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Molar mass of silicon is determined

Especially interesting for
  • analytical and physical chemistry
  • semiconductor (particularly silicon) technology

Within the scope of the Avogadro Project, PTB has measured – in a novel way – the molar mass of silicon with a relative measurement uncertainty of 1 · 10–8. The mass-spectrometric method developed for this purpose is also interesting for the determination of isotopic distributions of other elements in analytical chemistry and in semiconductor technology.

In order to determine the exact isotopic composition and – thus – the molar mass of the highly enriched 28Si crystal, a novel measuring  method was developed on a theoretical and experimental basis. Thereby, samples of the 28Si material are wet-chemically converted quantitatively in a single step in alkaline silicate solutions. Then, the ratio of the isotopes 30Si and 29Si is determined, which are present in extremely small amounts in the "Avogadro crystal". In this way it is possible to avoid the very difficult direct determination of the 28Si fraction.

The method is based on modified isotope dilution mass spectrometry (IDMS). The silicon sample of the "Avogadro crystal" was mixed with a second silicon sample – highly enriched in 30Si – in such a way that the isotopes 30Si and 29Si were present in a ratio of about 1:1.

The resulting relative measurement uncertainty of less than 1 · 10–8 could only be achieved through the use of a Multicollector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) with simultaneous ion current determination.

Both the ICP-MS and also the modified IDMS have for the first time been used for the determination of a molar mass. The method is generally suitable for the determination of the molar mass of elements with several isotopes which have one isotope with a heavy surplus or deficiency – as is, for example, also the case with calcium or strontium.

The novel exact correction factor determination for the ICP-MS is interesting for analytical chemistry. Possible industrial applications are, for example, purity determinations for semiconductor (particular silicon) technology.


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Physikalisch-Technische Bundesanstalt
Abbestr. 2-12
10587 Berlin

Scientific publications:

Rienitz, O.; Pramann, A.; Schiel, D.: Novel concept for the mass spectrometric deter - mination of absolute isotopic abundances with improved measurement uncertainty: Part 1 – theoretical derivation and fea- sibility study. Int. J. Mass Spectrom. 289 (2010) 47-53 Mana, G.; Rienitz, O.; Pramann, A.: Measurement equations for the determi - nation of the Si molar mass by isotope dilution mass spectrometry. Metrologia 47 (2010) 460-463 Pramann, A.; Rienitz, O.; Schiel, D.; Güttler, B.: Novel concept for the mass spectrometric determination of absolute isotopic abundances with improved measurement uncertainty: Part 2 – de - velopment of an experimental procedure for the determination of the molar mass of silicon using MC-ICP-MS. Int. J. Mass Spectrom. 299 (2011), 78–86