Papes for the Peeps – Analysis of Antioxidants in FuelsPosted: February 21, 2014
This is the latest in a series of posts where I attempt to translate my published research into a format suitable for a non-specialist audience.
My paper “Synthetic Phenolic Antioxidants in Conventional and Alternatively-Derived Middle Distillate Fuels Analysed by Gas Chromatography with Triple Quadrupole and Quadrupole Time of Flight Mass Spectrometry” was recently published in the ACS journal Energy and Fuels (paywalled).
This piece of work describes two new methods for determining antioxidant compounds in jet and diesel fuels. Antioxidants are added to some fuels to stop the fuel reacting with oxygen while in storage. When fuels react with oxygen, they can become unsuitable for use and cause engine problems. Although these antioxidants serve an important purpose, they are only permitted to exist in the fuel up to a certain concentration. Sometimes, if a fuel is suspected to be reacting with oxygen, the users might want to add antioxidant to stop the fuel from going bad – but if they don’t know how much antioxidant is in there (if any), how will they know how much to add without going over the limit?
The antioxidants are present in the fuel at very low concentrations, which makes it difficult to measure them without the bulk of the fuel interfering in the analysis. It’s possible to extract the antioxidants from the fuel, which then makes the measurement easier, but the extraction process is often long, resource intensive (uses lots of solvent) and frequently doesn’t work well enough. My laboratory recently acquired two new GC-MS (gas chromatography – mass spectrometry) instruments with advanced detection systems so I decided to see how these instruments would go at detecting antioxidants in fuels at low levels, and without any sample treatment.
Left: generic structure of these antioxidants, where ‘R’ can represent a methyl or tertiary butyl group in 1-3 of these R positions. Right: BHT, a common antioxidant used in fuels, foods and other products, where the R group opposite the OH is a methyl and the two R groups adjacent to the OH are tertiary butyl.
I have posted before about how gas chromatography and mass spectrometry work, and in this study it is the mass spectrometers that play a key role in the detection of the antioxidant compounds. The two different instruments I used are able to exploit different characteristics of the target molecules, in order to detect them at low levels, without interference.
The QQQ achieves excellent sensitivity by fragmenting molecules in the mass spectrometer more than once. For example, using the antioxidant shown in the picture above, the spectrum for this compound is
Which means that ordinarily, I would use the strong signal from the ion with a mass of 205 to look for this compound. But fuels have so many other moelcules in them, that there are loads of other compounds that also generate a signal at 205 and these swamp the signal from the target compound. So I can program the QQQMS to collect the strong ions, and perform another fragmentation on it. This generates a new mass spectrum with a new set of fragment ions. In this case, the fragmentation of 205 produces a signal at 145. So I can get the QQQMS to monitor these specific fragmentations, and keep track of the transition of each ion into another ion as it is broken apart in the spectrometer. So while there may be many compounds that have a signal at 205, there is only one molecule which has a signal of 205 fragmenting to 145. By using this approach, I can be very specific in my identification and measurement of my target compounds and this specificity brings with it excellent sensitivity and low detection limits.
The QTOF is able to detect very specific compounds because it can measure their mass very accurately. The other mass spectrometers in our lab are able to measure the weight of ions to one atomic mass unit (amu). Using the example above, the most accurate mass of the main ion we can obtain with these instruments is 205 amu. And again, there will be many other compounds with fragment ions of the same molecular weight. However, if we calculate the mass of this fragment (C14H21O) accurately, it comes out as 205.1587. Another possible ion with the same molecular weight is C13H19NO, but the accurate mass of this ion is 205.1461. This difference of 0.0127 amu is enough for the QTOF to distinguish between these two molecules, so I can program the instrument to look only for the accurate mass ion I’m interested in and discard the other closely matching, but interfering compounds.
Exploiting the strengths of these two mass spectrometers has allowed me to detect and measure low levels of antioxidant compounds in very complex fuel mixtures.