Characterization of Sulfur-Containing Compounds Derived from Jet Fuels Using (+)APCI Coupled to an Orbitrap Mass Spectrometer

In jet fuels, sulfur-containing compounds (SCCs) contribute to beneficial (i.e., lubricity) and adverse characteristics (i.e., fuel instability, emissions). Previous qualitative methods relied heavily on the combined use of sulfur-specific detectors and mass spectrometers (MS) to complete compositional analysis of SCCs in jet fuels. As the methods required measurements of known SCCs to facilitate identification, qualitative analysis was significantly limited. With SCCs influencing important fuel properties, novel qualitative methods are warranted. In this study, a method was developed to complete the qualitative characterization of SCCs in jet fuels by using positive-ion mode atmospheric pressure chemical ionization (+)APCI coupled to an orbitrap. Prior to fuel measurements, SCCs exhibiting different functionalities were measured to determine dominant ionization reaction(s). Tandem mass spectrometry measurements were also employed on ionized SCCs to determine the extent of structural information achievable and applicability for structural elucidation of SCCs. Upon MS1 fuel measurements, the elemental compositions, range and average m/z values, carbon numbers, ring and double bond equivalent values, and relative abundances for ionized SCCs in each identified homologue ion series (ions of the same empirical formula increasing by 14.01565 Da, a CH2 unit) were determined for each jet fuel. New ion series were identified. Further, the most abundant ion series were represented by empirical formulas of CnH2n+1S, CnH2n–1S, and CnH2n–3S. Furthermore, the detected SCCs exhibited greater ranges in carbon number (i.e., 7–19 carbons) and molecular weights than previously recorded. Kendrick mass defect plots were also made, which demonstrated clear qualitative differences for the fuels. Overall, the method was successful at providing more compositional information than past analytical methods and required only a simple dilution (S-methylation, extractions, and high-performance liquid chromatography (HPLC) separations were not required). Access to such detailed compositional data will likely prove critical for linking the SCC composition to fuel properties, such as emissions and fuel stability.