The mixture was vortex-blended for 30 s, and deposited by centrifugation at 5000 rpm for 10 min. The clear and colorless
supernatant was then transferred to a new tube, and 200 lL of sulphanilic acid solution (10 mg mL1) and 200 lL of
1-naphtylamine solution (1 mg mL1) were added.