Compound-Specific Chlorine Isotope Analysis of Tetrachloromethane and Trichloromethane by Gas Chromatography-Isotope Ratio Mass Spectrometry vs Gas Chromatography-Quadrupole Mass Spectrometry: Method Development and Evaluation of Precision and Trueness

Compound-specific chlorine isotope analysis of tetrachloromethane (CCl₄) and trichloromethane (CHCl₃) was explored by both, gas chromatography-isotope ratio mass spectrometry (GC-IRMS) and GC-quadrupole MS (GC-qMS), where GC-qMS was validated in an interlaboratory comparison between Munich and Neuchâtel with the same type of commercial GC-qMS instrument. GC-IRMS measurements analyzed CCl isotopologue ions, whereas GC-qMS analyzed the isotopologue ions CCl₃, CCl₂, CCl (of CCl₄) and CHCl₃, CHCl₂, CHCl (of CHCl₃), respectively. Lowest amount dependence (good linearity) was obtained (i) in H-containing fragment ions where interference of ³⁵Cl- to ³⁷Cl-containing ions was avoided; (ii) with tuning parameters favoring one predominant rather than multiple fragment ions in the mass spectra. Optimized GC-qMS parameters (dwell time 70 ms, 2 most abundant ions) resulted in standard deviations of 0.2‰ (CHCl₃) and 0.4‰ (CCl₄), which are only about twice as large as 0.1‰ and 0.2‰ for GC-IRMS. To compare also the trueness of both methods and laboratories, samples from CCl₄ and CHCl₃ degradation experiments were analyzed and calibrated against isotopically different reference standards for both CCl₄ and CHCl₃ (two of each). Excellent agreement confirms that true results can be obtained by both methods provided that a consistent set of isotopically characterized reference materials is used. (Graph Presented).