Deep eutectic solvent-based dispersive liquid-liquid microextraction: A sustainable approach for determination of...

Nitrogen-, sulfur-, and oxygen-containing heterocyclic aromatic compounds (NSO-HETs), such as carbazole, indole, dibenzothiophene, benzonaphthothiophene, and dibenzofuran, are recalcitrant and undesirable contaminants in fuels. In this study, a sustainable deep eutectic solvent-based dispersive liquid-liquid microextraction (DES-DLLME) method was developed for the simultaneous determination of NSO-HET in fuel samples. Initially, univariate experiments were performed to evaluate the extraction performance of six DES mixtures composed of choline chloride, tetrabutylammonium bromide, ethylene glycol, oxalic acid, and malonic acid. Subsequently, multivariate optimization using a simplex-centroid mixture design, a fractional factorial design (2^5-2), and a Doehlert matrix was employed to investigate the main parameters that influence the DES-DLLME (extraction time, solvent volume, and cleanup). Under optimal conditions, the method was validated and showed adequate limits of detection (LOD<21.31 μg L^-1) and quantification (LOQ<70.98 μg L^-1), good selectivity, suitable ruggedness, precision (RSD<20%), and accuracy (50-118%). The use of greenness (AGREEprep), practicality (BAGI), and sustainability (SPMS) metrics demonstrated the sustainability and applicability of the proposed DES-DLLME. Finally, the method was applied to marine diesel and diesel containing 500 ppm and 10 ppm of sulfur. Ten NSO-HET were quantified in the samples with the highest concentrations in the marine diesel (86.9 μg L^-1 to 496 μg L^-1). No significant differences between the diesel fuels were observed, and Spearman's rank correlation revealed significant relationships involving quinoline, indole/benzofuran, 2,1-benzonapthothiophene, dibenzothiophene, and 2-methylbenzofuran, suggesting similarities among the samples. To the best of our knowledge, this study is the first to employ DES-DLLME for the quantification of NSO-HETs in real fuel samples. This sustainable analytical strategy can be applied to fuel derivatives, contributing to greener practices in fuel analysis.