桂建业,陈宗宇,张永涛
A novel approach to compound-specific stable chlorine isotope
analysis is presented to overcome many of the major drawbacks of existing
methods. Contemporary methods often require either several laborious off-line
procedures prior to isotope analysis or complicated mathematical data
calculations and evaluation schemes to derive the chlorine isotope ratios from
the mass spectra. In contrast to previous methods, our on-line setup was based
on the use of gas chromatography-negative chemical ionisation mass spectrometry.
The ion traces of 35Cl-(m/z 35) and 37Cl-(m/z 37) were recorded to
directly determine the isotopic ratios of chlorine. The chlorine fragments were
obtained from each specific chlorinated compound using the appropriate energy
and an optimised acquisition mode, and the method completely avoided isobaric
interferences. The principle and the mechanism of this approach were presented
and explained using trichloroethene. Optimisation of the gas chromatography-negative
chemical ionisation mass spectrometry parameters and method validation were
achieved using chlorinated alkenes, chlorinated benzenes, and organochlorine
pesticides. The sensitivity of the method over previous methods was
significantly improved, and the ultra-trace injection amounts of 0.4-473.5 pmol
resulted in standard deviations (1σ) ranging from 0.55-2.15‰. The present study
suggested that the gas chromatography-negative chemical ionisation mass
spectrometry method offered an innovative approach to compound-specific
chlorine isotope analysis, and it could facilitate the application of
compound-specific stable chlorine isotope analysis in environmental fields.