After this, it was filtered through a 0.45 μm nylon membrane filter and 1 mL of this solution was placed into a 1.5 mL injection vial.
A 500 μg mL−1 standard stock solution containing the target compounds was prepared employing a 1% acetic acid solution adjusted to pH 5 as solvent. From this solution, working standard solutions from 1 to 50 μg mL−1 were daily prepared in the same solvent.