21 Table 2: Precision of D8071 analysis run on four gasoline samples, with 12 runs per sample. All %RSD values are below 6%, and most are below 2.5%.
4.2. Conjugated Diolefins Analysis
Though not officially within the scope of ASTM D8071, conjugated diolefins can be analysed using the same GC-VUV acquisition method as D8071, in fact using the same quantitative analysis as well. The conjugated diolefins are spectrally distinct from any of the saturates, as well as the olefins and mono- aromatics, in that they have good absorbance past 200 nm (Figure 2). This makes the spectral deconvolution rather straightforward (Figure 3). Detection limits for the C5
-C8
conjugated diolefins range from 0.01-0.05% mass, with isoprene (2-methyl-1,3-butadiene) having the lowest detection limit.
4.3. Verified Hydrocarbon Analysis
Verified hydrocarbon analysis, or VHA, is a GC-VUV analogue of the DHA method ASTM D6730. This method was initially developed using similar conditions to D6730, which includes a 100-meter column and a cryogenic (5°C) oven start but no precolumn, as the critical separations like benzene and methylcyclopentene can be done spectroscopically. Eventually the method was translated to a 60-meter column in order to reduce run time while still maintaining good separation and similar elution order, reducing the run time from 174 minutes to 49 minutes. Currently the method reports up to 151 compounds by both mass and volume percent, and more compounds are being added to the spectra library. These 151 compounds account for approximately 90% of all gasoline components, giving good coverage across the hydrocarbon classes and carbon number (from C3
to C15 ).
Figure 2: VUV absorbance spectra of several hydrocarbon species. The selective VUV absorption in the 200-240 nm region for the conjugated diolefin isoprene (2-methyl-1,3-butadiene) makes it spectrally distinct from any of the PIONA spectra.
Table 3: Comparison of D8071 and two VHA methods for gasoline analysis. Both VHA methods correlate with D8071 well, especially the shorter (60m) method.
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