Natural chelators such as citrate and vitamin C can be ingested in large quantities and then safely excreted. Drug based chelators are a different story. Lipoic acid is a pretty good chelator, but it should be taken with an RDA worth of biotin or more if quantities exceeding 100 mg lipoic acid are taken per day.
The amount of heavy metal chelated by natural chelators such as vitamin C and citrate is probably small in most individuals because their body burden of heavy metals is low.
But when the body burden is high, the amount of ingested chelator and the amount of chelated heavy metals in the urine may be high enough to measure with ultrasensitive techniques.
Would it not be a hoot if all the worry about mercury in fish -especially during pregnancy -could have been avoided by co-ingestion of higher levels of natural chelators such as citrate and vitamin C? Fish has so much going for it to let concerns about mercury be blown out of proportion.