Given ppm can be expressed as mg/m3, assuming your percentage is weight in g per 100ml volume and 1ml of water is 1cm3. Therefore 2% would be 20,000g per cubic meter which is 20million mg. Therefore the ppm would be 20x106.

you can not convert IC to concentration

Dear Pangastuti, for the demonstration of antioxidant capacity result in terms of IC50; you must have percent inhibition versus concentration data. Percent concentration of your sample extract will not be useful for literature comparison, i.e. you should know amount of sample -rich in antioxidant compound- per a specified volume of solution (e.g. mg/ml).