They are abundant in any soil or sediment that has supported biological activity, and they can bind to ancient bone, introducing carbon from the burial environment that does not reflect the age of the bone itself.

The XAD resins used in the purification process selectively bind humic acids while allowing purified collagen to pass through.

The result is a collagen fraction that is substantially freer of humic contamination than what earlier extraction methods could achieve.

For the Arlington Springs femora, which had spent approximately 13,000 years in alluvial sediments through which water had been moving — water that carried dissolved organic matter from the surrounding vegetation and soils — this purification was critical.

The contamination problem was severe enough that the osteocalcin fraction, extracted without XAD purification, gave a date thousands of years younger than the purified collagen.