Direct Compound-Specific Stable Chlorine Isotope Analysis of Organic Compounds with Quadrupole GC/MS Using Standard Isotope Bracketing

A method has been developed for the direct determination of the stable chlorine isotope composition (delta Cl-37) of organochlorines that eliminates sample preparation, achieves precision comparable to earlier techniques while improving the sensitivity, and makes use of benchtop gas chromatography-quadrupole mass spectrometry instruments (GCqMS). The method is based on the use of multiple injections (n = 8-10) of the sample, bracketed by a molecularly identical isotopic standard with known delta Cl-37, determined using off-line thermal ionization mass spectrometry (TIMS). Mass traces of two isotopologues differing by one chlorine isotope were used to calculate delta Cl-37 values. Optimization of mass spectrometry and peak integration parameters as well as method validation was achieved using tetrachloroethene (PCE), p,p'-dichlorodiphenyl-trichloroethane (DDT), and pentachlorophenol (PCP),spanning a delta Cl-37 range of -5.5 to +3.2 parts per thousand vs SMOC. Injecting 1.6-1100 pmol resulted in standard deviations (1 sigma) of 0.6-1.3 parts per thousand, and the delta Cl-37 results agreed with values independently measured with TIMS. The method was tested by determining the Rayleigh fractionation during evaporation of pure liquid PCE, resulting in a chlorine isotopic enrichment factor of epsilon(Cl) = -1.1 +/- 0.4 parts per thousand. Furthermore, position-specific delta Cl-37 analysis based on analysis of DDT mass fragments was evaluated. The GCqMS-delta Cl-37 method offers a simplified yet sensitive approach for compound-specific chlorine isotope analysis.