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.