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516 A · ANALYTICAL CHEMISTRY, VOL. 57, NO. 4, APRIL 1985
Effects from radioactivity and other anomalies in isotopic abundances Unfortunately, most of the other elements with stable or quasi-stable isotopes are much more uncertain and, therefore, of greater justifiable con cern to most chemists. Nuclides with half-lives greater than 4 Χ 108 years are called quasi-stable (4). Radioac tive decay of these nuclides into other elements cannot cause a significant change of the atomic weight of the original element to the precision of its IUPAC tabulation in a million years. However, one has to watch the daugh ter elements. Because of nature's effi ciency in separating elements by their properties, the radioactive daughter in the geological past is typically deposit ed in isolation from the parent. This daughter element is produced over geologic time in only one of its iso topes. Take the example of rubidium; the less common of its two isotopes, 87 Rb, decays by the loss of a beta par ticle to 87 Sr. Now imagine the evapo ration of an ocean lagoon millions of years ago. Dissolved strontium is far less soluble than rubidium, so rubi dium is concentrated in the solids of the very last evaporation process long after virtually all the strontium has been deposited. Over the eons on dry land, the rubidium now produces strontium isotope with an anomalous atomic weight. Strontium-87 that is isotopically almost pure is found only in traces as an anomaly—accorded no more than a footnote in the IUPAC table. The atomic weight of this most unusual strontium sample will be 86.91, not 87.62 as in the IUPAC table. One cannot really be happy about this treatment of highly anomalous oc currences. After all, analysts are called upon to analyze the most unusual specimens, too; and they are apt to take the published IUPAC values for the atomic weights without a thought for the footnotes. Alternative treat ments of these very unusual isotopic compositions of naturally occurring specimens seem even less desirable. For example, one surely would not fa vor lowering the tabulated precision of A r (Sr) to 87.3(4), the (4) indicating uncertainty in the last digit. The di lemma is compounded because, as technology progresses, artificially in duced anomalies in isotopic composi tion will become more widespread. Consequently, analysts are also facing samples with inadvertent or undis closed changes in isotopic composi tion. Of course, the IUPAC table was never meant to be applicable to such altered specimens, but such samples represent a potential pitfall to ana lysts. Fission product palladium, if it enters the market in metallo-organic compounds, will have an atomic
