Combined oxygen and silicon isotope analysis of biogenic silica

There is increasing interest in the use of biogenic silica O and Si isotope ratios to
understand climate and environmental processes. Virtually all of the fairly substantial body of literature
deals with either oxygen or silicon. This is partly because measurement of oxygen isotope composition
is done using either vacuum dehydration, isotope exchange or stepped fluorination techniques, while
increasingly researchers are turning to multicollector inductively coupled plasma mass spectrometry
(MC-ICP-MS) for Si isotope analysis, even though Si isotope analysis can be done using fluorination
methods used for the oxygen isotope measurements. Here we describe a procedure for simultaneous
determination of isotopic abundances of oxygen and silicon from the same aliquot of biogenic silica.
Pure silica is dissociated into O and Si compounds using a fluorination technique, in which reaction
with bromine pentafluoride (BrF5) produces oxygen (O2, subsequently converted to CO2), silicon
tetrafluoride (SiF4) and other fluorine by-products (e.g. BrF3). These compounds are cryogenically
separated using cold traps. Yields for oxygen and silicon recovery are 70–80% for biogenic silica
depending on the nature of the hydrous layer and 97–99% for pure quartz. Reproducibility of the
oxygen isotopic composition is ca. 0.3% and silicon between 0.06–0.12%.