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\bibitem{Toeroek_etal1986} {\textquotedblleft}\textit{Heterogeneity effects in direct XRF analysis of traces of heavy metals preconcentrated on polyurethane foam sorbents}{\textquotedblright}. T{\"o}r{\"o}k S, Braun T, Van Dyck P, Van Grieken R, X-ray spectrometry \textbf{15}, 7 (1986).   http://doi.org/10.1002/XRS.1300150104

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