Sample volume – 3 lL of chloroform phase was directly injected
into the HPLC system. Next on-line sample clean-up was achieved on the pretreatment precolumn Chromolith
Guard Cartridge RP-18e (Merck), 10 4.6 mm, with a washing mobile phase (methanol:water,
92:8, (v/v)) at a flow rate of 1.5 mL/min