His-tag resins and purification kits can be pre-charged with nickel ions for convenience. Uncharged resins and kits that can be charged by the user are also available. These can be used with various metals to determine the best combination.

His tags are strings of molecules of the amino acid histidine that are added to either the C- or N- end (terminus) of a recombinant protein while it is being produced. Histidine has a tendency to bind to metal ions (nickel, cobalt, copper, and zinc), so the presence of these tags makes it possible to separate the tagged proteins from other peptides and proteins in the sample.

Recombinant proteins are produced by using viruses or plasmids to modify the gene sequences of the host bacteria to produce large quantities of a specific protein. This is a basic biotechnology tool for producing proteins.

After the proteins have been produced, the bacterial cells are lysed or burst. To collect only the desired recombinant protein, it must be separated from other proteins from the host cells. 

An affinity resin (made from sepharose/agarose) containing bound nickel or cobalt ions is added to the mixture. The resin is also available in bead, microplate, gravity-flow or spin columns, and other formats.

Following an incubation period, the resin or column is washed with phosphate buffer to remove any proteins that have not interacted with the metal ions. This purification technique is known as immobilized metal affinity chromatography or IMAC. 

The his-tagged proteins are often eluted from the resin using imidazole, which competes with the his-tag for binding sites on the resin. If imidazole is not suitable, lowering the pH of the column will strip the metal ions and therefore the bound protein at the same time.