Alumina cartridges were previously conditioned
with 5 mL of ultrapure water, 5 mL of methanol and 10 mL
of ethyl acetate, percolated at 3 mL min−1, consecutively. The
sample was collected in a glass vial and the ethyl acetate
was evaporated under a gentle nitrogen stream until dryness at 35 ◦C. The residue was redissolved, first with 500 L of
methanol containing the IS at the concentration of 20 mg L−1
and thoroughly vortexed. Then 1.5 mL of ultrapure water was
added and vortexed again.