Chemical quantitation via GCxGC-FID
The quantitative chemical characterization of the oil samples was carried out using a comprehensive two-dimensional gas chromatography system Agilent 7890B GC (GCxGC) coupled to a flame ionization detector (FID). A thermal modulator cooled with liquid nitrogen (LECO Corporation, Saint Joseph, MI), an Agilent 7683B series injector, and a HP 7683 series autosampler were also used. A reversed phase column configuration was selected having a primary mid-polar column DB-17ms (30 m x 0.25 mm x 0.25 µm) and a secondary nonpolar column DB-1ms (0.8 m x 0.25 mm x 0.25 µm). Both columns were provided by Agilent (Santa Clara, CA). Ultrahigh purity helium (99.9999 %) was used as the carrier gas at a constant flow rate of 1.5 mL/min, and a front inlet septum purge flow of 3 mL/min and a front inlet gas saver flow of 20 ml/min were set in the GC x GC system. A split inlet liner with glass wool suitable for low pressure drop provided by Agilent (Santa Clara, CA) was used to protect the system due to the nature of the samples under study. The temperature of the front inlet, initial oven temperature, and the target oven temperature were set at 280 °C, 40 °C (hold time of 0.2 min), and 260 °C (hold time of 5 min), respectively. An oven temperature ramp rate of 3 °C/min was considered for a total GC method time of analysis of 4,712 seconds (78.5 min) per sample. Secondary oven and modulator temperature offsets were set at 50 °C and 15 °C, respectively. A modulation period of 2.5 s was used to avoid wraparound. Samples of 0.5 µL were injected to the system using a manual dilution factor of 100 (10 µL of sample in 1 ml of pentane) and automatic split ratio of 20:1. The development and refinement of the GCxGC-FID classification map (Supplemental Figure 9 ) was made based on the methodology described in a previous study.67 Thus, the chemical characterization encompassed nine hydrocarbon groups and carbon numbers as follows: n-paraffins (C6-C32), iso-paraffins (C6-C32), monocyclo- (C6-C30), dicyclo- (C8-C20), and tricyclo-paraffins (C10-C16), alkylbenzenes (C6-C20), cycloaromatics (C9-C18), naphthalenes (C10-C19), and triaromatics (C14-C19). Data were processed in ChromaTOF software version 4.71.0.0 optimized for GC x GC – FID (LECO Corporation, Saint Joseph, MI) with a signal-to-noise ratio of 50. Weight percentage (wt. %) relative to each hydrocarbon class and carbon number was calculated via normalizing the peak area by integration of the GC x GC chromatograms extracting solvent and column bleed peaks using Microsoft Excel - 365.