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\bibitem{Johansson_etal1975} {\textquotedblleft}\textit{Elemental trace analysis of small samples by proton-induced X-ray-emission}{\textquotedblright}. Johansson TB, Van Grieken RE, Nelson JW, Winchester JW, Analytical chemistry \textbf{47}, 855 (1975).   http://doi.org/10.1021/AC60356A035

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