Determination of sulfur in diesel via CS molecule by high-resolution molecular absorption spectrometry applying palladium nanoparticles as chemical modifier

Abstract

Crude oil contains a high concentration of sulfur, an element that poses serious environmental and health risks. Sulfur occurs mostly in heavier petroleum fractions, like diesel, and it is possible to reduce its concentration by using methods such as catalytic hydrodesulfurization. High-resolution continuum source molecular absorption spectrometry (HR-CS MAS) allows for determination of non-metals. Sulfur is generally analyzed as the CS molecule at 258 nm, which corresponds to the electronic transition X¹Σ⁺ → A¹Π. This study aimed to apply graphite furnace HR-CS MAS to determine sulfur in diesel, using palladium nanoparticles as chemical modifier. To evaluate the proposed method, seven diesel samples (including two reference samples) were employed as emulsions with surfactants and alcohols. Direct diesel analysis was also evaluated, in an attempt to improve sulfur detection. Sodium sulfate, thiourea, and 2-mercaptoethanol were used as sulfur standards. Two types of synthesized palladium nanoparticles were compared with other chemical modifiers in graphite furnace vaporization. Measurements were conducted at 258.056 nm, with detection of three pixels. The ethanol emulsion was stable (4 h). PdCl₂ nanoparticles provided the best results as chemical modifier, and thiourea was the most suitable sulfur standard under the experimental conditions. The following figures of merit were obtained: limit of detection (LOD) = 120 mg kg⁻¹, limit of quantification (LOQ) = 400 mg kg⁻¹, R² = 0.9972, and relative standard deviation = 1–5%. The method's accuracy, evaluated with two reference samples, agreed with a confidence interval of 95%. It was not possible to determine sulfur in one sample only (sulfur concentration 10 mg kg⁻¹), due to the LOD. This drawback was circumvented by applying direct diesel analysis. Using Pd nanoparticles as chemical modifier it was possible to obtain better sensitivity, LOD (7.2 ng absolute or 3 mg kg⁻¹ for direct analysis), and LOQ (24 ng or 9 mg kg⁻¹).