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ANALYTICAL CHEMISTRY
topic abundance measurements by the use of synthetic isotopic mixtures having very nearly the composition of the natural element." He adds that this "procedure . . . is, in many respects, hardh7 less exacting than the determination ot an atomic weight by chemical methods." At the risk of being presumptuous this precautionary statement may be expanded somewhat. One of the great advantages of mass spectrometry is that the measurements are not likely to be affected by chemical impurities. Impurities, other than those that are isobaric with the nuclides being measured, are separated and discarded by the instrument. For the same reason the stoichiometry of the substances used is of little importance. However, in synthesizing an isotopic mixture the composition of the compounds used in the mixture must be accurately known, both chemically and stoichiometrically. Thus it is necessary to make sure that pure isotopic preparations have adequate chemical purity. It is necessary also to convert them to reliable weighing forms in order to make sure that the synthesized mixture will contain the isotopes in the desired proportions. If separated isotopes of requisite isotopic purity are available, and if the proper preparation of the synthetic mixture can be assured, improvements in atomic weights can probably be realized most easily by mass spectrometry. The usefulness of the procedure has been recently demonstrated with silver (3> and chlorine, two elements that had been much studied chemically but whose atomic weights were nevertheless somewhat in doubt. The costs of separated isotopes are usually high, especially if the preparations have a high level of isotopic purity. They must be available in sufficient quantities for the necessary chemical purification, followed by conversion into weighing forms suitable for synthesizing a mixture of accurately known isotopic proportions. The discrepancy between the chemical and physical values for the atomic weight of germanium is of particular interest in view of Dr. Cameron's detailed analysis of the results of several isotopic abundance measurements for the clement, from which he concludes that the value 72.63 cannot reasonably be in error by as much as the difference between this and the chemical value, 72.59. The writer would not presume to question this conclusion, especially since the averages of five series of completely independent abundance measurements are reasonably concordant. However it is also difficult to under-
stand why the average of the eight series of chemical determinations should be in error by as much as 0.04. Both the chloride and the bromide of germanium were compared with silver and the respective silver halides. The work was done in two laboratories, using totally different starting materials, and showed an extreme range of 0.027. Review of the publications did not disclose likely sources of error large enough to account for the discrepancy. If the discrepancy is to be resolved by absolute abundance measurements, there will have to be provided at least two, and preferably all five, of the natural isotopes of germanium at a high level of isotopic purity. This will be a formidable and costly task. Of the ten elements for which atomic weights arc based on chemical determinations, seven besides germanium exist in nature in from 5 to 10 isotopic forms. For this group as a whole there would seem to be little prospect of getting better values soon from mass spectrometry. It is therefore desirable to consider whether new and useful evidence can be obtained from chemical determinations or, alternatively, from measurements of the density and lattice dimensions of crystals (see later discussion). At the outset, it seems reasonable to say that in any new work on atomic weights the accuracy target should bebetter that 1 part in 10,000 and, if possible, approach 1 part in 100,000. Such accuracy was attained in some of the earlier chemical work. Present-day chemical procedures have numerous refinements and other advantages over those in use a generation ago. Perhaps the greatest advantage is knowing that the atomic weights of the 21 mononuclidic elements now provide a network of reliable landmarks in a territory where once nothing was completely fixed except the defined atomic weight of oxygen. Among others may be mentioned more powerful techniques for preparing pure substances and for evaluating their purity by physical methods and by trace analysis. In reviewing the earlier publications one does not find reference to good fractionating columns, to cryometric methods for measuring purity, to zone refining, to the preparations of solids in the form of single crystals, or to the examination of solids for stoichiometric imperfections by the techniques now employed in research on the solid state. To be as sure as possible of attaining a sufficiently high level of purity, the earlier workers often were forced to much more time-consuming and laborious operations than would be necessarv todav
