Briefly, 100 μl of sample were mixed with 450 μl of distilled water in a test tube followed by addition of 30 μl of 5% NaNO2 solution. After 6 min, 60 μl of a 10% AlCl3.6H2Osolution were added and allowed to stand for another 5 min before 200 μl of 1 M NaOH were added. The mixture was mixed well using a vortex.