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\bibitem{Eltayeb+VanGrieken1989} {\textquotedblleft}\textit{Preconcentration and XRF-determination of heavy metals in hair from Sudanese populations}{\textquotedblright}. Eltayeb MAH, Van Grieken RE, Journal of radioanalytical and nuclear chemistry \textbf{131}, 331 (1989).   http://doi.org/10.1007/BF02060598

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