PURIFICATION, RECOVERY, AND LASER-DRIVEN FLUORINATION OF SILICON FROMDISSOLVED AND PARTICULATE SILICA FOR THE MEASUREMENT OF NATURAL STABLE-ISOTOPE ABUNDANCES

A procedure for the purification, recovery, and determination of isoto pic abundances of silicon from biogenic and lithogenic particulate mat ter and dissolved silicic acid is reported, Purification involves the reaction of acid molybdate with dissolved silicon in natural waters or that produced by the dissolution of particulate silica by hydrofluori c acid. The resulting silicomolybdic acid is then quantitatively preci pitated by reaction with triethylamine hydrochloride, The silicon is r ecovered as silicon dioxide through stepwise combustion of the dried p recipitate, Fluorination of the product for isotopic analysis is accom plished by laser heating under pure fluorine generated by the decompos ition of a fluorine-based salt, The resulting silicon tetrafluoride is separated from hydrogen fluoride and other fluorination byproducts cr yogenically using a variable-temperature cold trap, Yields for silicon recovery are 99.9% for precipitation and greater than 95% for the pur ification/fluorination procedure. Reproducibility of the isotopic comp osition for pure quartz granules processed through the procedure is +/ -0.1 parts per thousand for delta(30)Si.