Cryogenic zone compression for the measurement of dioxins in human serum by isotope dilution at the attogram level using modulated gas chromatography coupled to high resolution magnetic sector mass spectrometry.

A liquid nitrogen jet-cooled thermal modulator dedicated to comprehensive two-dimensional gas chromatography has been mounted in a GC oven coupled to a high resolution magnetic sector mass spectrometry instrument. The data acquisition parameters of the slow double-focusing magnetic sector MS instrument have been optimized to accommodate the description of the narrow modulated GC peaks. Acquisition rates were increased to 20Hz, while maintaining high mass resolution. Selected ion monitoring (SIM) descriptors, typically including several ions for both native and labeled analytes, were thus reduced to one or two to ensure enough MS cycle time. For maximization of the sensitivity enhancement due to cryogenic zone compression (CZC), the entire GC peak of interest was trapped and remobilized in one event. Optimization of the method resulted in the ability to detect low attogram (ag) amounts of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) (313ag gives a S/N of 400:1), a level that had not yet been attained using classical GC-HRMS. An isotope-dilution calibration curve was constructed using (13)C(12)-2,3,7,8-TCDD as the internal standard over the range of 500ag/μL to 35,000ag/μL (R(2)=0.9953). Analyses of a standard natural human reference serum-matrix NIST SRM 1589a containing 223ag of 1,2,3,7,8-pentachlorodibenzo-p-dioxin (1,2,3,7,8-PeCDD) (70% recovery rate assumed) resulted in a peak with a S/N of 188:1 (4 sigma, m/z=355.8546). Measurement of 2,2-bis (4-chlorophenyl-1,1,1-trichloroethane) (DDE) and 2,2',4,4',5,5'-hexabromobiphenyl (BB-153) in human dried-blood spot (DBS) samples is also reported to illustrate the usefulness of such a sensitive technique. Finally, some of the challenges related to sample preparation, blank levels, and to the fact of measuring of such a limited number of molecules (less than 600,000 TCDD molecules) are discussed.