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.
The latest research article to come out of my group is now available online in the American Chemical Society journal Energy and Fuels (paywalled). The title “Oxidation of neat synthetic paraffinic kerosene fuel and fuel surrogates: quantitation of dihydrofuranones” is probably more than enough to put off any non-specialist reader, but I think it is a reasonable summation of the entire paper. So in breaking down the title, I will explain what the paper is all about.
Oxidation: The word ‘oxidation’ can be used to describe lots of different chemical reactions, and will mean different things to people even within different disciplines of chemistry. In the context of this work, oxidation refers to the reaction of the molecules in fuel with oxygen from the atmosphere, or dissolved in solution, to incorporate the oxygen into the structure of the molecules.
Fig.1 An example of one type of oxidised fuel molecule.
Fuels oxidise naturally over time, but generally get used up (burned in an engine) long before it could become a problem. However, oxidation, like most chemical reactions, happens much more quickly at high temperatures. A lot of modern aeroplane and ships are designed in such a way that the fuels are cycled through hot areas of the engine or fuel system before they get burned. This means that the fuels can become oxidised within the fuel system in a matter of minutes or hours. The oxidation reactions lead to the formation of solid deposits and gums which can damage engine parts, making it run less efficiently and require more maintenance.
Neat: ‘Neat’ is one of those weird words that means something completely different in the scientific vernacular to regular conversation. Rather than meaning ‘nifty, good, or tidy’, the scientific ‘neat’ refers to something being pure, unadulterated, or unblended.
It is quite common for fuels to be blended with other fuels or additives before use, for lots of different reasons (I will talk more about this in the next section). But the fuels that we used in this study were used ‘as is’ – unblended and more or less pure. This makes it a little easier for us to investigate, eliminating the introduction of possible unknown quantities into the fuel.
Synthetic Paraffinic Kerosene Fuel (SPK): SPK is a generic term for jet fuel which has been created from a non-crude oil source, usually via one of two processes; synthesis from carbon monoxide and hydrogen gases (Fischer-Tropsch process), or processed biological oils and fats.
- Synthetic: This really just refers to the fact that these fuels are not traditional fossil fuels, made from dead dinos and dug up out of the ground.
- Paraffinic: Paraffin is just another name for hydrocarbon, the molecules which make up fuels.
- Kerosene: Kerosene is a generic name for a mixture of hydrocarbons with characteristics which makes it suitable for use as an aviation fuel.
Fuel Surrogates: This is a term we use for mixtures that resemble a fuel in a particular way, but have been simplified in order to study them in more detail. Fuels can contain over a million different chemicals, so it’s often necessary to create ‘model fuels’ from a reduced selection of chemicals which are far less complex and easier to analyse.
In this study we’ve used two different fuel surrogates. One is a single component surrogate, one pure compound which we studied to determine if the oxidation was dependent on reaction with other molecules in the fuel. Turns out it’s not, all you need is one hydrocarbon, oxygen and heat. The other surrogate has nine components, representing the main classes of compounds found in real fuels. This was able to give us a better idea of the range of chemicals that are formed when a real fuel is oxidised.
Quantitation: Quantitation is just a fancy way of saying ‘measured with a known amount of accuracy and precision’. Generally, chemical analysis can be qualitative (what is it?), quantitative (how much is there?), or both. In this paper we have used some different techniques to try and quantify the compounds of interest
- Fourier Transform Infrared Spectroscopy (FTIR)
An FTIR instrument uses molecular vibrations to look at the different functional groups within molecules. Generally the functional groups that exist in oxidised compounds would be well suited to FTIR analysis, but in this case the complexity of the fuel mixture coupled with the very low concentrations of the compounds of interest makes it quite difficult to get accurate determinations. This is why the 2 separation techniques described below are more useful.
- High Performance Liquid Chromatography (HPLC)
A liquid chromatograph is often used for quantifying lots of different chemicals and has a huge range of applications across many industries. In this case the HPLC was used purely for its separating power, in order to facilitate quantitation with another technique (GC-MS, below). The interactions that occur between the fuel sample and the instrument allow the compounds of interest to be (mostly) separated from the fuel matrix.
- Gas Chromatography-Mass Spectrometry (GC-MS)
GC operates on the same principle as HPLC above – that is, interactions between the instrument and the sample allow for separations of mixtures to occur. Here, GC is really useful because the separations are very high quality and made even better by the rough separation already carried out by the HPLC. The coupling of a GC to another instrument (mass spectrometer, MS) increases the power even more as it allows for fast and simple identification of the molecules in the mixture.
