Spatial concentrations of trace elements in keratin

Bulk analysis of trace elements in hair can be unreliable because there are two sources of trace elements: (1) an endogenous source, which is physiologically incorporated into hair structure during hair growth; and (2) an exogenous source, which accumulates on the surface layer and may then diffuse into the hair interior. Here, I studied the spatial distribution and diffusive behavior of trace elements to understand these two sources. Concentration profiles of cross-sections of hair and horn for Al, Mg, Zn, and Mn were obtained by laser ablation analysis. Internal trace element concentrations in animal horns were used as reference values for the physiological level of trace elements in keratin. In addition, trace element concentrations in both abraded and unabraded segments from elephant and giraffe tail hair were measured. By comparing those measurements, eleven measured trace elements are classified into four groups: (1) insoluble group, Al and Ti; (2) physiological group, Cu, Zn, and Se; (3) alkaline earth group, Mg, Ca, Sr, and Ba; and (4) special group, Mn and Pb. As a case study, two sources of Sr with distinct isotopic compositions were identified in a horse tail hair, using sequential, longitudinal sampling, and mixing model analysis. The classification of trace elements and Sr isotopic analysis provide guidelines for future studies on trace elements in hair.