Miniaturized liquid-liquid extraction for selective separation and enrichment of non- or low-alkylated aromatics from complex mineral oil mixtures

Mineral oil aromatic hydrocarbons (MOAH) are a complex mixture of aromatic compounds present in mineral oils and are of toxicological concern due to the possible presence of genotoxic and carcinogenic species, particularly polycyclic aromatics with three or more rings (≥3R) and a low degree of alkylation. Current analytical techniques, such as LC-GC-FID, show limited chromatographic resolving power when applied to MOAH, which are dominated by highly alkylated mono- and di-aromatic hydrocarbons that can mask trace-level polyaromatics, even when advanced comprehensive two-dimensional gas chromatographic (GC × GC) approaches are employed. This study presents a novel application of a miniaturized liquid-liquid extraction (LLE) procedure as a selective pre-separation and enrichment step to isolate and enrich the most toxic MOAH fraction. The implemented protocol, consisting of a double first extraction step (DMF/water 9:1 v/v) followed by back extraction with hexane after dilution with a 4% NaCl solution, promotes the selective redistribution of aromatic compounds primarily according to their degree of alkylation and polarity. Non- and low-alkylated aromatics, including most ≥3R PAHs, are efficiently transferred to the back extract, whereas highly alkylated mono- and di-aromatic hydrocarbons remain predominantly in the first extract. Using PAH standard mixtures, the method achieved an average cumulative recovery of approximately 92% for ≥3R PAHs, while mineral oil saturated hydrocarbons (MOSH) were almost quantitatively retained in the first extract (96.7% ± 3.3%). Application to model mineral oil mixtures further demonstrated a strong reduction of squalene and highly alkylated MOAH interferences. Overall, this miniaturized and cost-effective LLE approach generated aromatic extracts, substantially mitigating chromatographic masking effects and improving GC × GC-based characterization of toxicologically relevant MOAH fractions.