Isotopes in biogenic silica

We use a Sercon Geo 20-22 Mass Spectrometer with a coupled self-constructed laser fluorination line for analyzing stable oxygen isotopes in biogenic silica samples (diatoms, radiolaria, sponge spicules). Our research focuses mainly on Arctic lake systems. For measuring stable oxygen isotopes in biogenic silica, a sediment sample undergoes several preparation steps incl. wet chemistry and heavy liquid separation to achieve a pure opal sample (> 97% SiO₂).

Prior to isotope analyses, biogenic samples of ca. 1.5 mg are thermally dehydrated under inert gas flow conditions to remove exchangeable oxygen groups. For this purpose, we use the so called Inert Gas Flow Dehydration (IGFD) technique with step-wise heating to 1100°C in a horizontal ceramic tube furnace (Carbolite GmbH) flushed with inert gas i.e. Argon or Helium 5.0 (Linde AG). The dehydrated silica sample is transferred to the reaction chamber, where it reacts with a CO₂ laser under BrF₅ atmosphere (this process is called laser fluorination). The generated O₂ und SiF₄ of the sample is separated of by-products and directed to a molecular sieve. From the molecular sieve the sample enters the Sercon Geo 20-22 mass spectrometer for oxygen isotope analysis. The by-product SiF₄ is transferred to a borosilicate glass ampoule for silicon isotope analytics (δ²⁹Si und δ³⁰Si) in a Finnigan MAT 252 mass spectrometer in the ISOLAB facility. Hence, δ¹⁸O and δ³⁰Si can be measured at the same sample aliquot - a technique not widely available. 

The following publication deals with the laboratory routine of the oxygen isotope measurements of silicate material at AWI Potsdam:

B. Chapligin, H. Meyer, H. Friedrichsen, A. Marent, E. Sohns and H.-W. Hubberten (2010): A high-performance, safer and semi-automated approach for the δ¹⁸O analysis of diatom silica and new methods for removing exchangeable oxygen– Rapid Communicatons in Mass Spectrometry 24: 2655–2664.