9
Figure 1: a) Schematic of VUV spectrometer. Compounds eluting from the GC column are introduced to VUV-range light in the flow cell, and the resulting absorbance spectrum is recorded. b) Example spectra of a seven-carbon paraffin (heptane), isoparaffin (2-methylhexane), olefin (3-methyl-1-hexene), naphthene (methyl- cyclohexane), and aromatic (toluene). c) Example spectra of six closely-related aromatics. Each compound has a unique spectrum, but because they are all aromatics, they share the same aromatic spectral shape.
Figure 1a
Figure 1b
Figure 1c
deconvolution, allowing for additional chromatographic flexibility [20]. ASTM D8071 uses GC-VUV to analyse the paraffin/isoparaffin/olefin/ naphthene/aromatic (PIONA) and oxygenate content of fuels samples in a single run, and because it uses a simple setup with a run time under 34 minutes, it is an ideal solution for fuels analysis [21].
This study aims to demonstrate GC-VUV’s ability to analyse gasoline samples containing NTGAs. Rather than tailoring the method conditions to the new compounds, information on each NTGA was simply added to the spectral library and the samples were analysed using the method conditions listed in ASTM D8071 [21].
2. Experimental:
All samples were analysed with an Agilent 6890 gas chromatograph (Wilmington, DE, USA) paired with a VUV Analytics VGA-100 spectrometer (Cedar Park, TX, USA). The GC was equipped with a Restek Rxi-1ms column (Bellefonte, PA, USA) with dimensions of 30m x 0.25 mm ID x 0.25 µm and a Restek Topaz 4mm ID Precision inlet liner with wool. Helium with a purity of 99.999% was used as the carrier gas for the GC. Nitrogen with a purity of 99.999% was used as the makeup gas for the spectrometer.
All samples were run under the conditions specified by ASTM D8071 as follows:
Gas chromatograph
• Constant flow mode: 1.0 mL/min He • GC inlet: 250°C, split 300:1 • Injection volume: 1 µL
• Oven: 35°C (10 min), 7°C/min to 200°C VGA-100
• Makeup gas: 0.25 psi N2 • Acquisition range: 125-240 nm • Acquisition rate: 4.5 Hz • Transfer line temperature: 275°C • Detector flow cell: 275°C
Oxygenate-free gasoline was obtained from Valero (San Antonio, TX, USA), and E10 gasoline (containing 10% ethanol) was obtained from a local gas station. All NTGA compounds and other reagents were purchased from Sigma Aldrich (St. Louis, MO, USA) with a purity of >99%. Each compound analysed as an NTGA is listed in Table 1.
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