Absolute Silicon Mass Measurements in support of the International Avogadro Project and the Redefinition of the Kilogram
Robert D. Vocke Jr., Savelas A. Rabb, Gregory C. Turk
As the Planck and Avogadro constants are tied by the molar Planck constant, any redefinition of the kilogram using an accurately determined Planck constant (h) must also yield an accurate Avogadro constant (NA), and vice versa. We present the results of an absolute molar mass determination of the highly enriched 28Si crystal (AVO28) produced by the International Avogadro Project (IAP) using the virtual element approach pioneered by PTB. However, unlike previous studies, we use Tetramethylammonium Hydroxide (TMAH) to dissolve and dilute our samples. Silicon isotopic measurements were made on a high resolution multicollector inductively coupled plasma mass spectrometer. The use of TMAH as the matrix solution enhanced the Si signal sensitivity and also provided a rigorous cleaning procedure between samples. A unique approach to instrument tuning and hardware improved the quality of the Si isotopic measurements, particularly for the extremely low abundance Si isotopes, 29Si and 30Si, in the AVO28 material. The molar mass of AVO28 and the Avogadro constant were 27.976 969 72(12) gmol-1 and 6.022 140 69(14) x 1023 mol-1, respectively. Furthermore, the results of the IAP effort for the Avogadro constant, when the data from PTB, NRC and NIST are combined, meet the CCM requirements for metrological comparability and yield a value of 6.022 140 62(30) x 1023 mol-1 and a value for the Planck constant of 6.626 070 31(23) x 10-34 Js.