The IDA molecule has 3 sites and the NTA molecule has 4 sites available for interaction with the Ni2+ metal ions. As a result generally, the NTA offers a stronger His-tag protein binding and slightly less metal ion leaching during purification process and therefore can withstand harsher purification buffers. However, it is more difficult to elute a protein from an NTA media due to these strong interactions. The His-tag protein binding is softer with the IDA media than with the NTA media providing a purification mechanism for His-tag proteins which is easier. There is a slightly higher metal leaching level for IDA media however as a result they can be recycled better and more so than NTA without loss of binding activity. Typically these cartridges are stripped and reloaded when a new His-tag purification is undertaken. This is the major reason for IDA being selected as the IMAC support of choice for the OmniSep A-Ni Dash cartridges. Iminodiacetic acid (IDA) is a tridentate chelating agent, covalently coupled to 6% highly cross-linked agarose beads and loaded with Ni2+ ion, which selectively and tightly bind histidine residues as can be seen below: