Stock solutions of each amine at the concentration of 1 mg mL1
were prepared by diluting the standards in 0.1 N hydrochloric acid
(HCl); the stock solutions were stored at 471 °C. After this procedure,
1 mL of each stock solution was transferred to a volumetric
flask, resulting in a standard solution with a final concentration of
0.143 μg mL1 for each amine.