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42 May / June 2021


separation, it is particularly useful for petroleum analysis [6, 7, 14].


Petroleum biomarker analysis provides an example of the value of soft ionisation for GCxGC-HRTOFMS. Biomarkers are organic compounds that are present in petrochemicals and have characteristic distributions that reflect the original biological material from which the petrochemicals were formed [15]. Biomarkers measured by GC-MS and GCxGC-MS include steranes, phytane, and pristane. Biomarker analysis is used by petroleum geochemists to obtain information about the age, geographic origin and geographic distribution of oils. This information is important for petroleum exploration and forensic investigations of oil spills [16].


One-dimensional gas chromatography alone is insufficient to separate biomarkers in crude oil, so biomarker characterisation is commonly carried out by combined gas- chromatography and mass spectrometry. GC-MS methods rely on selected ion monitoring or better yet, high-resolution selected ion monitoring. Tandem mass spectrometry (GC-MS/MS) with selected reaction monitoring is an alternative approach that relies on the selectivity of MS/MS for high sensitivity measurements. On the other hand, comprehensive two- dimensional gas chromatography combined with high-resolution time-of-flight mass spectrometry provide greater separation, better signal-to-noise, and greater selectivity for isomeric biomarkers [17]. Additionally, FI and PI can be applied to biomarker analysis with GCxGC-HRTOFMS to simplify data interpretation by reducing fragmentation [5]. In this work, petroleum biomarkers were identified in crude oil by using FI and PI with GCxGC-HRTOFMS.


Experimental


An Agilent 7890B gas chromatograph fitted with a Zoex ZX-2 GCxGC thermal modulator was interfaced to a JEOL JMS-T200GC time- of-flight mass spectrometer equipped with a combination EI/PI ion source and EI/FI/FD ion source. A nonpolar BPX5 column (SGE Corporation, 30 m length, 0.25 mm I. D., 0.25 µm liquid phase thickness) was used for the first column. A more polar BPX50 column (SGE Corporation, 3 m length, 0.1 mm I. D., 0.1 µm liquid phase thickness) was used for the second column.


Experimental conditions are given in Table 1.


Table 1. Experimental Conditions. [Sample]


Crude Oil (NIST SRM1582, 5mg/mL (Hexane)) [GC condition] GC:


7890B (Agilent)


GCxGC modulator: ZX2 Thermal modulator (Zoex) GC column:


Modulation period: 6 sec Duration time: Inlet mode: Carrier gas: Oven:


[MS condition] MS:


Ion source: EI Ionisation: PI Ionisation:


FI Ionisation: Mass range:


1st: BPX5, 30 m x 0.25 mm, 0.25 µm 2nd: BPX50, 3 m x 0.1 mm, 0.1 µm


400 msec COC, Oven track mode


He, Constant Flow, 1.75 mL/min (350kPa) 60°C (0.1min)->3°C /min->300°C (20min)


AccuTOF GCv 4G


Dedicated EI ion source with a direct EI chamber, 200C Ionisation voltage: 70 eV, Ionisation current: 300 µA lamp: L7292 (Hamamatsu Photonics)


PI+, D2


Spectral distribution: 115 - 400 nm (10.8 eV@115 nm) Cathode: -10 kV, Emitter (CARBOTEC 5eV@115 mA/5 msec m/z 50-700


Recording interval: 0.04 sec (25 Hz)


Figure 1. EI mass spectra for cholestane (A) and stigmastane (B) from the NIST 2020 Mass Spectral Database, PI mass spectra for cholestane (C) and stigmastane (D), and FI mass spectra for cholestane (E) and stigmastane (F). The red arrows in (C) and (D) denote the characteristic sterane fragment ion C16


H26 Results and Discussion


Figure 1 shows the EI mass spectra for 5-a-cholestane (C27


H48 (C29 H52 ) and 5-a-stigmastane


) from the NIST 2020 Mass Spectral Database and the measured photoionisation


+ .


and field ionisation mass spectra for these compounds. Although the EI mass spectra exhibit molecular ions, extensive fragmentation is also observed, including


methyl loss to produce fragment ions 12


C27 H37 + and the isotope 13 C1 12C26 H37 + .


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