The most precise method of chlorine isotope analysis described to date is based on the isotope ratio mass spectrometry (IRMS) of chlorine quantitatively converted into chloromethane, CH(3)Cl. This gas can be produced from several chlorine-containing compounds and analyzed by IRMS. However, the mass spectrum of chloromethane is rather complicated and the ratio of the most abundant ions (mass-52/mass-50) differs from the (37)Cl/(35)Cl isotope ratio. This difference becomes significant when the delta37 Cl exceeds 10 per thousand. Moreover, the electron ionization source yields approximately 80% of all the ionic species at the useful masses 50 and 52. To overcome these drawbacks, we have devised a negative ion mass spectrometer which retains all the best features of IRMS, including a dual-inlet system with changeover valve, dual collector assembly and CH(3)Cl gas as analyte. In the modified ion source we have replaced the ionization chamber with an electron beam by a metal tube with a hot metal filament inside it. Within this tube the (35)Cl(-) and (37)Cl(-) ions are produced with an efficiency dependent on the filament material and its temperature. No other ionic species were found in the mass spectrum except of traces at masses 26 and 28 at ppm levels, probably due to the formation of CN(-) and CO(-). The minimal amount of Cl used in our method is of the order of 5 micromol (3 mg AgCl) and the precision is better than 0.005 per thousand (1sigma).
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