Carbon Isotope Analyses of Semivolatile Organic Compounds in Aqueous Media Using Solid-Phase Microextraction and Isotope Ratio Monitoring GC/MS

Solid-phase microextraction (SPME) was used to facilitate the measurement of stable carbon isotope compositions (at natural abundance) of six organic compounds representing four compound classes in aqueous solution. Toluene, methylcyclohexane, hexanol, and acetic, propionic, and valeric acids were extracted from aqueous solutions with appropriate SPME phases and thermally desorbed into the split/splitless inlet of an isotope ratio monitoring gas chromatograph/mass spectrometer (irmGC/MS). Hydrophobic compounds (toluene, methylcyclohexane, hexanol) extracted by a nonpolar SPME phase were slightly (≤0.5‰) enriched in (13)C while organic acids extracted with a polar phase were depleted in (13)C to a somewhat greater degree (≤1.5‰) relative to material remaining in the aqueous phase. Isotopic fractionation was not observed to vary systematically as a function of equilibration time or solute concentration. Further, isotope fractionation did not vary consistently with the partition coefficient (K(fw)). However, both salinity and cosolvent effects, which altered the partition coefficients of the solutes, also yielded a reduction in the magnitude of isotopic fractionation (to ≤0.4‰ for the hydrocarbons, ≤0.5‰ for the organic acids). We conclude that fractionations are most likely associated with the interactions of organic compounds with the organic phase coating SPME fibers and are specifically due to mass-dependent energy shifts upon solution of each analyte into the organic phase. In addition, fractionations are also influenced by energy shifts associated with electrostatic forces acting on the analyte in the water phase during the partitioning process. The magnitude of isotopic fractionations can be minimized under conditions appropriate for the analysis of natural waters, and with careful calibration, SPME and irmGC/MS should be a valuable means for isotopic analyses for a wide range of organic constituents in aqueous samples.