Polyhistidine-tags are often used for affinity purification of polyhistidine-tagged recombinant proteins expressed in "Escherichia coli" and other prokaryotic expression systems. Bacterial cells are harvested via centrifugation and the resulting cell pellet lysed either by physical means or by means of detergents and enzymes such as lysozyme or any combination of these. At this stage, raw lysate contains the recombinant protein among many other proteins originating from the bacterial host. This mixture is incubated with an affinity resin containing bound divalent nickel or cobalt ions, which are available commercially in different varieties. Nickel and cobalt have similar properties and as they are adjacent period 4 transition metals ("v. iron triad"). These resins are generally sepharose/agarose functionalized with a chelator, such as iminodiacetic acid (Ni-IDA) and nitrilotriacetic acid (Ni-NTA) for nickel and carboxyl-methyl aspartate (Co-CMA) for cobalt, which the polyhistidine-tag binds with micromolar affinity. Ernst Hochuli et al. coupled 1987 the NTA ligand and Nickel-ions to agarose beads. The resin is then washed with phosphate buffer to remove proteins that do not specifically interact with the cobalt or nickel ion. With Ni-based methods, washing efficiency can be improved by the addition of 20 mM imidazole (proteins are usually eluted with 150-300 mM imidazole). Generally, nickel-based resins have a higher binding capacity, while cobalt-based resins offer the highest purity. The purity and amount of protein can be assessed by SDS-PAGE and Western blotting.