22 May / June 2019
GC-VUV seemed a 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
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).
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.
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.
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.
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).
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
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).
Initial GC-VUV results for jet fuel analysis correlate favourably with D1319. Ten proficiency samples were analysed using GC-VUV, D1319 (FIA), D5186 (SFC), and D6379 (HPLC-UV). While both the SFC and HPLC methods biased high on every sample for total aromatics, GC-VUV was evenly distributed on either side of the D1319 results, with four results slightly higher than, four slightly lower than, and
Table 4: Precision of GC-VUV jet fuel analysis run on six jet samples, with 8 runs per sample. All %RSD values are below 3.5%, and most are below 1%.
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