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A method has been developed for the direct determination of the stable chlorine isotope composition ([delta][sup.37]Cl) 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][sup.37]Cl, determined using off-line thermal ionization mass spectrometry (TIMS). Mass traces of two isotopologues differing by one chlorine isotope were used to calculate [delta][sup.37]Cl 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][sup.37]Cl range of -5.5 to +3.2%0 vs SMOC. Injecting 1.6-1100 pmol resulted in standard deviations (1[sigma]) of 0.6-1.3[per thousand], and the [delta][sup.37]Cl 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].sub.Cl] = -1.1 [+ or -] 0.4[per thousand]%0. Furthermore, position-specific [delta][sup.37]Cl analysis based on analysis of DDT mass fragments was evaluated. The GCqMS-[delta][sup.37]Cl method offers a simplified yet sensitive approach for compound-specific chlorine isotope analysis. 10.1021/ac902445f

Improved sensitivity in the analysis of stable chlorine isotopes of organochlorines ([[delta].sup.37]C1-OC1) has been established using sealed tube combustion in conjunction with thermal ionization mass spectrometry (TIMS). TIMS of chlorine isotopes was performed on