Good question. The tag is there for the sake of affinity purification, so it doesn't have to interact with the ion exchange resin at all. However, the tag's interaction with the ion exchange resin could be useful for the purification (if you haven't removed it first by proteolysis). I think you should just try some different pHs to see which gives the best result, if the pIs of the tag and the target protein are substantially different

Adam B Shapiro Thank you so much!!