Milli q ro4 water purification system

All organic solvents, methanol and acetonitrile, were from J.T. Baker (Deventer, Holland) HPLC-grade. Water was purified by a Milli-Q-RO4 water purification system (Millipore, Bedford, MA, USA) with a resistivity of 10 MΩ·cm. The standard AA was purchased from Fluka (Deisenhofen, Germany). Potassium hexacyanoferrate (II) trihydrate and zinc sulfate heptahydrate were obtained from Merck (Darmstad, Germany). Oasis HLB 200 mg/6 mL SPE cartridges were obtained from Waters (Milford, MA, USA). Bond Elut-Accucat (200 mg/3 mL) SPE cartridges were purchased from Varian (Chicago, IL, USA) and used with a SPE vacuum manifold (Visiprep, Supelco, Bellefonte, PA, USA). Amber glass auto-sampler vials with septum screw caps were obtained from Agilent Technologies (Wilmington, DE, USA). The analytical column (Atlantis dC18, 250 × 4.6 mm/5) with pre-column and syringe filters (0.45 μm PVDF) were from Waters (Milford, MA, USA).
A standard AA stock solution (1.0 mg/mL) was prepared by dissolving 1.0 mg of AA in 1.0 mL of Milli-Q water. To weigh AA, an electronic analytical balance (Radwag, PCE group, 0.01 mg Radom, Poland) was used. The AA stock solution was diluted in amber glass volumetric flasks to prepare calibration standards at 50, 100, 200, 500, 800 and 1000 ng/mL respectively and stored at 4°C until use.