Application of multivariate techniques for optimization of direct method for determination of lead in naphtha and petroleum condensate by electrothermal atomic absorption spectrometry

A direct method is proposed for the determination of lead in naphtha and petroleum condensate by electrothermal atomic absorption spectrometry ( ET AAS) using palladium as a permanent modifier. The procedure includes the dilution of 3 mL of sample ( naphtha or petroleum condensate) to a final volume of 10 mL with xylene, and direct injection of 30 mL of this solution into the graphite furnace. The optimization of the instrumental conditions was performed using multivariate techniques. Firstly, a 2 3 full factorial design was performed for preliminary evaluation of the factors: pyrolysis time, pyrolysis temperature and atomization temperature. This experiment showed that in the studied levels only the factors pyrolysis time and atomization temperature were significant. Then, a 3 2 full factorial design was performed for the determination of the critical conditions of these variables. The method allows the determination of lead using the standard calibration technique with a calibration curve from 2.6 to 30 mu g L-1 ( correlation coefficient higher than 0.998). A limit of detection (3 sigma) of 0.8 mu g L-1 and a characteristic mass of 35 pg were obtained in the presence of palladium as modifier. The precision expressed as relative standard deviation (RSD) was 1.5 and 0.8% for lead concentrations of 3.0 and 30 mu g L-1 (n = 10). Recovery studies demonstrate that lead can be determined in naphtha and petroleum condensate using calibration with organic standard solutions. This method was applied for the determination of lead in three petroleum condensate and two naphtha samples. The concentrations found for the petroleum condensate was between 2.7 and 5.7 mu g L-1, while the naphtha samples did not contain any detectable lead.