Heistand, Robert N.. "The Fischer Assay, standard method?" (PDF). Symposium on oil shale, tar sands, and related materials — production and utilization of synfuels. Diakses pada 18 Agustus 2008. "Salinan arsip"(PDF). Diarsipkan dari versi asli(PDF) tanggal 2012-02-11. Diakses tanggal 2014-06-03.
Heistand, Robert N.. "The Fischer Assay, standard method?" (PDF). Symposium on oil shale, tar sands, and related materials — production and utilization of synfuels. Diakses pada 18 Agustus 2008. "Salinan arsip"(PDF). Diarsipkan dari versi asli(PDF) tanggal 2012-02-11. Diakses tanggal 2014-06-03.