6
4. GC-VUV Fuels Applications 4.1. PIONA Analysis of Gasoline (ASTM D8071)
ASTM D8071 was officially approved in 2017 as a test method for determination of hydrocarbon group types, along with several select hydrocarbons and oxygenates, in gasoline-range fuels using GC-VUV. Most analytes are classed into one of the five PIONA hydrocarbon group types. Certain specific analytes are called out individually: the octane boosters methanol, ethanol, and isooctane, and light VOCs such as BTEX (benzene, toluene, ethylbenzene, and xylene isomers), naphthalene, and the methylnaphthalenes [13].
This method utilises a single 30-meter 100% PDMS GC column for its 33.6-minute analysis, compressing the chromatography and relying heavily on VUV’s ability to spectrally deconvolve coeluting analytes and quantitate by hydrocarbon class and carbon number during the data processing step (Figure 1).
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%.
Table 1. Relative response factors for the PIONA hydrocarbon classes and select speciated compounds.
Figure 1. VUV absorbance spectra from 125-240 nm for C6 compounds of each of the PIONA hydrocarbon class types. The shapes of these
spectra are representative of their respective class across the gasoline range.
Data is analysed using a novel automated quantitative method, time interval deconvolution (TID)[14]. Unlike a typical chromatographic quantitative analysis which sets quantitation windows in which a peak is measured in some capacity, TID divides the entire chromatogram into equal time segments, typically 0.02 minutes wide. For each time interval, the spectra within that interval are summed, and the summed spectrum is matched against the spectral library (approximately 770 compounds) for the best combination of 1 - 3 spectra (called a tiered search), depending on whether the addition of the second or third component improves the fit metric by a defined amount. In order to speed up the analysis, a given time segment is searched within a user-defined retention index (RI) window (typically ± 25 RI units) in the library [14, 15].
Using this iterative process, the total spectral response for each class/analyte is determined. Quantitative data are calculated without the need for calibration standards. Instead, relative response factors are used to calculate percent mass: for each PIONA class, an experimentally determined average response factor is used; for the individual target analytes, an experimentally determined response factor specific for that analyte is used (Table 1). Further conversion can be made to percent volume using either a class-based average density or an analyte-specific density.
This method operates with a high level of precision. Four ASTM gasoline proficiency samples were run 12 times each and analysed for each PIONA class as well as 9 targeted analytes. All %RSD values are below 6%, with all but four values below 2.5% (Table 2).
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.
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.
4.3. Verified Hydrocarbon Analysis Verified hydrocarbon analysis, or VHA, is a GC-VUV analogue
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124
