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A systematic search has been made for the existence of previously unobserved naturally occurring isotopes of low abundance. A two-stage mass analyzer was employed. With the exception of Ta¹⁸⁰, none were found. The improved abundance sensitivity of the two-stage spectrometer, however, allowed determinations of lower experimental "upper limits" to be placed on a large number of isotopes. The elements, sodium, aluminum, scandium, niobium, and cesium were confirmed to be monoisotopic. A comparison has also been made of abundance measurements obtained with this instrument to those obtained in previous investigations.
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White et al. (1956) studied this question.
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