Analysis of unresolved complex mixtures of hydrocarbons extracted from Late Archean sediments by comprehensive two-dimensional gas chromatography (GCGC) Gregory T. Ventura a, * , Fabien Kenig a , Christopher M. Reddy b , Glenn S. Frysinger c , Robert K. Nelson b , Ben Van Mooy b , Richard B. Gaines c a University of Illinois at Chicago, Department of Earth and Environmental Sciences, M/C 186, 845 West Taylor Street, Chicago, IL 60607-7059, USA b Department of Marine Chemistry and Geochemistry, MS#4, Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1543, USA c Department of Science, US Coast Guard Academy, New London, CT 06320-8101, USA article info Article history: Received 22 October 2007 Received in revised form 3 March 2008 Accepted 4 March 2008 Available online 26 March 2008 abstract Hydrocarbon mixtures too complex to resolve by traditional capillary gas chromatography display gas chromatograms with dramatically rising baselines or ‘‘humps” of coeluting compounds that are termed unresolved complex mixtures (UCMs). Because the constitu- ents 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 of the southern Abitibi Province of Ontario, Canada contain UCMs with different appearances or ‘‘topologies” relating to the intensity and retention time of the compounds comprising the UCMs. These topologies appear to have some level of stratigraphic organization, such that samples collected at any strati- graphic formation collectively are dominated by UCMs that either elute early- (within a window of C 15 –C 20 n-alkanes), early- to mid- (C 15 –C 30 n-alkanes), or have a broad UCM that extends through the entire retention time of the sample (from C 15 –C 42 n-alkanes). Compre- hensive two-dimensional gas chromatography time-of-flight mass spectrometry (GCGC– MS) was used to resolve the constituents forming these various UCMs. Early- to mid-elut- ing UCMs are dominated by configurational isomers of alkyl-substituted and non-substi- tuted polycyclic compounds that contain up to six rings. Late eluting UCMs are composed of C 36 –C 40 mono-, bi-, and tricyclic archaeal isoprenoid diastereomers. Broad UCMs spanning the retention time of compound elution contain nearly the same com- pounds observed in the early-, mid-, and late-retention time UCMs. Although the origin of the polycyclic compounds is unclear, the variations in the UCM topology appear 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. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The identification of individual molecular constituents within complex organic mixtures of petroleum and sedi- ment solvent extracts is typically achieved by a series of chemical and chromatographic separations. The most com- mon technique employs capillary gas chromatography (GC), often coupled to a mass spectrometer, in order to facilitate the separation, quantitation, and identification of individual molecular components in complex mixtures. Unfortunately, the chromatographic resolution afforded by capillary GC is insufficient to resolve some complex mixtures, which appears as a pronounced rising baseline or a series of rising baselines in a gas chromatogram (e.g. Gough and Rowland, 1990). In oils, the number of uniden- 0146-6380/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2008.03.006 * Corresponding author. Tel.: +1 508 289 5262. E-mail address: gventura@whoi.edu (G.T. Ventura). Organic Geochemistry 39 (2008) 846–867 Contents lists available at ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem