A method is provided for purifying a protein comprising the steps of providing a heme tag with an open coordination site and tagging the recombinant protein of interest with the heme tag. A resin framework is used, wherein a base that binds to heme is immobilized to the resin, and the open coordination site of the heme tag is capable of reversibly binding to the base immobilized to the resin. The tagged protein is reversibly bound to the resin, then eluted from the resin and quantified. The method enables the tagged protein to be tracked during protein expression or purification, and can be used to identify secretion of a protein to the periplasm or to tag proteins in the cytoplasm.