In order to purify a desired protein some physical methods are used to increase its concentration with respect to other proteins or impurities. The most widely used method is to precipitate some of the contamination proteins by addition of ammonium sulphate so that the desired proteins either remains in the supernatant or in the precipitate.  Which fraction contain the desired in the supernatant or in the precipitate. Which fraction contains the desired protein can be determined by measuring the specific activity of the protein. If the supernatant contains the desired proteins, the precipitate fraction is discarded . The active fraction (supernatant) is subjected to another round of salt precipitation so that unwanted proteins are eliminated.