Identification and Chemometric Profiling of C₁₀- and C₁₁-Decalins as Novel Hydrodesulfurization Products for Diesel Spill Environmental Forensics

This study introduces decahydronaphthalenes (C₁₀- and C₁₁-decalins; C₀De and C₁De) as a novel class of hydrodesulfurization (HDS)-associated forensic markers. Diesel-contaminated soil residues from 12 gas station spills and fresh ultra-low-sulfur diesel (ULSD) reference samples (2018-2025) in Taiwan were evaluated. The decalin abundances, derived from gas chromatography/mass spectrometry (GC/MS) peak areas, were converted into diagnostic ratios (DRs) using a conserved C₁₅ sesquiterpane (BS3) internal reference. These ratios were correlated with conventional fingerprint parameters and alternative HDS products to support source identification and manufacturing-era estimation. Notably, Pearson correlations revealed that C₀De and C₁De weathering behaviors mimic those of short- (CH-6/CH-7) and medium-chain (CH-9) n-alkylcyclohexanes, respectively. Furthermore, C₁De's negative correlation with CxD proxies indicates the premise that rigorous HDS processing lowers sulfur concentration while elevating these hydrogenated side products. Expanding on a framework using dimethylbiphenyls (C₂B) and trimethyltetralins (C₃T), adding these novel decalin markers in principal component analysis (PCA) maintained manufacturer-specific source discrimination, with collinear loading vectors confirming C₁De and C₃T as codependent HDS markers. Cross-validated (CV) linear discriminant analysis (LDA) showed that replacing C₃T with C₀De or C₁De improved discrimination among broad sulfur-regulatory manufacturing-era groups, although residual classification uncertainty remained for strongly weathered samples. These results support C₀De and C₁De as complementary HDS-associated markers for semi-quantitative manufacturing-era assessment rather than exact chronological dating.