Zinc chelates seem to recognize two exposed histidine tags in vicinal position and Copper chelates will recognize one single exposed histidine tag. This kit is recommended for proteins that are difficult to separate. It includes: 2ml of HIGH Density ZINC, 2ml of LOW Density ZINC and 2ml of LOW Density COPPER. HIGH Density ZINC provides a higher amount of available groups and usually allows for higher binding capacity. LOW Density ZINC & LOW Density COPPER have a lower amount of groups and it usually improves the selectivity/specificity.