Determination of halogens and sulfur in pitch from crude oil by plasma-based techniques after microwave-induced combustion

Abstract

This work demonstrates the feasibility of halogen and sulfur determination in pitch produced from crude oil residues by inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP-OES), respectively, after digestion by microwave-induced combustion (MIC). Using the proposed system, it was possible to achieve a very effective digestion of pitch (up to 400 mg) using 20 bar of oxygen and 50 μL of 6 mol L⁻¹ of ammonium nitrate as an ignition aid. Water and NH₄OH solutions (10 to 100 mmol L⁻¹) were evaluated for analyte absorption and a reflux step was applied after combustion in order to ensure quantitative analyte absorption. The volume of absorbing solution (6, 8, 10 and 12 mL) was also evaluated and best results were obtained using 6 mL confirming the results of previous studies. Accuracy was evaluated by comparison of results with those obtained using a total Cl and S analyzer, X-ray fluorescence (XRF, for Cl and S determination) and pyrohydrolysis followed by ICP-MS determination (Br and I). By using 50 mmol L⁻¹ NH₄OH as the absorbing solution the agreement with reference values ranged from 95 to 103% for all analytes. The results obtained using MIC presented an agreement of 101 and 100% with those obtained using a total Cl and S analyzer, respectively. The results were also compared with XRF analysis (for Cl and S) and agreement was 102 and 92%, respectively. In the same way, the results obtained by MIC were in agreement with those obtained by the pyrohydrolysis method (agreement of 97 and 105% for Br and I, respectively). The residual carbon content of MIC digests was always below 1% showing the high efficiency of pitch digestion. Using 400 mg of sample mass and 6 mL of 50 mmol L^-1 NH₄OH as the absorbing solution, limits of detection were 3.1 μg g⁻¹ for S by ICP-OES and 3.2, 0.02 and 0.002 μg g⁻¹ for Cl, Br and I by ICP-MS, respectively.