Acetonitrile and methanol (LC/MS grade), anhydrous magnesium sulfate (99.5% powder; MgSO4) and sodium chloride were purchased from Merck (Darmstadt, Germany); the Sepra C18-E sorbent (50 µm, 65 Å) used in the QuEChERS process was obtained from Phenomenex (Torrance, CA, USA).
The standards (certified reference materials) of THC (1.0 mg/mL in methanol T-005 Cerilliant), THC-d3 (1.0 mg/mL in methanol T-011 Cerilliant), 11-OH-THC (1.0 mg/mL in methanol H-027 Cerilliant), 11-OH-THC-d3 (100 µg/mL in methanol H-041 Cerilliant), 11-COOH-THC (1.0 mg/mL in methanol T-010 Cerilliant) and 11-COOH-THC-d3 (100 µg/mL in methanol T-004 Cerilliant) were acquired from Sigma-Aldrich (St. Louis, MO, USA). Deionized water was purified on a Milli-Q system (MilliporeSigma, Bedford, MA, USA). Working solutions were prepared in methanol. They were all kept under stable conditions at − 20 °C.
A standard mixture was prepared from individual stock standard solutions to obtain a concentration of 1 μg/mL in acetonitrile. All other working solutions for determining calibration curves, recovery percentages, and limits of detection were prepared by diluting the standard mixture in acetonitrile or in a blank blood sample.
The standards (certified reference materials) of THC (1.0 mg/mL in methanol T-005 Cerilliant), THC-d3 (1.0 mg/mL in methanol T-011 Cerilliant), 11-OH-THC (1.0 mg/mL in methanol H-027 Cerilliant), 11-OH-THC-d3 (100 µg/mL in methanol H-041 Cerilliant), 11-COOH-THC (1.0 mg/mL in methanol T-010 Cerilliant) and 11-COOH-THC-d3 (100 µg/mL in methanol T-004 Cerilliant) were acquired from Sigma-Aldrich (St. Louis, MO, USA). Deionized water was purified on a Milli-Q system (MilliporeSigma, Bedford, MA, USA). Working solutions were prepared in methanol. They were all kept under stable conditions at − 20 °C.
A standard mixture was prepared from individual stock standard solutions to obtain a concentration of 1 μg/mL in acetonitrile. All other working solutions for determining calibration curves, recovery percentages, and limits of detection were prepared by diluting the standard mixture in acetonitrile or in a blank blood sample.