Trace element concentrations in horn: Endogenous levels in keratin and susceptibility to exogenous contamination

As a recorder containing both physiological and environmental information, keratinized tissues, such as hair and feather, can be used to reveal geographical information, to monitor the exposure to pollutants, and to reconstruct dietary history. However, trace element analysis of keratinized tissues is complicated by the lack of reference endogenous ranges of trace element and the lack of understanding of the susceptibility of each element to exogenous contamination. The interior of animal horn is the cleanest of all keratinized tissues with minimum exogenous contamination because of its large size. Thus, the trace element concentrations in horn interior samples can provide reliable endogenous concentration ranges. Here we measured the concentrations of trace elements in horn interior samples of cattle and wild animals, which we propose to be used as the reference ranges for endogenous levels of trace elements in keratin. We calculated the enrichment factors of 30 trace elements in horn interior samples relative to the continental crust, which we considered the average exogenous contamination. We compared the ranges of elemental concentrations measured in horn interior samples, in the order of decreasing enrichment factor, to their reference ranges in hair, fingernails, and toenails, as well as their concentrations in caprine horns. Such comparison validates the use of the enrichment factor as an indicator of the susceptibility of an element to contamination: an element with a high enrichment factor is generally less likely to be affected by contamination and vice versa.