18 May / June 2019
Gas Chromatography -
Vacuum Ultraviolet Spectroscopy: A Versatile Tool for Analysis of Gasoline and Jet Fuels
Alex Hodgson, Jack Cochran, James Diekmann, Ryan Schonert VUV Analytics, Inc, 1500 Arrow Point Blvd, Suite 805, Cedar Park, TX 78613
Gas chromatography (GC) is a powerful analytical tool, particularly in the petrochemical industry. Current fuels analysis is performed using a variety of GC methodologies, including detailed hydrocarbon analysis (DHA) and multidimensional GC for gasolines, as well as fluorescent indicator adsorption (FIA) for both gasoline and jet fuels. However, these methods have several drawbacks, including long run times, complex configurations, and, in the case of FIA, grave concerns over the quality of the materials required to perform the analysis.
Vacuum ultraviolet (VUV) spectroscopy is a relatively new analytical methodology that utilises molecules’ unique spectral absorbance fingerprints in the vacuum ultraviolet wavelength range (125-240 nm) to identify and quantitate analytes. Using a single hardware configuration, GC-VUV can perform PIONA-class quantitative analysis - including speciation of conjugated dienes - in gasoline samples, as well as measure total saturates, aromatics, and di-aromatics in jet fuel samples.
1. Introduction
Characterisation of petroleum products, from liquefied petroleum gases (C1
-C4 gasoline (C5 -C12
fuel and diesel (C10 oils and waxes (C20
) to
) to middle distillates like jet -C20
+) is one of the highest
priorities for refiners. They must not only comply with various government-instituted environmental and public safety regulations but also constantly gain advantages over their competitors. In the downstream refining sector, determining fuel content can help refine process procedures and streamline quality control of their finished fuels [1, 2].
Since the early 1950s, GC has been the primary tool for analysing fuels. In the ensuing 70 years, many different detector types, foremost among them flame ionisation detection (FID) and mass
) and even the heavier
spectrometry (MS), have been used with gas chromatography to determine boiling point distribution, hydrocarbon class type, and, in certain cases, even speciation of petroleum products [3]. Current PIONA methods class most gasoline components into one of five hydrocarbon group types: paraffins (linear alkanes), isoparaffins (branched alkanes), olefins (alkenes), naphthenes (cycloalkanes), and aromatics.
While many of the current analytical methods are widely accepted as ‘gold standards’, technologies and methods are constantly being improved and updated. The most common shortcoming most GC methodologies experience is long run times. Petroleum samples are some of the most complex matrices: gasoline contains hundreds of compounds, and some of the higher-carbon cuts can contain thousands. Most detectors cannot provide any qualitative information of eluting analytes, requiring baseline separation of peaks to obtain the most accurate data. Even those detectors that can identify compounds (e.g., mass spectrometry) are flow-limited, and deconvolution software is not entirely reliable.
VUV spectroscopy is a relatively new GC detection methodology that combines qualitative spectral identification - similar to mass spectrometry - with faster flow rates, allowing for shorter run times while still maintaining high accuracy and precision. This paper details several GC-
VUV petrochemical applications that are impacting the petrochemical industry.
2. Current Analysis of Fuels 2.1. Detailed Hydrocarbon Analysis
Detailed hydrocarbon analysis (DHA), under the ASTM D6730 method, is a widely utilised methodology for gasoline-range fuels analysis. This method purports speciation of up to 600 compounds, though not all are named. It employs a 100-meter 100% poly-dimethylsiloxane (PDMS) column - with a 5% diphenyl PDMS ‘tuning’ precolumn - connected to a flame ionisation detector (FID), a cryogenic starting oven temperature (5°C), and a run time of 174 minutes to maximise baseline separation of analytes, allowing for a high degree of speciation for PIONA compounds and select oxygenates [4]. The method has since been refined and the run time shortened, down to as low as 38 minutes in some cases.
The major drawback of DHA is that it relies solely on peak retention time for identification, since FID response does not provide any qualitative information of its own. The ‘tuning’ precolumn is necessary
to help provide baseline separation (Rs > 1.5) of known coelutions like benzene/ methylcyclopentene and m-xylene/p- xylene. Also, any unexpected coelutions cannot be quantified, since the interfering compound(s) cannot be identified [5].
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