Chromatography 7
natural fit given its aptitude for analysing gasoline.
Jet fuel is a middle distillate, ranging from C6 to C21, making it a much more complex matrix than gasoline. As the carbon number increases, the number of possible isomers increases exponentially,
cresting well over 60,000 branched-chain isomers by C18. Also, the absorbance spectra of higher carbon-number saturates (starting
around C10) become too similar to distinguish. Because of these complexities, speciation is not practical, nor even the PIONA-type class analysis of gasoline. Therefore, the only hydrocarbon groups reported are total saturates and total aromatics, which is further subdivided into total mono-aromatics and total di-aromatics, analogous to data reported by D1319 and D1840.
Figure 3. Spectral deconvolution of these coeluting analytes is straightforward, as each compound’s VUV absorbance spectrum contributes to different regions of the acquired spectrum (note: asterisk colours correspond to compound name’s colour in legend).
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).
To check the accuracy of this VHA method, a well-characterised gasoline sample was analysed using both the 174-minute and 49-minute VHA methods as well as D8071. Mass percent data of all reported D8071 classes and species were compared for the three methods (individual compound values from VHA analyses were summed to get PIONA values). Both VHA methods compare well to the D8071 values, especially the 49-minute method (Table 3).
Table 3. Comparison of D8071 and two VHA methods for gasoline analysis. Both VHA methods correlate with D8071 well, especially the shorter (60m) method.
Figure 4. Similar to Figure 3, each hydrocarbon class analysed in the GC-VUV jet method is spectrally distinct, increasing the accuracy of deconvolution during data analysis.
Initial GC-VUV results for jet fuel analysis correlate favourably with D1319. Ten proficiency samples were analysed using GC-
Olefins, while reported in D1319 and some of the other alternate methods, are typically not present in jet fuel, and no olefins have been positively detected in any jet fuel samples analysed by GC- VUV. The VUV absorbance spectra of olefins have response in both the saturate (125 - 160 nm) and aromatic (170 - 200 nm) regions, which means that any reported olefin values are likely a misidentification of saturate-aromatic coelutions; therefore, olefins are not included in the searchable spectra library.
This method utilises the same hardware configuration as D8071 in a 14.1-minute analysis. The chromatography is compressed to an even greater degree, since the spectral deconvolution in the data analysis must only resolve saturates, mono-aromatics, and di- aromatics, all of which are spectrally distinct (Figure 4).
4.4. Jet Fuel Analysis by GC-VUV
Until recently, gas chromatography has not played a role in analysis of jet fuels. However, with D1319 no longer viable as the referee method, alternative methods were needed, and GC-VUV seemed a
Figure 5. A comparison of measured total aromatics content for 10 jet fuel samples. Note GC-VUV’s better correlation to D1319 than the other two methods.
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
