Chromatography
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 fl uorescent indicator adsorption (FIA) for both gasoline and jet fuels. However, these methods have several drawbacks, including long run times, complex confi gurations, 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 fi ngerprints in the vacuum ultraviolet wavelength range (125-240 nm) to identify and quantitate analytes. Using a single hardware confi guration, 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 liquefi ed petroleum gases (C1-C4) to gasoline (C5-C12) to middle distillates like jet fuel and diesel (C10-C20) and even the heavier oils and waxes (C20+) is one of the highest priorities for refi ners. 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 refi ning sector, determining fuel content can help refi ne process procedures and streamline quality control of their fi nished 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 fl ame ionisation detection (FID) and mass 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 fi ve hydrocarbon group types: paraffi ns (linear alkanes), isoparaffi ns (branched alkanes), olefi ns (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 fl ow-limited, and deconvolution software is not entirely reliable.
VUV spectroscopy is a relatively new GC detection methodology that combines qualitative spectral identifi cation - similar to mass spectrometry - with faster fl ow 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 fl ame 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 refi ned 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 identifi cation, since FID response does not provide any qualitative information of its own. The ‘tuning’ precolumn is
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