8 Analytical Instrumentation
GCxGC Analysis of Complex Petroleum Hydrocarbons: Sulphur Speciation in Diesel
Jack Cochran and Jan Pijpelink, Restek Corporation 110 Benner Circle, Bellefonte, PA, US
Comprehensive 2D GC, also known as GCxGC, is a powerful technique with a great deal of potential for improving separations of complex petroleum and petrochemical samples that contain hundreds—or even thousands—of components. While GCxGC is still considered emerging technology and is used primarily in research and development labs, it has undergone significant growth in the past few years.
As evidenced by recent literature, industrial applications for GCxGC are growing in a number of areas; however, its ability to increase peak capacity and advance separations in complex matrices makes it particularly useful in the petro industries. GCxGC can be used to analyse hydrocarbons ranging in volatility from C4 (butane) to C40 (tetracontane), although with automatic flow programming of the cold jet in the thermal modulator analyses up to C48 have been reported [1]. Other notable recent advances include a full characterisation of middle distillates using supercritical fluid chromatography [2], a high temperature analysis of vacuum gas oils up to nC60 [3], and a partial characterisation of military fog oil [4]. Military fog oil is an extremely complex middle distillate product containing mainly aliphatic compounds. Using GCxGC-TOFMS, Kohl et al. were able to identify a wide range of low concentration aromatics from what would typically be an unresolved complex mixture. Compounds reported included alkanes, cyclohexanes, hexahydroindanes, decalins, adamantanes, bicyclohexanes, alkylbenzenes, indanes, tetrahydronaphthalenes, partially hydrogenated polycyclic aromatic hydrocarbons, biphenyls, dibenzofurans, and dibenzothiophenes. In these and other examples, the analytical power of comprehensive 2D GC is used to achieve a more advanced separation of complex matrices than is possible with 1D GC.
Technical Overview of GCxGC
In comprehensive 2D GC, two independent separations are applied to a single sample injection. A summary of the technique is given here; however, recent review papers provide a more complete description, focusing on recent advances and industrial applications [5,6]. In GCxGC, the first separation is usually based on boiling point and uses a standard nonpolar phase. Next, a thermal or valve modulator is used to focus the effluent from the first column onto the second column, which is a short (1-2 m) column and typically is a polar phase. Inverse polarity column set-ups are also sometimes employed, but with either approach the key to maximising use of the separation space is to choose orthogonal columns that differ significantly in selectivity. Separation results are displayed as a contour plot, which is a three dimensional representation of intensity (z) across the retention times of both column 1 (x) and column 2 (y). The result is a technique with increased peak capacity that can be used to separate compounds which coelute in the first dimension. Ultimately, the increased peak capacity obtainable with GCxGC allows better characterisation of complex petro fractions such as naphtha, gasoline, and diesel.
Sulphur Speciation in Diesel
The analysis of sulphurs, such as dibenzothiophenes, in diesel offers a timely example of the benefits of GCxGC for complex hydrocarbon samples. In recent years, new vehicle emissions standards have been adopted across the globe; these requirements have driven the development of ultra low sulphur diesel that is compatible with new emission control technology. In many countries, specifications for total sulphur in diesel have been reduced from 50 ppm to 10-15 ppm, making the analysis of various sulphur components even more crucial for the refinery industry. Using the analysis of dibenzothiophenes as an example, it is clear that 1D GC is insufficient to distinguish sulphur compounds due to coelutions with other components in the diesel matrix (Figure 1). An MS detector was used for this work because it offers the potential to identify a broad range of analytes from a single injection, compared to the limited information that can be obtained using a sulphur-specific detector. However, even with the power of the MS, the diesel sample was too complex to yield usable results. The spectrum taken at the retention time for dibenzothiophene confirms that the separation of components is not sufficient and the sulphur compounds cannot be identified (Figure 2).
In contrast, when GCxGC is used for the analysis of sulphurs in diesel, individual dibenzothiophenes are resolved both from the interferences that obscured them in the one- dimensional analysis and also from each other. The structured chromatogram (contour plot) that
Figure 2: Mass spectrum taken at dibenzothiophene retention time for one-dimensional GC of diesel. The mass spectrum is not representative of dibenzothiophene at all due to sample complexity.
Figure 1: Elution area of dibenzothiophenes in a one-dimensional GC analysis of diesel. The sample is so complex that no distinct chromatographic peaks can be seen.
August / September 2011 •
www.petro-online.com
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