylformamide (Sigma-Aldrich^®, Merck Group, Darmstadt, German), polyacrylonitrile (Aldrich^®, Merck KgaA, Darmstadt, Germany), 10 μm silica particles bonded with octyl (Luna^® C8(2), Phenomenex^® Inc., Torrance, CA, USA), and 3-cyanopropyl (Luna^® CN, Phenomenex^® Inc., Torrance, CA, USA) ligands.
Certified reference materials were purchased as 1 g L⁻¹ stock solutions in methanol. Fentanyl and zolpidem (tartrate) were purchased from LGC Standards (LGC Poland, Łomianki, Poland), and (±) methadone was purchased from Cerillant^® (Merck Life Science Sp.z.o.o, Poznań, Poland). Deuterium-labeled reference materials of Fentanyl D5, (±)-methadone D3, and zolpidem D6 were acquired from Cerillant^® (Merck Life Science Sp.z.o.o, Poznań, Poland) as 100 mg L⁻¹ stock solutions in methanol.