|Ventura, G.T., Kenig, F., Reddy, C.M., Frysinger, G.S., Nelson, R.K., Van Mooy, B., and Gaines, R.B. , Resolving the composition and formation of unresolved complex mixtures extracted from late Archean metasediments by comprehensive two-dimensional gas chromatography (GC×GC)., Organic Geochemistry, 2008; v39, 846-867.|
Hydrocarbon mixtures too complex to be resolved using traditional capillary gas chromatrography (GC) display gas chromatograms with dramatically rising baselines or “humps” of coeluting compounds that are termed unresolved complex mixtures (UCMs). Because the constituents of UCMs are not ordinarily identified, a large amount of geochemical information is never explored. Gas chromatograms of saturated/unsaturated hydrocarbons extracted from Late Archean argillites and greywackes from the southern Abitibi Province of Ontario, Canada contain UCMs with different appearances or “topology”, relating to the intensity and retention time of the compounds comprising them. The topology appears to have some level of stratigraphic organization, such that samples collected at any stratigraphic formation collectively are dominated by UCMs that either elute early (within a window of C15-C20 n-alkanes), earlymid (C15-C30 n-alkanes), or have a broad UCM that extends through the entire retention time of the sample (C15-C42 n-alkanes). Comprehensive two dimensional gas chromatography time-offlight mass spectrometry (GC×GC-MS) was used to resolve the constituents forming these various UCMs. Early- to mid- eluting UCMs are dominated by configurational isomers of alkylsubstituted and non-substituted polycyclic compounds that contain up to six rings. Late eluting UCMs are composed of C36-C40 mono-, bi- and tricyclic archaeal isoprenoid diastereomers. Broad UCMs contain nearly the same compounds as observed in the early-, mid-, and late retention time UCMs. Although the origin of the polycyclic compounds is unclear, the variation in UCM topology appears to depend on the concentration of initial compound classes that have the potential to become isomerized. Isomerization of these constituents may have resulted from hydrothermal alteration of organic matter.