Pulsed voltammetric/amperometric detection of polycyclic aromatic sulfur heterocycles (PASHs) at the gold disc electrode for studies in petroleum asphalts

This work describes the voltammetric and amperometric behavior of a high number of PASHs (sulfides, thiophenes, benzothiophenes, dibenzothiophenes, indenothiophenes, naphtothiophenes, thienothiophenes, phenanthrothiophenes, and acenaphtothiophenes) at gold disc electrodes aiming at their identification and determination in petroleum asphalts. The adsorption/redox processes expected for sulfur compounds at gold electrodes could be observed in all the studied PASHs in DMSO and hydromethanolic medium. Differential pulse (DP) voltammetry in non-aqueous solutions (0.1 mol L⁻¹ NaClO₄ in DMSO) was approached for determining non-volatile PASHs in asphalts submitted to different aging processes. It was found herein that the DP voltammetric monitoring of PASH oxidation at + 0.7 V (vs. Ag/AgCl/LiCl 3 mol L⁻¹) for virgin/aged asphalts can be used for the comparative study of asphalts based on the consumption of PASHs. Additionally, pulsed amperometric detection (PAD) in hydroalcoholic solution (10 mmol L⁻¹ acetate buffer in 65% methanol) coupled with a chromatographic separation was approached for determining volatile PASHs in asphalts submitted to thermal decomposition processes. A detection cycle of 2 s involving oxidative (0.4 s at + 0.4 V) and reductive (1.2 s at − 1.0 V) cleaning pulses after a detection pulse of − 0.8 V (0.4 s) applied successively to the gold electrode (vs. Pd/PdO) was found to be optimal for regenerating the gold surface during successive chromatographic runs of PASHs. Thus, reversed-phase liquid chromatography (LC)–coupled PAD was found useful to separate a complex mixture of PASHs. The optimized PAD and LC separation was further applied to investigate the presence of electroactive PASHs as volatile compounds in asphalt fumes generated at 260 °C.