BIST (Bridge Ion Separation Technology) columns are a new type of ion exchange columns. When double-charged buffer ions are present in the mobile phase, the surface can switch its polarity. This can only occur when the concentration of water in the mobile phase is relatively low. If the water content is high the ions form a solvation shell that prevents the surface from switching its polarity. This means that the charge of the surface can only be controlled by the amount of water in the mobile phase. The less water contained in the mobile phase, the higher the retention of analytes with the same charge. The more water contained in the mobile phase, the higher the retention of oppositely charged analytes.