2.2. Preparation and storage of standards
Standard stock solutions containing 1000 mg/L each of BA or SA
was prepared in water (Milli-Q). The mixture was shaken well until
a homogenous and clear solution formed. If necessary, a small
amount of methanol was added to help dissolution. The same
method was applied to prepare a 500 mg/L internal standard CA
stock solution. All stock solutions were covered with aluminum
foil, stored in a freezer (4 C) and away from light (ready for a
maximum usage of one month). Before each experiment, BA and
SA standard solutions were prepared in the same volumetric flask,
and then diluted to five different concentrations in the range 20–
100 mg/L at constant intervals, where 50 mg/L internal standard
(CA) was added to every diluted standard solution.