The fidelity of elemental concentrations and mass-dependent (δ18O, δ13C, δ15N, δ2H, δ34S, δ42/44Ca) and radiogenic isotopic compositions (87Sr/86Sr, Pb isotopes) in human hair in seawater and fresh water and implications for forensics, anthropology, and environmental exposure monitoring

Abstract:

Human hair is an important biological matrix that can record environmental exposure to heavy or toxic metals with a high temporal resolution. Isotope ratios of oxygen, hydrogen, strontium, and lead are used to trace mobility, while isotopes of carbon and nitrogen can show a history of diet, including starvation. In order to interpret measurements in hair, endogenous from exogenous sources must be separated and isotopic compositions should be retained during growth.

This study examined hair exposed to seawater doped with small amounts of heavy metals and drinking water for 30 minutes, 2 hours, 1 day, 1 week, and 1 month of exposure. Samples were rinsed and elemental concentration, δ¹⁸O, δ¹³C, δ¹⁵N, δ²H, δ³⁴S, ⁸⁷Sr/⁸⁶Sr, Pb isotopes were measured in the bulk hair. To evaluate the effectiveness of an aggressive leach to remove exogenous contaminants, splits of each sample were leached with 1 M trace metal grade hydrochloric acid, sonicated three times sequentially, and both the solid residual material and the leachate solution were measured for elemental composition and ⁸⁷Sr/⁸⁶Sr and Pb isotopes. Hair sampled sequentially from scalp to tip to evaluate potential changes due to equilibration with shower water was also measured.

Isotope compositions of elements structurally incorporated into the hair keratin protein (C, N, O, S) were retained even after one month of exposure to seawater. Other elements, including Pb and other heavy metals, were adsorbed outside of error within 30 minutes. Despite the aggressive leach which removed the majority of lead and strontium, the isotopic composition of both the leachate and the residual solid indicated that the hair elemental concentrations and isotopic compositions rapidly equilibrated with water. Increases in elemental concentrations from root to tip, including calcium and strontium, suggests the majority of metals are from exogenous sources. Trophic level changes in δ^42/44Ca values are well known; δ^42/44Ca values along the length of hair shows calcium is consistent with water sources at the tip, and approach known δ^42/44Ca values for blood nearer the scalp. These results have important implications for the use of hair as an indicator of mobility and environmental exposure in forensics, anthropology, environmental exposure monitoring, and nutrient deficiency.