MRM chromatograms of the quantifier and qualifier transitions of three isobaric compounds, in elution order, 3-methylfentanyl (351.2 > 202.2, 351.2 > 105.1), isobutyryl fentanyl (351.2 > 188.2, 351.2 > 105.2) and butyryl fentanyl (351.2 > 188.2, 351.2 > 105.2), at 30 pg/mg in hair.