Graphene deposited onto aligned zinc oxide nanorods as an efficient coating for headspace solid-phase microextraction of gasoline fractions from oil samples☆
Introduction
Gasoline fractions generally refer to a liquid hydrocarbon (C4-C10) mixture initially distilled at <200 °C from crude oil. In most crude oil, gasoline fractions are rich in content and generally occupy its total 20%-40% [1]. In addition, gasoline fractions contain abundant n-alkanes, isoparaffin, naphthenes, aromatics, etc. Therefore, the content of gasoline fractions is not only an important indicator for gasoline products, but also an indispensable fundamental data for oil refining and processing. At present, analytical methods commonly used for the analysis of petroleum include gas chromatography-flame ionization detection (GC-FID) [2], [3], GC-mass spectrometry (GC–MS) [4] and GC-isotope ratio mass spectrometry (GC-IRMS) [5]. Generally, direct injection of crude oil into gas chromatograph is basically not recommended since the components with high boiling point will unavoidably remain on the column, thereby reducing the lifetime of the column and possibly causing contamination to subsequent analyses. Therefore, prior to GC analysis, selection of appropriate sample pretreatment methods is critical for the analysis of gasoline fractions in oil samples.
Traditional sample preparation techniques include liquid–liquid extraction and solid-phase extraction [6], [7], [8]. Although these methods are easy to carry out in routine labs, they require a large amount of organic solvents. Purge and trap is a solvent-free technique particularly suitable for the sampling of volatile gasoline fractions but it requires a specialized device compatible to GC [9], [10]. Single drop microextraction is another alternative feasible for gasoline extraction with very low solvent consumption [11], [12]. Since the organic microdrop is not stable during the extraction, an extra operation care must be taken to reduce the extraction irreproducibility. In addition, the solvent peak might overlap with some hydrocarbons with low molecular weight. Headspace solid-phase microextraction (HS-SPME) is a sampling tool in which a fiber coated with a small amount of adsorbent is placed above the samples to extract volatile compounds from sample matrix. This technique is considered to be an ideal method for the extraction of gasoline fractions from oil samples, since these compounds (C4-C10) has a high vapor pressure and tend to exist as gaseous forms, while those of macromolecular hydrocarbons are present with trace level at headspace due to their low vapor pressure. To date, very few studies have been reported using HS-SPME for the extraction of gasoline fractions from oil samples with commercial PDMS fibers [1], [13]. These commercial fibers provide good extraction reproducibility but suffer from being costly, fragile and nonresistant to organic solvent and high temperature. The purpose of this study is to tailor-make a SPME fiber, which is easy to prepare, chemically and mechanically stable, and more importantly, possesses high extraction efficiency towards gasoline fractions.
Since the gasoline fractions are mainly hydrophobic and volatile, our goal is to prepare SPME coatings with hydrophobic surfaces and high specific surface area. Aligned ZNRs have high specific surface area and they are easy to in-situ grown on a solid substrate using a hydrothermal method. However, the surface of ZNRs is considered as polar and so they show low efficiency towards nonpolar analytes [14]. Graphene (G) is an emerging two-dimensional carbon nanomaterial and it has been prepared as a SPME coating, which has proved to show excellent adsorption efficiency towards hydrophobic compounds due to its hydrophobicity and п-п localized system [15], [16], [17]. Our approach was to deposit a thin layer of G onto the surface of ZNRs as a composite SPME coating. By this means, the virtual surface area of G was effectively enlarged and the surface of ZNRs was changed from polar to non-polar. The G/ZNRs coated fiber was used to extract gasoline fractions from different kinds of oil samples and simulated oil-polluted sea water with HS-SPME mode, followed by the separation and detection with GC-FID.
Section snippets
Reagents and solutions
All solvents were of analytical grade and used as received. Graphite powder, potassium permanganate, concentrated sulfuric acid, sodium nitrate, hexamethylene tetramine and zinc nitrate hexahydrate were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Stainless-steel wires (SSWs) having O.D of 0.15 mm were provided by Shanghai Gaoge industry and Trade Co., Ltd. All oil samples were provided from the Shengli oil-field located at Dongying, China.
Instrumentation
All GC analyses were performed
Characterization of G, ZNRs and G/ZNRs coated fibers
G, ZNRs and G/ZNRs coated SPME fibers were prepared for comparison. As shown in Fig. 1a–b, a smooth and flat layer of G was coated onto the SSW. Fig. 1c–d shows that the ZNRs were well aligned on the SSW and they had hexagonal-like structures with a diameter of 200–300 nm. After depositing with G, a very thin layer was coated on the surface of ZNRs without changing their orientation and morphology (Fig. 1e–g). The EDS result (Fig. 1h) shows the presence of C (44.84%), Zn (38.72%) and O (16.44%)
Conclusions
Using aligned ZNRs as “hard templates”, a thin layer of G was immobilized onto the ZNRs as a SPME coating. This coating is mechanically and chemically stable and it combines the advantages of the high surface area of ZNRs and hydrophobic properties of G. Therefore, it shows higher extraction efficiency towards gasoline fractions compared to those of G, ZNRs and PDMS coated fiber. With HS-SPME mode, this coating can efficiently extract gasoline fractions from different kinds of oil samples and
Acknowledgements
This work was supported by the Applied Fundamental Research Program of Qingdao (15-9-1-94-JCH, 17-1-1-79-jch), the Natural Scientific Foundation of Shandong (ZR2016BQ23), and the Fundamental Research Funds for the Central Universities (No. 17CX02055), which are gratefully acknowledged.
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Selected paper from the 19th International Symposium on Advances in Extraction Technologies (ExTech 2017), 27-30 June 2017, Santiago de Compostela, Spain.