Appraisal of the chemical modification process for the determination of lead by ultrasonic slurry sampling-electrothermal atomic absorption spectrometry

Conventional and permanent modifiers separately and joined together were investigated for the determination of lead in coal fly ash samples by ultrasonic slurry sampling-electrothermal atomic absorption spectrometry (SS-ETAAS). Thorium in solution alone and combined with rhodium as a permanent modifier provided the most consistent performance in terms of precision and accuracy. Evidence about the action mechanism of the chemical modifiers was derived by scanning electron microscopy (SEM)-energy dispersive (ED) X-ray spectrometry from the solid residue on the graphite platforms that had previously been submitted to several temperatures. Mixtures of Th–Pb and Rh–Pb oxides were stated to be the most plausible precursor species of the lead gas phase atoms for Th-based modifiers. By contrast, oxygen-free Rh–Pb metallic species were found using the permanent modifier alone. The effects of the modifiers, the matrix components and the slurry parameters, as well as the pyrolysis and atomization temperatures, on the Pb atomic absorption signals were also studied. Calibration was carried out against aqueous standard solutions. The reliability of the whole procedure was validated by analyzing various coal fly ash certified reference materials. The between-batch precision (n = 7) was better than 4.5% for lead concentrations of 50 μg l−1. The detection limits and characteristic masses were 0.17 μg l−1 and 18 pg with thorium added in solution and 0.25 μg l−1 and 27 pg with rhodium, as permanent modifier, combined with thorium in solution for a 10 μl volume injected.