Isotopes in biogenic silica
We use a PDZ Europa 2020 Mass Spectrometer (MS-2020; now supplied by Sercon Ltd., UK) with a coupled self-constructed laser fluorination line for analyzing stable oxygen isotopes in biogenic silica samples (diatoms, radiolaria, sponge spicules). Our research focuses maily on Arctic lake systems.
The data you receive were measured with a PDZ Europa 2020 mass spectrometer (MS-2020; now supplied by Sercon Ltd., UK). Prior to isotope analyses, all biogenic samples were thermally dehydrated under inert gas flow conditions by the so called Inert Gas Flow Dehydration (IGFD) technique in a horizontal ceramic tube furnace (Carbolite GmbH) flushed with helium or argon of quality 5.0 (Linde AG). For this, about 1,5 to 2 mg of sample material are weighed onto a Ni plate, which is step-wise heated to 1100°C under continuous Helium/Argon supply. This process immediately transports away any exchangeable oxygen from the sample, thus, not allowing it to re-interact with the sample. The dehydrated silica sample is transferred to the reaction chamber, where it is reacted by laser fluorination under BrF5 atmosphere. The generated O2 of the sample is separated of by-products and directed to a molecular sieve. From the molecular sieve the sample enters the mass spectrometer for isotope analysis.
For measuring stable oxygen isotopes in biogenic silica, a sediment sample is undergoes several preparation steps incl. wet chemistry and and heavy liquid separation to achieve a pure opal sample (> 97% SiO2).
Prior to isotope analyses, all biogenic samples are thermally dehydrated to under inert gas flow conditions to remove exchangeable oxygen group (i.e. 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 Argon 5.0 (Linde AG).
The liberated sample O2 is transferred to a through a cold-trap system to a molecular sieve and then inserted to the PDZ Europa 2020 mass spectrometer, when the oxygen isotope composition is measured against a standard of known isotope composition. The by-product SiF4is transferred to a borosilicate glass ampoule for silicon isotope analytics (δ29Si and δ30Si) in a Finnigan MAT 252 mass spectrometer at the Opal Isotope Lab in Bremerhaven.
Hence, δ18O and δ30Si can be measured at the same sample aliquot in close cooperation with the Opal Isotope Lab in Bremerhaven.
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-18O analysis of diatom silica and new methods for removing exchangeable oxygen– Rapid Communicatons in Mass Spectrometry 24: 2655–2664.
Dehydrated samples are then combusted under BrF5 atmosphere by means of a CO2 laser to quantitatively transform the biogenic opal to O2 and SiF4 (this process is called laser fluorination).