A valve arrangement is disclosed for providing a switchable fluid connection between a fluid delivery system for driving a mobile phase and a stationary phase adapted for separating compounds of a sample fluid comprised in the mobile phase. In one purification method, substantially purified IFN-\u03b2 or variant thereof is precipitated and then dissolved in a guanidine hydrochloride solution. The cache occupancies of each of the threads for each of the scenarios are predicted.