Organic solvents used were purchased from Merck Romania: acetonitrile, methylene chloride and diethyl ether. Phosphoric acid, potassium dihydrogen phosphate, sodium hydroxide, hydrochloric acid 37%, ammonia 25% and ultrapure water (HPLC-grade), were purchased from Merck Romania. The hydro-organic RPLC mobile phases contained one of the following ionic liquids: 1-butyl-3-methylimidazolium hexafluorophosphate BMIM[PF6] and 1-butyl-3-methylimidazolium tetrafluoroborate BMIM[BF4] (Acros Organics, USA) (Table 1 ). For the purpose of the experiment, the non-soluble ionic liquid BMIM[PF6] was dissolved in a small quantity of about 1 mL of acetonitrile and mixed in the mobile phase.
Analytical standards nicotine and cotinine were purchased form Lipomed GmbH, Weil am Rhein, Germany.
Individual standard stock solutions (1 g mL−1) of nicotine and cotinine were prepared in HPLC-grade methanol, and working solutions were prepared daily by appropriate dilution of the stock solution with water. Both the stock solutions and the working solutions were stored at 4 °C.
Analytical standards nicotine and cotinine were purchased form Lipomed GmbH, Weil am Rhein, Germany.
Individual standard stock solutions (1 g mL−1) of nicotine and cotinine were prepared in HPLC-grade methanol, and working solutions were prepared daily by appropriate dilution of the stock solution with water. Both the stock solutions and the working solutions were stored at 4 °C.