I analyzed the caffeine antioxidant activity with 100 µL serial concentration 1;1.5;2;2.5;3;3.5 mg/mL of caffeine add with 100 µL DPPH 0.2mMolar (Ethanol as solvent) using 96 wall plate while 100 µL DPPH solutions as blank. After 30 minutes incubation, the absorbances of the caffeine + DPPH showed higher value compare to DPPH solutions and the colour of caffeine mixture still remains in purple colour with wavelength settings of microplate readers at 516nm.

So I wonder, is there any possibility of colour interferences during the preparation or do you think if DPPH assays might be not suitable assay for caffeine