Paper plate-assisted two-step - fractionation and speciation - for the evaluation of nickel and vanadium porphyrins via laser ablation inductively coupled plasma mass spectrometry.

This manuscript presents a novel two-step separation method for the fractionation and speciation of nickel (Ni) and vanadium (V) metalloporphyrins in light, medium, and heavy crude oil samples using a simple paper-based chromatographic platform (5 × 5 cm). Initially, the SARA (Saturates, Aromatics, Resins, and Asphaltenes) method was employed to separate crude oil into asphaltene and maltene fractions. The asphaltene fraction underwent direct speciation, while the maltene fraction was subjected to a sequential two-step separation-fractionation followed by speciation, on the same paper-based plate. Only 1 μL of each crude oil sample and 4 mL of solvent (hexane, toluene, dichloromethane, methanol, or their mixtures) were required for the separation, which was completed in approximately 5 min. A solvent gradient elution system was used, with solvent polarity increasing incrementally every 1 cm along the plate. After separation, the analyte-containing spots were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to detect Ni and V associated with metalloporphyrins. Among the detected compounds, octaethylporphyrin-Ni and oxygenated-V porphyrins were identified in the more polar fractions. Their identities were confirmed by comparison with a porphyrin standard and further validated using high-resolution mass spectrometry (HRMS). This cost-effective and environmentally friendly method offers a viable alternative to conventional non-paper-based chromatographic techniques. Additionally, it facilitates the direct identification of metalloporphyrins in complex crude oil matrices, including asphaltenes, with minimal sample preparation and solvent consumption.