Integrated SARA-Fluorescence Analysis of Crude Oil with Fraction-Specific Calibration

The compositional complexity and variability of crude oil demand rapid, reliable analytical methods for both refinery optimization and environmental forensics. Fluorescence spectroscopy is a sensitive, non-destructive alternative to conventional chromatographic and thermal techniques, yet its potential for quantitative, fraction-specific analysis remains under-exploited. In this work, SARA (saturates, aromatics, resins, asphaltenes) fractionation was integrated with systematic fluorescence profiling to characterize Iraqi Basra crude oil. Each fraction exhibited a distinct, reproducible fluorescence signature reflecting its underlying photophysics: the emission intensity of the saturate (alkane) and aromatic fractions increased linearly with concentration (0.001-0.04% w/v and 0.001-0.02% w/v, respectively; R² > 0.998), whereas the resin and asphaltene fractions displayed inverse linear responses (0.005-0.04% w/v and 0.005-0.08% w/v, respectively; R² > 0.997) attributable to aggregation-induced quenching. Fraction-specific linear calibration equations were established, and their slopes, intercepts, 95% confidence intervals, limits of detection and limits of quantification are reported. A systematic bathochromic shift in the emission maxima, from the saturate fraction (388 nm) and aromatics (435 nm) to asphaltenes (455 nm) and resins (527 nm) under 350 nm excitation, provides a spectroscopic ruler for the degree of polycyclic aromatic hydrocarbon (PAH) condensation, as confirmed by sub-fractionation of the aromatic fraction into mono-, bi-, and polycyclic classes (λ_max = 411, 450 and 480 nm, respectively). The method delivered good accuracy (90-105% recovery) and precision (intra-day RSD < 5%). Cross-validation against GC-MS on ten real petroleum samples showed no statistically significant bias (paired t-test, p = 0.75) or difference in precision (F-test, p = 0.78), confirming that the fluorescence-derived aromatic contents are statistically indistinguishable from the reference method. Because the individual SARA signatures overlap in untreated whole crude oil, the method is applied after SARA fractionation. By linking spectral fingerprints to chemical composition, this work advances fluorescence spectroscopy beyond qualitative fingerprinting toward a validated tool for rapid crude oil characterization, with promising potential for future field-deployable applications in spill assessment, pipeline monitoring and refinery feedstock evaluation, pending validation on a broader range of crude oil types.