Identification of Source Environment from the Analysis of Rock Oil Extracts Utilizing the High Selectivity of Triple Quadrupole GC-MS (2024)

It has long been known that a range of non-native, stable, saturated hydrocarbon biomarkers present in crude oil extracts could be used to identify the original facie from which the oil was formed. Routine analysis is completed using Gas Chromatography (GC) with Mass Spectrometric (MS) detection using a single quadrupole analyser. Selective Ion Recording (SIR) of the 217Da fragment ion is used to identify the steranes of interest. However, this type of analysis is prone to interferences from other families of compounds found in rock oil extracts including hopanes, methyl steranes, and bicadinanes. In this paper we will show, through the use of a Waters Micromass Quattro micro GC Triple Quadrupole Mass Spectrometer, that specificity can be improved through the use of GC-MS/MS to selectively identify branched steranes (and hence the rock oils facie) while eliminating interferences seen during SIR analysis.

Figure 1. The sterane structure, relevant side chain length, the associated organism and hence environment from which the compound was produced.

These trace components from crude oil source rock extracts can be difficult to specifically identify due to the complexity of the extract. The current method used for sterane analysis is GC-MS (single quadrupole instrument) using Selective Ion Recording (SIR) of a known fragment (217 Da) from the sterane molecule. However, it is known that a number of compounds, e.g. bicadinanes from tree resins, hopanes and methyl steranes, can co-elute with many of the steranes, and produce a fragment ion of 217 Da. These compounds can be at much higher concentrations than the steranes of interest swamping the signal and making the unambiguous identification of these biomarkers difficult.

Two South East Asia samples were prepared for analysis from significantly different facies, these were a marine ecosystem and a terrigenous (higher land plant) ecosystem. The source rock was solvent extracted and the resulting residue was fractionated by polarity to produce a non-polar hexane fraction. The samples were analysed on an Agilent 6890 GC attached to a Waters Micromass Quattro micro GC Triple Quadrupole Mass Spectrometer operated in EI+ mode (Figures 2 & 3).

Figure 2. Waters Micromass Quattro micro GC MS System.

Figure 3. Waters Micromass Quattro micro GC schematic.

The samples were injected splitless (1 uL injection). Helium was used as carrier gas with a constant flow rate of 1.0 mL/min. The injector temperature used was 250 °C. The GC capillary column employed was a J&W Scientific DB-5MS column, 30 m x 0.25 mm i.d. x 0.25 um film thickness. The temperature of the transfer line was held at 280 °C during the chromatographic run. The temperature ramp rate employed was 120 °C (1 min hold) to 300 °C (8 minutes hold) at 15 °C/min, producing a total run time of 38 minutes.

Table 1. Compound type, their associated SIR fragment mass, fragment formula and dwell time.

Table 2. Compound type, their associated MRM transitions, molecular formula, fragment formula, dwell time, and collision energy.

Figures 4 and 5 show the SIR analyses of oils formed in a marine and terrigenous environment respectively showing the selected ions described in Table 1. The 85 Da channel shows the oil sample containing a high concentration of alkane compounds while the 369 Da and 191 Da channels show the presence of a number of bicadinane and hopane components. The bicadinanes are significantly more concentrated inthe terrigenous sample (Figure 5, 369 Da ion) due to the fact that the compounds originate from a resin exuded from higher plants.³ The 217 Da channel shows the presence of not only C27-C30 steranes but also any bicadinane, hopanes and methyl sterane interferences.

Figure 4. SIR analyses of an oil formed in a marine environment.

Figure 5. The SIR of an oil formed in a terrigenous environment.

Figures 6 and 7 show the MRM analysis of the marine and terrigenous oils with the five MRM transitions described in Table 2. The presence of the four steranes (their associated isomers) are confirmed from the relevant transitions displayed. The presence of C28 steranes in figure 6 indicates an algal contribution and with the significant quantities of C30 steranes this confirms the marine environment. The presence of C29 steranes in this sample also identifies the presence of some plant life. However, the low levels of the bicadinanes confirm the environment to be non terrigenous. Figure 7 shows a higher quantity of C29 steranes compared to the C28 sterane (possibly from freshwater algae or bacteria) and indicates a terrigenous rather than a marine environment. The lack of any C30 steranes and the presence of a significant quantity of bicadinane material further confirm this.

Figure 6. MRM analysis of marine oils.

Figure 7. MRM analysis of terrigenous oils.

Through the use of the Waters Micromass Quattro micro GC Triple Quadrupole Mass Spectrometer we have been able to specifically identify C27-C30 steranes and use the information to accurately identify the facie of the rock oil under investigation. Comparison with the SIR analyses clearly identifies the difficulties encountered when undertaking such an investigation on a single quadrupole instrument and that for rapid unambiguous routine analysis a triple quadrupole mass analyzer is required. The bicadinane interferences seen in the SIR experiments are clearly identified and shown to cause no interference when the same samples are analysed by MRM.