In purifying proteins by column chromatography, elution can be performed in different ways, depending on the type of matrix. For example, bound proteins can be eluted by gradually increasing or decreasing salt concentration in the solution applied to the column. For which of the following matrices do you think increasing and decreasing salt concentration, respectively, are used for elution?
A) Ion-exchange; hydrophobic B) Affinity; ion-exchange C) Hydrophobic; affinity D) Hydrophobic; ion-exchange E) Ion-exchange; gel-filtration