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Representative selected ion monitoring chromatograms of: a a blank blood sample and b a spiked blood sample with the four fentanyl analogues at the respective limit of quantification (LOQ) concentrations (1.00 ng/mL for furanylfentanyl and ocfentanil, and 0.50 ng/mL for acetylfentanyl and butyrfentanyl). Rt retention time
