Abstract
Solid-phase microextraction (SPME) coupled with gas chromatography-isotope ratio mass spectrometry (GC-IRMS) has already been applied to collect and identify volatile light hydrocarbons in oil and source rocks. However, this technology has not yet been used to analyze volatile light hydrocarbons in dry gas (natural gas with C1/C2+ > 95%). In this study, we developed a method to measure the molecular and carbon isotope composition of natural gas using divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) fiber. This fiber proved to be suitable for extracting C1–C9 hydrocarbons from natural gas without inducing carbon isotopic fractionation. Notably, the extraction coefficients of the analytes were not the same but rather increased with the increasing carbon number of the hydrocarbons. Nevertheless, we successfully identified 24 hydrocarbons from the in-lab standard natural gas, while also obtaining the carbon isotope composition of C1 to C9 hydrocarbons with satisfying repeatability. The relative standard deviation (RSD) of the molecular composition data was in the range of 0.06–0.74%, with the RSDs of the carbon isotope composition data not exceeding 1‰. Finally, seven natural gas samples, collected from different sedimentary basins, were successfully analyzed and the stable carbon isotope compositions of C1–C9 hydrocarbons present in these were determined through this method. Overall, the new approach provides a simple but useful technique to obtain more geochemical information about the source and evolution of natural gas.
Highlights
Volatile light hydrocarbons (C1–C9) are important components of crude oil because they possess a large scope of geochemical information that is of great significance to oil and natural gas exploration
After the extraction, the relative concentration (%) of C5+ hydrocarbons increased significantly, with their extraction coefficients ranging from 10 to 2000 (Figure 6). us, after the extraction, almost every hydrocarbon reached a concentration between 1% and 10%, which indicated that the fiber can balance the content of C1–C9 hydrocarbons in natural gas
An Solid-phase microextraction (SPME) fiber coating with DVB/CAR/PDMS is suitable for the extraction of C1–C9 hydrocarbons from natural gas, thereby allowing the extraction of 24 types of hydrocarbons, with an extraction coefficient increasing significantly upon increasing the carbon number
Summary
Volatile light hydrocarbons (C1–C9) are important components of crude oil because they possess a large scope of geochemical information that is of great significance to oil and natural gas exploration. Considering that natural gas, oil, and source rocks contain light hydrocarbons (C1–C9), we expect that, instead of using an indirect deduction via the molecular and isotope fractionation theory [22,23,24], a direct study of these components would allow the detection of the correlation between gas, oil, and source rocks [15] Implementing this new method to measure the molecular and carbon isotope compositions of light hydrocarbons in natural gas is difficult, Journal of Analytical Methods in Chemistry due to the limit of detection (LOD) values of analytical instruments. E fiber inside the needle is coated with a special polymeric stationary phase, which could concentrate the organic analytes from the sample matrix [26] This device has been widely applied in sample pretreatment technology [27], as well as to analyze the hydrogen isotope composition of light hydrocarbons in natural gas [28] and crude oil [29]. Conditions for the Analysis Instruments. e molecular composition of the gas samples was determined using a gas SPME holder
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