Dihydrofuranones: Now, I’ve saved the most exciting part for last. A dihydrofuranone (or furanone for short), is a molecule which arises when a hydrocarbon becomes oxidised and eats its own tail, forming a cyclic molecule. In the example below, the yellow-coloured section could represent any hydrocarbon chain. These types of molecules have only been seen in fuels before where the oxidation temperature was much higher, or the oxidation time was much longer.
Fig. 2 Generic structure of a furanone molecule.
So what, I can hear you say, so what? There are two ‘so what’ aspects to this.
- These furanones have two oxygen atoms very close to each other, incorporated into the molecule. Normal unoxidised fuel hydrocarbons have no oxygen molecules in them. The presence of oxygen atoms in the molecule like this tends to attract water into the fuel from the atmosphere, particularly in humid environments. When water gets all friendly with the furanone molecules and becomes incorporated into the fuel, this is really bad news. Water in fuel can form ice crystals, which block the fuel system, and has been known to cause crashes. It also increases wear and corrosion, so is generally a very undesirable thing to have in your fuel and you definitely don’t want stuff in your fuel which increases the susceptibility to take up water.
- The oxidation reactions don’t stop once the furanones are formed. The fuel keeps on reacting with itself and creating new molecules which then go on to form insoluble particles and gums in the fuel. This is another way that engine blockages and wear can occur. So if we can figure out the mechanism of how the gums and particles are formed, we can work out ways to stop it happening in the first place.
So there you go, a whole scientific paper explained using only the title. The next paper I’m working on is about trying to figure out faster and more accurate ways of measuring oxidation products in fuels.
This is the first in what I hope to be an ongoing series of posts explaining in simple terms what my scientific publications are about.
The first paper A Method for the Identification and Quantitation of Hydraulic Fluid Contamination of Turbine Engine Oils by Gas Chromatography – Chemical Ionisation Mass Spectrometry, in the journal Lubrication Science has just been made available in the print version but has been available online (paywalled) since June.
This paper is my first publication where I am the first author. First author publications are important for early career scientists as it is a part of demonstrating your capability as an independent researcher. First authorship also usually implies that you have done the majority of the experimental work included in the publication, and also written the bulk of the manuscript itself.
In a way, this publication is very unusual in that it is not in any way related to my main research project which I devote >90% of my time to. However, my lab has been searching for a solution to this problem for many years now, so it was an important piece of work in its own right.
What’s it all about?
It is a relatively common occurrence for the lubricant systems of aircraft to become contaminated through leaks, or human error. In many aircraft, the turbine engine lubrication system is separate from the hydraulic lubrication system, and employs two separate oils for these purposes. If the contamination occurs on a bulk level, it is easy to determine – the oils are different colours, have different flash points and viscosities. But when the contamination is at a low level, these differences become diluted to the point that they are no longer useful indicators. Another difference between the two oils is the distinctive set of additives which are used in each. But using these to detect contamination is also unsatisfactory, because the additives are not used in consistent amounts in the oil, so while the contamination may be detected, the amount can’t be calculated.
In order to detect low levels of contamination, we exploited the chemical differences between these two lubricants. Often when we are analysing our samples, we use an instrument called a ‘Mass Spectrometer’ (MS). The MS breaks apart molecules into pieces, which, oddly enough, is a good way of figuring out what they looked like before they broke apart! But when different oils are put into the MS, they break apart into the exact same fragments so we can’t tell them apart. Annoying. To overcome this, we introduced methane gas into the MS, which attaches onto the oil molecules before they are broken apart in the instrument. The inclusion of the methane means that now the different oils will break apart differently, and we can tell them apart. Using this method we can detect contamination down to 0.5%, which is equivalent to about a teaspoon’s worth of contaminant in a bucket.
A lot of this paper is about the chemical analysis technique that was used, and proving that the method actually works, which is really important when you are developing a complete new way of measuring something. But it’s not that interesting to explain, it basically involves running a lot of samples under specific conditions, repeated the same thing loads of times, and complete some statistical analysis on the data, to make sure that the method is robust. We also made sure that the method worked on oils which were fresh, and those which might have been used, heated or contaminated with water.
I’ve already seen the use of this new method have a real impact on the day to day operation of aircraft. It enables flight crews and aircraft maintainers to make more informed decisions about whether to fly or ground an aircraft with suspected contamination of the turbine oil. This is something quite satisfying and rewarding for me, given that my main research project is slightly more esoteric, with applications likely to be many years in the future.