PRECISE DETERMINATION OF STABLE CHLORINE ISOTOPIC-RATIOS IN LOW-CONCENTRATION NATURAL SAMPLES

Investigation of stable chlorine isotopes in geological materials has been hindered by large sample requirements and/or lack of analytical p recision. Here we describe precise methods for the extraction, isolati on, and isotopic analysis of low levels of chlorine in both silicate a nd aerosol samples. Our standard procedure uses 2 mug of Cl for each i sotopic analysis. External reproducibility (1 sigma) is 0.25 parts per thousand for the Cl-37/Cl-35 measurements. Chlorine is extracted from silicate samples (typically containing at least 20 mug of Cl) via pyr ohydrolysis using induction heating and water vapor as the carrier, an d the volatilized chlorine is condensed in aqueous solution. Atmospher ic aerosols collected on filters are simply dissolved in water. Prior to isotopic measurement, removal of high levels of SO42-, F-, and orga nic compounds is necessary for the production of stable ion beams. Sul fate is removed by BaSO4 precipitation, F- by CaF2 precipitation, and organic compounds are extracted with activated carbon. Chlorine is con verted to stoichiometric CsCl by cation exchange, and isotopic ratios are determined by thermal ionization mass spectrometry of Cs2Cl+. We d emonstrate that the sensitivity and precision of this method allow res olution of natural variations in chlorine isotopic composition, and th ereby provide insight to some fundamental aspects of chlorine geochemi stry.