Studies of diesel system deposits continue to be the subject of interest and publications worldwide. The introduction of high pressure common rail systems resulting in high fuel temperatures in the system with the concomitant use of fuels of varying solubilizing ability (e.g. ULSD and FAME blends) have seen deposits formed at the tip of the injector and on various internal injector components. Though deposit control additives (DCAs) have been successfully deployed to mitigate the deposit formation, work is still required to understand the nature and composition of these deposits. The study of both tip and internal diesel injector deposits (IDID) has seen the development of a number of bench techniques in an attempt to mimic field injector deposits in the laboratory. One of the most used of these is the Jet Fuel Thermal Oxidation Tester or JFTOT (ASTM D3241). The tester was originally designed to assess the oxidation of jet fuel, based on the principle that low stability fuels produce deposits that form on metal surfaces. Recently it has been modified so that under suitable conditions it may be used to determine the deposit forming potential of diesel fuels. The JFTOT technique has been used by a number of groups to try and understand diesel injector deposits. The ineradicable nature of the material on the JFTOT tube has seen the deposits analyzed by laser scanning microscopy, ellipsometry and recently infra-red microscopy. Other methods have been invasive involving either solvent washing or scraping off the deposit. In this paper other techniques for the analysis of deposits will be described yielding both chemical and metrological characteristics of the deposits. Fourier Transform Infrared Microscopy (FTIRM), and Time-of-Flight Secondary Ion Mass Spectrometry (ToFSIMS) will be used to describe the surface characteristics. Measurements from a Profile meter will be used to estimate deposit surface roughness and data from Scanning Electron Microscopy (SEM) will be employed to describe the morphology. The final techniques described will be Direct Analysis In Real Time Mass Spectrometry (DARTMS) using ambient mass spectrometry. and Fourier Transform Ion Cyclotron Resonance Mass spectrometry (FTICRMS) The advantage of the DART method is that mixtures and objects can be subjected to mass spectrometric analysis with the minimum of pre-treatment and sample preparation. Thus the technique is well suited for analyzing deposits on JFTOT tubes as it requires little sample preparation. A number of studies of materials deposited on JFTOT tubes will be described showing the suitability of these techniques for analyzing and providing the potential characterization of JFTOT deposits. The FTICRMS will be used to assign species in the JFTOT test fuels both pre and post test.

A new ultra-high performance supercritical fluid chromatography -mass spectrometry (UHPSFC-MS) method has been developed using electrospray ionisation (ESI) to detect and quantify a new fiscal fuel marker, ACCUTRACE™ S10 that was introduced into fuel in the UK and Ireland from April 1st 2015. S10 is synthesised by the Dow Chemical Company and is used as a replacement for UV-visible fuel markers, such as quinizarin, Euromarker and Solvent Red 24. It is UV invisible, is doped at a low level (2.5 ppm) and was designed for detection using modern gas chromatography -mass spectrometry (GC-MS) instrumentation. All currently proposed methods for the determination of ACCUTRACE™ S10 fuel marker in diesel fuel [automotive diesel oil, current specifications EN 590 and U.S. ASTM D975] use gas chromatography-mass spectrometry (GC-MS), and 2-dimensional GC-MS (2-D GC-MS) is necessary to detect the marker at tank dilutions. However the lower limit of detection (LLOD) and lower limit of quantification (LLOQ) for S10 are severely compromised using legacy GC-MS instrumentation and measurement at tank dilutions is not feasible. ACCUTRACE™ S10 exhibited unusual ionisation characteristics when analysed by atmospheric pressure ionisation techniques, the ionisation is modelled and discussed and the strong ionisation response utilised to develop a new approach to detection and quantitation of the new fuel maker. The novel UHPSFC-MS method utilises this phenomenon, allows injection of undiluted fuels and affords detection and quantitation at doping and tank dilution levels.

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