Using a spark source mass spectrograph, a search has been made for undiscovered naturally occurring isotopes of low abundance. None was found, although the spark source instrument permitted examination of elements that have not been analyzed in this context for many years. The ability of the instrument to ionize all elements and the ease with which the rarer isotopes may be seen allowed upper limits of <0 000001 atom % to be placed on some isotopes. Be, P, Mn, Co, As, Rh, Au, and Bi were confirmed as monoisotopic.
F. D. Leipziger, "Some New Upper Limits of Isotopic Abundance By Mass Spectrometry," Appl. Spectrosc. 17, 158-160 (1963)
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