Atomic Weight of Silver Redetermined A new value of the atomic weight of s i l v e r ~ akey element in determining atomic weights of other elements-has been obtained by the National Bureau of Standards with the cooperation of the Atomic Energy Commission. The new value results directly from a highly accurate determination of the absolute isotopic abundance ratio of silver using the techniques of mass spectrometry. Natural silver consists of two isotopes of approximately equal abundance:, Agl@'and AgLoB. Although the mass of each isotope ia known in relative atomic units, until recently the isotopic abundance ratio (Ag^/AglO*) had not been measured accurately-that is, by using synthetic mixtures of silver isotopes as calibration samples. In general, the mass spectrometer will indicate slightly higher concentrations of one isotope than another, even though both isotopes may be present to the same extent. For this reason, it was necessary first of all to determine the instrumental bias. This bias was measured by calibrating the instrument with pure silver samples especially prepared to contain known abundance ratios of the isotopes. Separated silver isotopes were obtained from AEC's Oak Ridge National Laboratory for use as calibration samples. Five synthetic blends of silver were prepared with isotope ratios near that of naturally occurring silver. Once the mass spectrometer bias had been measured, the absolute isotopic abundance ratio of naturally occurring silver was accurately determined to be Ag^'/Ag109 = 1.07547 Â 0.0013. This ratio was measured for reference silver nitrate and for native silver from various parts of the
Volume 37, Number 10, October 1960
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