1 mL spiked serum or urine (c: 1 µg/mL) + 200 µL 2 % H3PO4

Column conditioning: 5 mL methanol, then 5 mL water (Do not let run the column dry!)

Sample aspiration: The prepared sample is passed through the column by vacuum or pressure.

Washing: With the following washing mixtures the impurities are removed: a) 2 mL 0.1 N HCl b) 2 mL methanol

Drying: With nitrogen or air

Elution: Analytes are eluted with 3 x 2 mL methanol / 5 % NH3.

Evaporation and reconstitution with 1 mL of mobile phase from subsequent HPLC.