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 measurem ent of stable carbon isotope compositions (at natural abundance) of si x organic compounds representing four compound classes in aqueous solu tion. Toluene, methylcyclohexane, hexanol, and acetic, propionic, and valeric acids were extracted from aqueous solutions with appropriate S PME phases and thermally desorbed into the split/splitless inlet of an isotope ratio monitoring gas chromatograph/mass spectrometer (irmGC/M S). Hydrophobic compounds (toluene, methylcyclohexane, hexanol) extrac ted by a nonpolar SPME phase were slightly (less than or equal to 0.5 parts per thousand) enriched in C-13 while organic acids extracted wit h a polar phase were depleted in C-13 to a somewhat greater degree (le ss than or equal to 1.5 parts per thousand) relative to material remai ning in the aqueous phase. Isotopic fractionation was not observed to vary systematically as a function of equilibration time or solute conc entration. Further, isotope fractionation did not vary consistently wi th the partition coefficient (K-fw). However, both salinity and cosolv ent effects, which altered the partition coefficients of the solutes, also yielded a reduction in the magnitude of isotopic fractionation (t o less than or equal to 0.4 parts per thousand for the hydrocarbons, l ess than or equal to 0.5 parts per thousand for the organic acids). We conclude that fractionations are most likely associated with the inte ractions of organic compounds with the organic phase coating SPME fibe rs and are specifically due to mass-dependent energy shifts upon solut ion of each analyte into the organic phase. In addition, fractionation s are also influenced by energy shifts associated with electrostatic f orces acting on the analyte in the water phase during the partitioning process. The magnitude of isotopic fractionations can be minimized un der conditions appropriate for the analysis of natural waters, and wit h careful calibration, SPME and irmGC/MS should be a valuable means fo r isotopic analyses for a wide range of organic constituents in aqueou s samples.