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Sampling preparation scheme. Abbreviations: ISTD, internal standard spiking solution; Conc., concentration. “Pre-Mix” included adding ISTD and enzymatic solution (if measuring “total” concentration) to urine samples followed by gentle mixing. “Hydrolysis” included enzymatic and alkaline procedures in order and was omitted to measure “free” (“unconjugated”) concentrations. “SPE Extraction” included SPE pre-equilibrium sample cleanup and elution.
