5
necessary to help provide baseline separation (Rs > 1.5) of known coelutions like benzene/methylcyclopentene and m-xylene/p- xylene. Also, any unexpected coelutions cannot be quantifi ed, since the interfering compound(s) cannot be identifi ed [5].
2.2. Multi-Dimensional Gas Chromatography
Multi-dimensional GC-FID, under the ASTM D6839 method and the trade name ‘Reformulyzer’ from PAC International, measures paraffi ns (alkanes), olefi ns (alkenes), naphthenes (cycloalkanes), and aromatics (PONA) compounds (paraffi ns and isoparaffi ns are not separated) and oxygenates in gasoline and gasoline blend streams [6]. While this method touts a 39-minute run time, the setup is extremely complex. A single system contains 4 different columns, each with a unique stationary phase, as well as a hydrogenator, 3 separate traps - alcohol, olefi n, and EAA (ether- alcohol-aromatic) - all connected via 7 valves, with temperature controllers for each component. With multiple columns and connections, there is a greater chance for leaks or restrictions, and instrumental problems take longer to troubleshoot and repair, leading to longer periods of down time.
2.3. Fluorescent Indicator Adsorption
Perhaps one of the oldest fuels analysis methods still in use today is fl uorescent indicator adsorption (FIA). FIA was developed in the 1940s and approved as ASTM method D1319 in 1954, and it is still the primary test method for measuring saturates, olefi ns, and aromatics in gasoline, jet fuel, and diesel. Fuel samples are physically separated using silica gel fractionation, and the length of each cut, marked by fl uorescent dyes, is measured with a ruler [7]. However, the boundaries between the cuts are not always clean, and manual measurement adds a level of human error to the analysis.
Recently D1319 has come under serious scrutiny: the most recent batch of dyes do not fl uoresce properly in the aromatics region, which is particularly troublesome for jet fuel and diesel, as aromatics comprise up to 30% of the total volume. Furthermore, the sole manufacturer of this dye no longer exists, and so far, alternate syntheses of the dye have been unsuccessful. Current alternative methods include ASTM D5186 and D6379, which use supercritical fl uid chromatography (SFC) and high-performance liquid chromatography (HPLC), respectively.
2.4. UV Spectrophotometry
Naphthalenes, which contribute heavily to soot formation during combustion, are monitored in jet fuel using ultraviolet (UV) spectrophotometry (ASTM D1840), a method developed in the early 1960s. Fuel samples are diluted and their total absorbance at 285 nm is measured [8]. However, this method cannot give any qualitative information on the sample, and its low absolute absorbance can lead to a relatively large error range. Additionally, non-naphthalene di-aromatics (e.g., dibenzothiophenes, biphenyls) and tri-aromatics are known to interfere with accurate measurements, as these compounds also have UV absorbance at 285 nm.
2.5. Maleic Anhydride Method The original method for determining conjugated diolefi ns in fuels
(and one that is still in use today) is UOP326, also known as the maleic anhydride method. Originally developed in 1965, this method uses maleic anhydride as a dienophile in a Diels-Alder reaction with conjugated diolefi ns in the sample. Excess maleic anhydride is added to the sample and heated in a refl ux for 3 hours; the remaining maleic anhydride is then converted to maleic acid and measured by colorimetric titration [9].
Although UOP326 is still used today in some capacity, it has several drawbacks. The method takes over 3 hours, whether done manually or automated. Certain nucleophiles like alcohols and thiols (which are commonly found in or added to fuels) will also react with maleic anhydride, positively skewing values. Conversely, some sterically-hindered diolefi ns like 2,5-dimethyl-2,4-hexadiene will not react at all, negatively skewing values. Because of this lack of selectivity, the method is only semiquantitative and cannot give qualitative information, particularly which diolefi n species are present.
More recently, an assortment of other methodologies for measuring conjugated diolefi ns have been implemented, including derivatised-sample GC-MS or GC-nitrogen chemiluminescence detection (NCD), HPLC, SFC-UV, nuclear magnetic resonance (NMR), near infrared (NIR) spectroscopy, and voltammetry [10].
3. Vacuum Ultraviolet Spectroscopy Theory
VUV absorption spectroscopy is a new addition to the fi eld of analytical chemistry, though the concept of measuring in this spectral region has been used in synchrotrons for decades. Traditionally this type of spectroscopy required a vacuum environment to properly analyse samples, as atmospheric molecules like water and oxygen absorb in this wavelength region and thus interfere with measurements. However, the GC-VUV detectors from VUV Analytics overcome this problem by keeping the optical and detector environments under a positive pressure of an inert gas such as nitrogen or helium, eliminating potential atmospheric interferences.
Photons in the ‘vacuum ultraviolet’ spectral region (i.e., 125-240 nm) are absorbed by a molecule’s electrons depending on the molecular orbital transition and the energy required to bridge that HOMO-LUMO gap. Higher energy, lower wavelength photons will cause transitions between the sigma bonding (σ) or non-bonding (n) orbitals and the sigma anti-bonding (σ*) orbitals; lower energy, higher wavelength photons cause transitions between pi bonding (π) and pi anti-bonding (π*) orbitals. Most molecules have at least a single sigma bond, which means nearly all molecules absorb in this wavelength region. Furthermore, because each molecular orbital has a specifi c position in three-dimensional space, each molecule probed by VUV light will have a unique spectral absorbance across the wavelength range, sometimes referred to as a ‘spectral fi ngerprint’ [11, 12].
Quantitation using VUV spectroscopy is straightforward, as it follows the Beer-Lambert Law (absorbance linearly proportional to concentration), akin to other light spectroscopy techniques. First- order quantitation means coelutions can be linearly deconvolved with a high degree of accuracy. This allows the chromatography of GC-VUV to be deliberately compressed, leading to signifi cantly shorter run times [5].
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