Chlorine stable isotopes: A comparison of dual inlet and thermal ionization mass spectrometric measurements

Chlorine stable isotope ratios, Cl-37/Cl-35, currently are measured using d ual-inlet and thermal-ionization mass spectrometry, These two different ana lytical techniques, however, have never been cross calibrated. A set of sam ples with chlorine stable isotope delta values ranging from -4.4 to +0.3 pa rts per thousand relative to standard mean ocean water chloride has been an alyzed using both of these techniques. Our data show that both techniques c an yield similar results within analytical uncertainty. CsCl thermal ioniza tion data are extremely sensitive to the amount of chlorine being measured and cannot be used to determine absolute ratios without an independent mean s of correcting for machine-induced mass fractionation, As long as standard s and samples are of equivalent size, however, the differences between samp les measured by thermal ionization remain constant. Dual inlet stable isoto pe mass spectrometry is suited best for samples of >10 mu mol Cl, yielding chlorine stable isotope data with less than or equal to 0.1 parts per thous and reproducibilities (2 sigma), Thermal ionization mass spectrometry easil y accommodates samples of similar to 0.1-0.3 mu mol Cl, with achievable unc ertainties of less than or equal to 0.2 parts per thousand (2 sigma).