Preferably, the binding agent of the class-specific reagent is selected so that unbound materials can be separated easily from the bound material if desired. [0072] In one embodiment, immobilized metal affinity chromatography (IMAC) is used to capture on a solid phase, such as beads, all phosphorylated proteins contained in a sample, such as a cell lysate, e.g. as disclosed in Holmes, J. Liquid Ch