Article pubs.acs.org/EF
Investigation of the Thermal Oxidation of Conventional and Alternate Aviation Fuels with Comprehensive Two-Dimensional Gas Chromatography Accurate Mass Quadrupole Time-of-Flight Mass Spectrometry Renée L. Webster,†,‡ Paul M. Rawson,†,§ Chadin Kulsing,‡ David J. Evans,† and Philip J. Marriott*,‡ †
Defence Science and Technology, 506 Lorimer Street, Fishermans Bend, Victoria, Australia 3207 Australian Centre for Research on Separation Science, School of Chemistry, Monash University, Wellington Road, Clayton, Victoria, Australia 3800 § School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, 124a Latrobe Street, Melbourne, Victoria, Australia 3000 ‡
ABSTRACT: Oxidation of aviation fuels prior to combustion affects the safe operation of high performance aircraft. This study reports the use of comprehensive two-dimensional gas chromatography (GC×GC) with accurate mass time-of-flight mass spectrometry (accTOFMS) to study oxidized species in complex thermally oxidized conventional and alternate aviation fuel matrixes. Tens of thousands of unique compounds were identified in the fuels, highlighting the need for comprehensive separations. GC×GC operation allowed a range of oxidized species to be isolated and identified, and in a conventional fuel, a series of homologous aldehydes and 2-ketones were found. 4-Methyl-2-hexanone and 1-pentanol were found to be strongly correlated with established methods for quantitatively assessing fuel stability.
1. INTRODUCTION Stability is a critical performance requirement of fuels used in modern jet aircraft and refers to the ability of a fuel to resist oxidation during heating or extended storage.1 The use of fuels as cooling fluid in high performance jet aircraft means that they are exposed to increased heat loads over an extended period of time, which induces autoxidation reactions of the fuel hydrocarbons.2 The formation of oxidized species generated in these oxidation reactions is implicated in the formation of insoluble deposits in fuel systems, and also the development of undesired fuel properties such as poor water separability.3 Synthetic and other alternatively derived fuels have been reported as having good thermal stability properties, resisting thermal oxidation much better than conventional jet fuel,4 notwithstanding the requirements of addition of materials such as synthetic phenolic antioxidants to the fuel.5 As the use of these fuels increases around the world, it is important to understand the process occurring at a molecular level.6 Thus, it is necessary that molecular speciation of the fuel is wellcharacterized. Jet fuels from both conventional and alternate feedstocks are complex mixtures, and it is difficult to identify oxidized species due to interferences arising from the bulk hydrocarbon matrix.5,6 Coelution of oxidized species and other trace heteroatomic compounds with hydrocarbons is impossible to avoid, and oxygen-specific detectors lack the sensitivity and robustness required to successfully characterize these analytes. Comprehensive two-dimensional gas chromatography (GC×GC) is a technique that is becoming increasingly utilized in the analysis of complex hydrocarbon mixtures, such as found in jet fuels.7 The advantages of GC×GC, which include high peak capacity, ordered elution of compound classes, increased © XXXX American Chemical Society
resolving power, and improved sensitivity, make it ideally applicable to the resolution of both the bulk and trace components of complex matrixes.8 Useful information about the composition of fuels, particularly the type and amount of the many different hydrocarbon and heteroatomic species, may be obtained using GC×GC, such that a group-type or “fingerprinting” type analysis is attainable for distinguishing between different fuel types.9−11 Traditionally, detailed information about the trace and nonhydrocarbon species in fuels has been acquired via lengthy and resource-intensive approaches. Poor selectivity, incomplete separations, and inadequate removal of interfering compounds have made detailed analysis of the trace components of fuels challenging. Employing a GC×GC approach addresses and may negate the need for extensive sample preparation and pretreatment techniques, such as fractionation or solid-phase or liquid extractions. GC×GC combined with time-of-flight mass spectrometers (TOFMS) providing unit mass resolution has previously been demonstrated as a worthwhile technique for the analysis of middle distillate fuels.12−14 The data acquisition rate of these detectors, usually at least 50 Hz, permits sufficiently fast sampling of the narrow peaks generated through modulation of first dimension peaks required to obtain high quality data.15 More recently, accurate mass capability TOFMS has been shown to contribute additional identification, particularly where analytes and fragments share similar unit masses and/or fragmentation patterns. This is due to the mass accuracy of the TOFMS being
