Quantification of heterocyclic aromatic compounds (NSO-HET) in fuels by offline HPLC-GC×GC-ToFMS

Polycyclic aromatic heterocycles containing nitrogen, sulfur and oxygen (NSO-HET) are toxic and persistent contaminants commonly found in fuels. The simultaneous determination of NSO-HET is hindered by matrix complexity, isomer overlap, and coelution with aromatic compounds. Conventional approaches rely on laborious steps like fractionation to reduce the complexity of the matrix before chromatographic analysis. The resolution power of the comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometric (GC × GC-ToFMS) allows fewer sample preparation steps; however, quantification studies by this technique are limited. This study addressed the analytical challenge of simultaneously quantifying NSO-HET in complex fuels by GC × GC-ToFMS with minimal sample preparation.

An offline preparative high-performance liquid chromatography (HPLC) method was applied to enrich the aromatic fraction and remove interferences, followed by GC × GC-ToFMS analysis of 55 NSO-HET across six fuel samples. Sample preparation was simplified, resulting in a reduction of solvent and sample manipulation. The method was validated and demonstrated adequate performance, with low limits of detection and quantification (0.34-70.34 ng mL^-1), recoveries from 56.6 % to 103 % and RSD values below 20 %. The method was successfully applied to both unfractionated and fractionated fuels. Marine diesel and diesel S-500 showed the highest concentrations, with 2,5,7-trimethylbenzothiophene and dibenzofuran at 943 ± 85.7 ng mL^-1 and 884 ± 31.7 ng mL^-1, respectively. Principal component analysis revealed significant discrimination among the samples based on compound class and concentration. This is the first study on the simultaneous quantification of a wide range of NSO-HET compounds in fuels samples by GC × GC-ToFMS.

The offline HPLC-GC × GC-ToFMS validated method enabled the accurate and simultaneous quantification of 55 individual NSO-HETs, including benzothiophenes, dibenzothiophenes, naphthobenzothiophenes, carbazoles, indoles, and dibenzofurans, in different fuels. It provides a robust alternative to traditional approaches for both industrial and research applications.