Arsenic determination in naphtha by electrothermal atomic absorption spectrometry after preconcentration using multiple injections

Abstract

Naphtha is a petroleum fraction containing C4–C15 hydrocarbon compounds and is used as feedstock for petrochemical processes which are affected profoundly by trace amounts of arsenic. A simple method for arsenic determination in naphtha using electrothermal atomic absorption spectrometry with polarized Zeeman-effect background correction was developed. Multiple injections were used for direct preconcentration in the graphite tube, eliminating sample pretreatment. Doehlert experimental designs were used to find the best settings for the furnace program parameters. Modifier concentration, number of multiple injections and sample volume were also optimized with the same multivariate approach. With three 45 µL sample injections, ashing temperature 1200 °C, atomization temperature 2700 °C and ashing time 60 s, a detection limit of 0.5 µg L⁻¹ and a characteristic mass of 61 pg were achieved, using 3000 mg L⁻¹ Pd(NO₃)₂ as the chemical modifier. The relative signal standard deviation was found to be 9.0% at the 2.3 µg.L⁻¹ level. Results from several naphtha test samples showed arsenic levels typically below 3.0 µg L⁻¹. The high degree of automation of the proposed method minimized technician sample handling allowing its application for routine analysis. This procedure has been used for arsenic determination in naphtha feeds processed in the Braskem Petrochemical (Salvador, Bahia, Brazil).