Application of chromatography and mass spectrometry to the analysis of gasoline

Abstract

The introduction of increasing stringent emission legislation placed on vehicle manufactures has resulted in the development of more efficient gasoline engines, namely, the development of gasoline direct injection (GDI) engines. GDI provides increased efficiency when compared to previous fuel injection systems. Although, GDI injectors are susceptible to injector nozzle deposits due to the elevated pressures and temperatures experienced within the combustion chamber. The presence of deposits within the injector nozzle impact the spray pattern produced and the flow of gasoline, reducing the efficiency of GDI. It is postulated that the formation of deposits is linked to gasoline composition, thus it is essential to comprehensively understand the chemical make-up of gasoline. Mass spectrometry and chromatography are extremely versatile tools to investigate gasoline composition, providing selectivity and detailed structural information for the compounds present.

EI GC-MS is the industry standard technique for gasoline analysis. It affords high chromatographic resolution and detailed structural information from energetic fragmentation for the hydrocarbon base fuel and volatile additives in gasoline. Although it is limited to thermally stable and low molecular weight compounds. Gasoline samples can often appear near identical, with just varying concentrations of the compounds present.

The use of atmospheric pressure ionisation mass spectrometry (API-MS) (i.e. electrospray ionisation, atmospheric pressure photo-ionisation and atmospheric pressure chemical ionisation) has permitted the detection of thermally labile and high molecular weight compounds within the gasoline matrix, while providing selectivity for the polar fraction of the gasoline, eliminating the hydrocarbon matrix. Differences between gasoline samples, which appeared identical by GC-MS are readily apparent, further selectivity was achieved by using ionisation additives.

Due to gasoline’s increase solubility in supercritical CO2 when compared to the mobile phases of liquid chromatography, ultra-high pressure supercritical fluid chromatography (UHPSFC) was used to separate components of the gasoline samples prior to ionisation to reduce the effect of ion suppression.

Two derivatised polyisobutylene detergent additives and one derivatised polypropylene glycol fluidiser additive. The additives were identified in finished gasoline samples at typically doping concentrations, without the use of pure standards for each polymeric additive. The gasoline samples required little sample preparation (i.e. a 5 % gasoline in methanol, and the addition of an ionisation additive).

Optical microscopy was used to identify performance reducing deposits within GDI injectors from test engines in an attempt to correlate the presence of deposits with compounds within the gasoline matrix.

SpectralWorks AnalyzerProTM was used to provide a gauge on the similarity of the GC-MS data of ten gasoline samples, which look near identical when interpreting manually. One gasoline was identified as being very different compared to the other nine gasoline samples. This gasoline is thought to be a fuel designed to produce deposits within a test engine, and is described by a reference fuel patent (US 8764854 B).

UHPSFC coupled to API-MS was used to investigate the presence deposit precursors, named by the patent, within the gasoline samples (diolefins and peroxides).

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Identifiers

ORCID for G. John Langley: orcid.org/0000-0002-8323-7235

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