Halas Stanislaw, Pelc Andrzej
Mass Spectrometry Laboratory, Maria Curie-Sklodowska University, 20-031 Lublin, Poland.
Rapid Commun Mass Spectrom. 2009 Apr;23(7):1061-4. doi: 10.1002/rcm.3970.
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).
迄今为止所描述的最精确的氯同位素分析方法是基于对定量转化为氯甲烷（CH₃Cl）的氯进行同位素比率质谱分析（IRMS）。这种气体可由几种含氯化合物产生并通过IRMS进行分析。然而，氯甲烷的质谱相当复杂，最丰富离子的比率（质量数52/质量数50）与³⁷Cl/³⁵Cl同位素比率不同。当δ³⁷Cl超过千分之10时，这种差异变得显著。此外，电子电离源在质量数50和52的有用质量处产生大约80%的所有离子种类。为了克服这些缺点，我们设计了一种负离子质谱仪，它保留了IRMS的所有最佳特性，包括带有转换阀的双进样系统、双收集器组件以及以CH₃Cl气体作为分析物。在改进的离子源中，我们用一根内部带有热金属丝的金属管取代了带有电子束的电离室。在该管内，³⁵Cl⁻和³⁷Cl⁻离子的产生效率取决于灯丝材料及其温度。在质谱中除了在百万分之一水平的质量数26和28处有微量痕迹外，未发现其他离子种类，这可能是由于CN⁻和CO⁻的形成。我们方法中使用的氯的最小量约为5微摩尔（3毫克AgCl），精度优于千分之0.005（1σ）。
